Can You Explain How Tarjan's Pseudocode Works to Someone Familiar with C or Java?












2












$begingroup$


The Short Story



A famous computer scientist Tarjan wrote a book years ago. It contains absolutely bizarre pseudo-code. Would someone please explain it?



The Long Story #



Tarjan is known for many acomplishments, including the fact that he was co-inventor of splay trees. He published a book, "Data Structures and Network Algorithms," during the 1980s.



All of the pseudo-code in Tarjan's book is written in a language of his own devising. The pseudo-code conventions are very regimented. It's almost a true language, and one could imagine writing a compiler for it. Tarjan writes that his language is based upon the following three:




  • Dijkstra's Guarded Command Language


  • SETL


  • ALGOL



I am hoping that someone familiar with one or two of the above languages, or the work of Tarjan, will be able to answer my question.



An example of a function written in Tarjan's language is shown below:



heap function mesh (heap nodes h1, h2);

if key(h1) > key(h2) → h1 ⟷ h2 fi;

right (h1) := if right(h1) = null → h2

|right(h1) ≠ null → mesh (right(h1), h2) fi;

if rank (left (h1)) < rank (right (h1)) → left(h1) ⟷ right(h1) fi;

rank (h1) := rank(right(h1)) + 1;

return h1,

end mesh;


I have seen lots of pseudo-code, but I have never seen anything like Tarjan's. How does Tarjan's pseudocode work? How can examples of Tarjan's pseudocode be re-written as something which looks more like C or Java? It need not even be C or Java. The if-else construct in Tarjan's language is not only different from C-family languages, but also different from Python, Matlab and many others.










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    2












    $begingroup$


    The Short Story



    A famous computer scientist Tarjan wrote a book years ago. It contains absolutely bizarre pseudo-code. Would someone please explain it?



    The Long Story #



    Tarjan is known for many acomplishments, including the fact that he was co-inventor of splay trees. He published a book, "Data Structures and Network Algorithms," during the 1980s.



    All of the pseudo-code in Tarjan's book is written in a language of his own devising. The pseudo-code conventions are very regimented. It's almost a true language, and one could imagine writing a compiler for it. Tarjan writes that his language is based upon the following three:




    • Dijkstra's Guarded Command Language


    • SETL


    • ALGOL



    I am hoping that someone familiar with one or two of the above languages, or the work of Tarjan, will be able to answer my question.



    An example of a function written in Tarjan's language is shown below:



    heap function mesh (heap nodes h1, h2);

    if key(h1) > key(h2) → h1 ⟷ h2 fi;

    right (h1) := if right(h1) = null → h2

    |right(h1) ≠ null → mesh (right(h1), h2) fi;

    if rank (left (h1)) < rank (right (h1)) → left(h1) ⟷ right(h1) fi;

    rank (h1) := rank(right(h1)) + 1;

    return h1,

    end mesh;


    I have seen lots of pseudo-code, but I have never seen anything like Tarjan's. How does Tarjan's pseudocode work? How can examples of Tarjan's pseudocode be re-written as something which looks more like C or Java? It need not even be C or Java. The if-else construct in Tarjan's language is not only different from C-family languages, but also different from Python, Matlab and many others.










    share|cite|improve this question







    New contributor




    Sam Muldoon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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      2












      2








      2





      $begingroup$


      The Short Story



      A famous computer scientist Tarjan wrote a book years ago. It contains absolutely bizarre pseudo-code. Would someone please explain it?



      The Long Story #



      Tarjan is known for many acomplishments, including the fact that he was co-inventor of splay trees. He published a book, "Data Structures and Network Algorithms," during the 1980s.



      All of the pseudo-code in Tarjan's book is written in a language of his own devising. The pseudo-code conventions are very regimented. It's almost a true language, and one could imagine writing a compiler for it. Tarjan writes that his language is based upon the following three:




      • Dijkstra's Guarded Command Language


      • SETL


      • ALGOL



      I am hoping that someone familiar with one or two of the above languages, or the work of Tarjan, will be able to answer my question.



      An example of a function written in Tarjan's language is shown below:



      heap function mesh (heap nodes h1, h2);

      if key(h1) > key(h2) → h1 ⟷ h2 fi;

      right (h1) := if right(h1) = null → h2

      |right(h1) ≠ null → mesh (right(h1), h2) fi;

      if rank (left (h1)) < rank (right (h1)) → left(h1) ⟷ right(h1) fi;

      rank (h1) := rank(right(h1)) + 1;

      return h1,

      end mesh;


      I have seen lots of pseudo-code, but I have never seen anything like Tarjan's. How does Tarjan's pseudocode work? How can examples of Tarjan's pseudocode be re-written as something which looks more like C or Java? It need not even be C or Java. The if-else construct in Tarjan's language is not only different from C-family languages, but also different from Python, Matlab and many others.










      share|cite|improve this question







      New contributor




      Sam Muldoon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.







      $endgroup$




      The Short Story



      A famous computer scientist Tarjan wrote a book years ago. It contains absolutely bizarre pseudo-code. Would someone please explain it?



      The Long Story #



      Tarjan is known for many acomplishments, including the fact that he was co-inventor of splay trees. He published a book, "Data Structures and Network Algorithms," during the 1980s.



      All of the pseudo-code in Tarjan's book is written in a language of his own devising. The pseudo-code conventions are very regimented. It's almost a true language, and one could imagine writing a compiler for it. Tarjan writes that his language is based upon the following three:




      • Dijkstra's Guarded Command Language


      • SETL


      • ALGOL



      I am hoping that someone familiar with one or two of the above languages, or the work of Tarjan, will be able to answer my question.



      An example of a function written in Tarjan's language is shown below:



      heap function mesh (heap nodes h1, h2);

      if key(h1) > key(h2) → h1 ⟷ h2 fi;

      right (h1) := if right(h1) = null → h2

      |right(h1) ≠ null → mesh (right(h1), h2) fi;

      if rank (left (h1)) < rank (right (h1)) → left(h1) ⟷ right(h1) fi;

      rank (h1) := rank(right(h1)) + 1;

      return h1,

      end mesh;


      I have seen lots of pseudo-code, but I have never seen anything like Tarjan's. How does Tarjan's pseudocode work? How can examples of Tarjan's pseudocode be re-written as something which looks more like C or Java? It need not even be C or Java. The if-else construct in Tarjan's language is not only different from C-family languages, but also different from Python, Matlab and many others.







      programming-languages






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      asked 2 hours ago









      Sam MuldoonSam Muldoon

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          $begingroup$

          Table of Contents #



          I will divide my explanation of Tarjan's pseudocode into the following sections:



          (1) Tarjan's If-else Blocks (the -> & | operators)



          (2) Assignment and Equality Tests (:= and =)



          (3) There is else if, but no else construct



          (4) Tarjan's Conditional Assignment Operator := if



          (4.5) Tarjan Arrays (or Lists)



          (5) Additional Examples of Tarjan's if and := if



          (6) Summary of Operators



          (7) Tarjan's Double-pointed Arrow Operator ()



          **(8) Tarjan's do-loops are like C/Java while-loops **



          **(9) Tarjan's Conditional-assignment operator with all false conditions **



          1) Tarjan's If-else Blocks #



          (the operators and | )



          The if-else construct is perhaps the most fundamental control structure in Tarjan's language. In addition to C-like if-blocks, if-else behavior is very nearly built-into in Tarjan's assignments and Tarjan's while loops. Tarjan's arrow operator -> (or →) is a delimiter between the condition of a if-statement and the execution block of an if-statement.



          For example, in Tarjan's language we might have:



          # Example One
          if a = 4 → x := 9 fi


          If we partially translate the line of Tarjan code above into C or Java, we get the following:



          if (a = 4)
          x := 9
          fi



          Instead of a right curly braces (as in C and Java) Tarjan ends an if-block with a backwards spelling of the key-word: fi



          If we continue translating our above example, we get:



          if (a = 4) {
          x := 9
          }


          (2) Assignment and Equality Tests (:= and =)#



          Tarjan uses = for equality tests, not assignments.



          Tarjan's = is like Java ==



          Tarjan uses := for assignment.



          Tarjan's := is like Java =



          Thus, if we continue translating our example, we have:



          if (a == 4) {
          x = 9
          }


          A vertical bar (or "pipe" or |) in Tarjan's language is equivalent to the else if keyword in C or Java.

          For example, in Tarjan's language we might have:



          # Example Two
          if a = 4 → x := 9 | a > 4 → y := 11 fi


          The Tarjan-code above translates to:



          if (a == 4) {
          x = 9
          }
          else if (a > 4) {
          y = 11
          }


          (3) else if only and no else construct



          Earlier, I covered the basics of if-statements without describing the nuances. However, we will not discuss a small detail. The last clause in a Tarjan-ian if-else block must always contain an arrow () operator. As such, there is no else in Tarjan's language, only else if. The closest thing to an else-block in Tarjan's language is to make the rightmost test-condition true.



          if a = 4 → x := 9 |  a > 4  → y := 11 | true  → z := 99  fi


          In C/Java, we would have:



          if (a == 4) {
          x = 9
          }

          else if (a > 4) {
          y = 11
          }
          else { // else if (true)
          z = 99
          }


          Examples are easier to understand than general descriptions. However, now that we have some examples under our belt, know that the general formal of a Tarjan's if-else construct is as follows:



          if condition
          → stuff to do
          | condition
          → stuff to do
          [...]
          | condition
          → stuff to do
          fi


          The character | is like if else



          The character separates the test-condition from the stuff-to-do.



          (4) Tarjan's Conditional Assignment Operator := if#



          Tarjan's if can be used two very different ways. So far, we have only described one of the uses of the Tarjanian if. Somewhat confusingly, Tarjan still uses the notation/syntax if for the second type of if-construct. Which if is being used is based on context. Analyzing the context is actually very easy to do as the second type of Tarjan-if is always pre-fixed by an assignment operator.



          For example, we might have the following Tarjan code:



          # Example Three
          x := if a = 4 → 9 fi


          Begin Digression###



          After working with Tarjan code for awhile, you get used to the order of operations. If we parenthesize test condition in the example above, we obtain:



          x := if (a = 4) → 9 fi



          a = 4 is not an assignment operation. a = 4 is like a == 4 -- it returns true or false.



          End Digression###



          It can help to think of := if as syntax for a single operator, distinct from := and if In fact, we will refer to the := if operator as the "conditional assignment" operator.



          For if we list (condition → action). For := if we list (condition → value) where value is teh right-hand-side value we might assign to the left-hand-side lhs



          # Tarjan Example Four
          lhs := if (a = 4) → rhs fi



          in C or Java might look like:



          # Example Four
          if (a == 4) {
          lhs = rhs
          }


          Consider the following example of "conditional assignment" in Tarjanian code:



          # Tarjan Instantiation of Example Five
          x := a = 4 → 9 | a > 4 → 11 | true → 99 fi



          In C/Java, we would have:



          // C/Java Instantiation of Example Five
          if (a == 4) {
          x = 9
          }
          else if (a > 4) {
          x = 11
          }
          else if (true) { // else
          x = 99
          }


          (5) Summary of Operators:



          So far, we have:




          • := ...... Assignment operator (C/Java=)


          • = ...... Equality test (C/Java ==)


          • ...... Delimiter between test-condition of an if-block and the body of an if-block


          • | ..... C/Java else-if


          • if ... fi ..... if-else block


          • := if... fi ..... Conditional assignment based on an if-else block



          (5.5) Tarjan Lists/Arrays:



          Tarjan's Language has built-in array-like containers. The syntax for Tarjan arrays is much more intuitive than the notation for Tarjan if else statements.



          list1  := ['lion', 'witch', 'wardrobe'];
          list2a := [1, 2, 3, 4, 5];
          list2b := [1, 2];
          list3 := ["a", "b", "c", "d"];
          list4 := [ ]; # an empty array


          Tarjan array elementa are accessed with parentheses (), not square-brackets



          Indexing begins at 1. Thus,



          list3  := ["a", "b", "c", "d"]
          # list3(1) == "a" returns true
          # list3(2) == "b" return true


          Below shows how to create a new array containing the 1st and 5th elements of [1, 2, 3, 4, 5, 6, 7]



          nums := [1, 2, 3, 4, 5, 6, 7]
          new_arr := [nums(1), nums(5)]


          The equality operator is defined for arrays. The following code prints true



          x := false
          if [1, 2] = [1, 2, 3, 4, 5] --> x := true
          print(x)


          Tarjan's way to test if an array is empty is to compare it to an empty array



          arr := [1, 2]
          print(arr = [ ])
          # `=` is equality test, not assignment


          One can create a view (not copy) of a sub-array, by providing multiple indices to operator () combined with ..



          list3  := ["a", "b", "c", "d"]

          beg := list3(.. 2)
          # beg == ["a", "b"]
          # beg(1) == "a"

          end := list3(3..)
          # end == ["c", "d"]
          # end(1) == "c"

          mid := list3(2..3)
          # mid == ["b", "c"]
          # mid(2) == "c"

          # `list3(4)` is valid, but `mid(4)` is not


          (6) Additional Examples of Tarjan's if and := if#



          The following is another examples of an Tarjan conditional assignment (:= if):



          # Tarjan Example Six
          a := (false --> a | true --> b | false --> c1 + c2 | (2 + 3 < 99) --> d)


          (true --> b) is the leftmost (cond --> action) clause having a true condition.
          Thus, the original assignment Example Six has the same assignment-behavior as a := b



          Below is our most complicated example of Tarjan code thus far:



          # Tarjan Example -- merge two sorted lists

          list function merge (list s, t);

          return if s = --> t
          | t = [ ] --> s
          | s != [ ] and t != and s(l) <= t(1) -->
          [s(1)]& merge(s[2..], t)
          | s != [ ]and t != [ ] and s(1) > r(l) -->
          [t(1)] & merge (s,t(2..))
          fi
          end merge;


          The following is a translation of Tarjan's code for merging two sorted lists. The following is not exactly C or Java, but it is much closer to C/Java than the Tarjan version.



          list merge (list s, list t) {



          if (s is empty) {
          return t;
          }
          else if (t is empty){
          return s;
          }
          else if (s[1] <= t[1]) {
          return CONCATENATE([s[1]], merge(s[2...], t));
          else { // else if (s[1] > t[1])
          return CONCATENATE ([t[1]], merge(s,t[2..]);
          }


          }



          Below is yet another example of Tarjan-code and a translation in something similar to C or Java:



          heap function meld (heap h1, h2);

          return if h1 = null --> h2
          | h2 = null --> h1
          | h1 not null and h2 not null --> mesh (h1, h2)
          fi
          end meld;


          Below is the C/Java translation:



          HeapNode meld (HeapNode h1, HeapNode h2) {

          if (h1 == null) {
          return h2;
          }
          else if (h2 == null) {
          return h1;
          } else {
          mesh(h1, h2)
          }
          } // end function


          (7) Tarjan's Double-pointed Arrow Operator (<-->)#



          Below is an example of Tarjan code:



          x <--> y    


          What Does a Double Arrow () Operator Do in Tarjan's Language?

          Well, almost all variables in Tarjan's Language are pointers.
          <--> is a swap operation. The following prints true



          x_old := x
          y_old := y
          x <--> y
          print(x == y_old) # prints true
          print(y == x_old) # prints true


          After performing x <--> y, x points to the object which y used to point to and y points to the object which x used to point to.



          Below is a Tarjan statement using the <--> operator:



          x := [1, 2, 3]
          y := [4, 5, 6]
          x <--> y


          Below is a translation from the Tarjan code above to alternative pseudocode:



          Pointer X     = address of array [1, 2, 3];
          Pointer Y = address of array [4, 5, 6];
          Pointer X_OLD = address of whatever X points to;
          X = address of whatever Y points to;
          Y = address of whatever X_OLD points to;


          Alternatively, we could have:



          void operator_double_arrow(Array** lhs, Array** rhs) {

          // swap lhs and rhs

          int** old_lhs = 0;
          old_lhs = lhs;
          *lhs = *rhs;
          *rhs = *old_lhs;
          return;
          }

          int main() {

          Array* lhs = new Array<int>(1, 2, 3);
          Array* rhs = new Array<int>(4, 5, 6);
          operator_double_arrow(&lhs, &rhs);
          delete lhs;
          delete rhs;
          return 0;
          }


          Below is an example of one of Tarjan's functions using the operator:



          heap function mesh (heap nodes h1, h2);
          if key(h1) > key(h2) → h1 ⟷ h2 fi;
          right (h1) := if right(h1) = null → h2
          |right(h1) ≠ null → mesh (right(h1), h2)
          fi;

          if rank (left (h1)) < rank (right (h1))
          → left(h1) ⟷ right(h1)
          fi;

          rank (h1) := rank(right(h1)) + 1;
          return h1;
          end mesh;


          Below is a translation of Tarjan's mesh function into pseudo-code which is not C, but looks more like C (relatively speaking). The purpose of this is to illustrate how Tarjan's operator works.



          node pointer function mesh(node pointers h1, h2) {

          if (h1.key) > h2.key) {

          // swap h1 and h2
          node pointer temp;
          temp = h1;
          h1 = h2;
          h2 = temp;
          }

          // Now, h2.key <= h1.key

          if (h1.right == null) {
          h1.right = h2;

          } else // h1.key != null {
          h1.right = mesh(h1.right, h2);
          }



          if (h1.left.rank < h1.right.rank ) {
          // swap h1.left and h1.right

          node pointer temp;
          temp = h1;
          h1 = h2;
          h2 = temp;
          }

          h1.rank = h1.right.rank + 1;
          return h1;
          }


          (8) Tarjan's do-loops are like C/Java while-loops #



          Tarjan's language if and for constructs are familiar for C/Java programmers. However, the Tarjan keyword for a while-loop is do. All do-loops end with the keyword od, which is the backwards spelling of do. Below is an example:



          sum := 0
          do sum < 50 → sum := sum + 1


          In C-style pseudocode, we have:



          sum = 0;
          while(sum < 50) {
          sum = sum + 1;
          }


          The above is actually not quite right. A Tarjan do-loop is really a C/Java while(true) with an if-else block nested inside. A more literal translation of the Tarjan code is as follows:



          sum = 0;
          while(true) {
          if (sum < 50) {
          sum = sum + 1;
          continue;
          // This `continue` statement is questionable
          }
          break;
          }


          Below, we have a more complicated Tarjan do-loop:



          sum := 0
          do sum < 50 → sum := sum + 1 | sum < 99 → sum := sum + 5


          C/Java-style pseudocode for the complicated Tarjan do-loop is as follows:



          sum = 0;
          while(true) {

          if (sum < 50) {
          sum = sum + 1;
          continue;
          }
          else if (sum < 99) {
          sum = sum + 5;
          continue;
          }
          break;
          }


          (9) Tarjan's Conditional-assignment operator with all false conditions



          Although the lengthy explanation above covers most things, a few matters are still left unresolved.
          I hope that someone else will someday write a new-improved answer based on mine which answers these quandries.



          Notably, when the conditional assignment operator := if is used, and no condition is true, I am not what value is assigned to the variable.



          x  := if (False --> 1| False --> 2 | (99 < 2) --> 3) fi


          I am not sure, but it is possible that no assignment is made to x:



          x = 0;
          if (false) {
          x = 1;
          }
          else if (false) {
          x = 2;
          }
          else if (99 < 2) {
          x = 3;
          }
          // At this point (x == 0)


          You could require that the left-hand-side variable seen in an := if statement be previously declared. In that case, even if all conditions are false, the variable will still have a value.



          Alternatively, perhaps all-false conditions represents a runtime error. Another alternative is to return a special null value, and store null in the left-hand argument of the assignment.






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            Never seen this before but I think I can infer what's meant from context.. Presumably the must be a swap operation, and if G1 -> S1 | G2 - >S2 | ... fi is an if/then/else-type construct that also returns a value, like the ternary ?: operator in C and Java.



            With that in hand we could write the above function in a Java-like language like so:



            HeapNode mesh(HeapNode h1, HeapNode h2)
            {
            if(h1.key > h2.key)
            {
            // swap h1 and h2

            HeapNode t = h1;
            h1 = h2;
            h2 = t;
            }

            // One of the two cases has to hold in this case so we won't get to the
            // exception, but it'd be an exception if none of the cases were satisified
            // since this needs to return *something*.

            h1.right = (h1.right == null) ? h2
            : (h1.right != null) ? mesh(h1.right, h2)
            : throw new Exception();

            if(h1.left.rank < h1.right.rank)
            {
            // swap h1.left and h1.right

            HeapNode t = h1.left;
            h1.left = h1.right;
            h1.right = t;
            }

            h1.rank = h1.right.rank + 1;

            return h1;
            }





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              $begingroup$

              Table of Contents #



              I will divide my explanation of Tarjan's pseudocode into the following sections:



              (1) Tarjan's If-else Blocks (the -> & | operators)



              (2) Assignment and Equality Tests (:= and =)



              (3) There is else if, but no else construct



              (4) Tarjan's Conditional Assignment Operator := if



              (4.5) Tarjan Arrays (or Lists)



              (5) Additional Examples of Tarjan's if and := if



              (6) Summary of Operators



              (7) Tarjan's Double-pointed Arrow Operator ()



              **(8) Tarjan's do-loops are like C/Java while-loops **



              **(9) Tarjan's Conditional-assignment operator with all false conditions **



              1) Tarjan's If-else Blocks #



              (the operators and | )



              The if-else construct is perhaps the most fundamental control structure in Tarjan's language. In addition to C-like if-blocks, if-else behavior is very nearly built-into in Tarjan's assignments and Tarjan's while loops. Tarjan's arrow operator -> (or →) is a delimiter between the condition of a if-statement and the execution block of an if-statement.



              For example, in Tarjan's language we might have:



              # Example One
              if a = 4 → x := 9 fi


              If we partially translate the line of Tarjan code above into C or Java, we get the following:



              if (a = 4)
              x := 9
              fi



              Instead of a right curly braces (as in C and Java) Tarjan ends an if-block with a backwards spelling of the key-word: fi



              If we continue translating our above example, we get:



              if (a = 4) {
              x := 9
              }


              (2) Assignment and Equality Tests (:= and =)#



              Tarjan uses = for equality tests, not assignments.



              Tarjan's = is like Java ==



              Tarjan uses := for assignment.



              Tarjan's := is like Java =



              Thus, if we continue translating our example, we have:



              if (a == 4) {
              x = 9
              }


              A vertical bar (or "pipe" or |) in Tarjan's language is equivalent to the else if keyword in C or Java.

              For example, in Tarjan's language we might have:



              # Example Two
              if a = 4 → x := 9 | a > 4 → y := 11 fi


              The Tarjan-code above translates to:



              if (a == 4) {
              x = 9
              }
              else if (a > 4) {
              y = 11
              }


              (3) else if only and no else construct



              Earlier, I covered the basics of if-statements without describing the nuances. However, we will not discuss a small detail. The last clause in a Tarjan-ian if-else block must always contain an arrow () operator. As such, there is no else in Tarjan's language, only else if. The closest thing to an else-block in Tarjan's language is to make the rightmost test-condition true.



              if a = 4 → x := 9 |  a > 4  → y := 11 | true  → z := 99  fi


              In C/Java, we would have:



              if (a == 4) {
              x = 9
              }

              else if (a > 4) {
              y = 11
              }
              else { // else if (true)
              z = 99
              }


              Examples are easier to understand than general descriptions. However, now that we have some examples under our belt, know that the general formal of a Tarjan's if-else construct is as follows:



              if condition
              → stuff to do
              | condition
              → stuff to do
              [...]
              | condition
              → stuff to do
              fi


              The character | is like if else



              The character separates the test-condition from the stuff-to-do.



              (4) Tarjan's Conditional Assignment Operator := if#



              Tarjan's if can be used two very different ways. So far, we have only described one of the uses of the Tarjanian if. Somewhat confusingly, Tarjan still uses the notation/syntax if for the second type of if-construct. Which if is being used is based on context. Analyzing the context is actually very easy to do as the second type of Tarjan-if is always pre-fixed by an assignment operator.



              For example, we might have the following Tarjan code:



              # Example Three
              x := if a = 4 → 9 fi


              Begin Digression###



              After working with Tarjan code for awhile, you get used to the order of operations. If we parenthesize test condition in the example above, we obtain:



              x := if (a = 4) → 9 fi



              a = 4 is not an assignment operation. a = 4 is like a == 4 -- it returns true or false.



              End Digression###



              It can help to think of := if as syntax for a single operator, distinct from := and if In fact, we will refer to the := if operator as the "conditional assignment" operator.



              For if we list (condition → action). For := if we list (condition → value) where value is teh right-hand-side value we might assign to the left-hand-side lhs



              # Tarjan Example Four
              lhs := if (a = 4) → rhs fi



              in C or Java might look like:



              # Example Four
              if (a == 4) {
              lhs = rhs
              }


              Consider the following example of "conditional assignment" in Tarjanian code:



              # Tarjan Instantiation of Example Five
              x := a = 4 → 9 | a > 4 → 11 | true → 99 fi



              In C/Java, we would have:



              // C/Java Instantiation of Example Five
              if (a == 4) {
              x = 9
              }
              else if (a > 4) {
              x = 11
              }
              else if (true) { // else
              x = 99
              }


              (5) Summary of Operators:



              So far, we have:




              • := ...... Assignment operator (C/Java=)


              • = ...... Equality test (C/Java ==)


              • ...... Delimiter between test-condition of an if-block and the body of an if-block


              • | ..... C/Java else-if


              • if ... fi ..... if-else block


              • := if... fi ..... Conditional assignment based on an if-else block



              (5.5) Tarjan Lists/Arrays:



              Tarjan's Language has built-in array-like containers. The syntax for Tarjan arrays is much more intuitive than the notation for Tarjan if else statements.



              list1  := ['lion', 'witch', 'wardrobe'];
              list2a := [1, 2, 3, 4, 5];
              list2b := [1, 2];
              list3 := ["a", "b", "c", "d"];
              list4 := [ ]; # an empty array


              Tarjan array elementa are accessed with parentheses (), not square-brackets



              Indexing begins at 1. Thus,



              list3  := ["a", "b", "c", "d"]
              # list3(1) == "a" returns true
              # list3(2) == "b" return true


              Below shows how to create a new array containing the 1st and 5th elements of [1, 2, 3, 4, 5, 6, 7]



              nums := [1, 2, 3, 4, 5, 6, 7]
              new_arr := [nums(1), nums(5)]


              The equality operator is defined for arrays. The following code prints true



              x := false
              if [1, 2] = [1, 2, 3, 4, 5] --> x := true
              print(x)


              Tarjan's way to test if an array is empty is to compare it to an empty array



              arr := [1, 2]
              print(arr = [ ])
              # `=` is equality test, not assignment


              One can create a view (not copy) of a sub-array, by providing multiple indices to operator () combined with ..



              list3  := ["a", "b", "c", "d"]

              beg := list3(.. 2)
              # beg == ["a", "b"]
              # beg(1) == "a"

              end := list3(3..)
              # end == ["c", "d"]
              # end(1) == "c"

              mid := list3(2..3)
              # mid == ["b", "c"]
              # mid(2) == "c"

              # `list3(4)` is valid, but `mid(4)` is not


              (6) Additional Examples of Tarjan's if and := if#



              The following is another examples of an Tarjan conditional assignment (:= if):



              # Tarjan Example Six
              a := (false --> a | true --> b | false --> c1 + c2 | (2 + 3 < 99) --> d)


              (true --> b) is the leftmost (cond --> action) clause having a true condition.
              Thus, the original assignment Example Six has the same assignment-behavior as a := b



              Below is our most complicated example of Tarjan code thus far:



              # Tarjan Example -- merge two sorted lists

              list function merge (list s, t);

              return if s = --> t
              | t = [ ] --> s
              | s != [ ] and t != and s(l) <= t(1) -->
              [s(1)]& merge(s[2..], t)
              | s != [ ]and t != [ ] and s(1) > r(l) -->
              [t(1)] & merge (s,t(2..))
              fi
              end merge;


              The following is a translation of Tarjan's code for merging two sorted lists. The following is not exactly C or Java, but it is much closer to C/Java than the Tarjan version.



              list merge (list s, list t) {



              if (s is empty) {
              return t;
              }
              else if (t is empty){
              return s;
              }
              else if (s[1] <= t[1]) {
              return CONCATENATE([s[1]], merge(s[2...], t));
              else { // else if (s[1] > t[1])
              return CONCATENATE ([t[1]], merge(s,t[2..]);
              }


              }



              Below is yet another example of Tarjan-code and a translation in something similar to C or Java:



              heap function meld (heap h1, h2);

              return if h1 = null --> h2
              | h2 = null --> h1
              | h1 not null and h2 not null --> mesh (h1, h2)
              fi
              end meld;


              Below is the C/Java translation:



              HeapNode meld (HeapNode h1, HeapNode h2) {

              if (h1 == null) {
              return h2;
              }
              else if (h2 == null) {
              return h1;
              } else {
              mesh(h1, h2)
              }
              } // end function


              (7) Tarjan's Double-pointed Arrow Operator (<-->)#



              Below is an example of Tarjan code:



              x <--> y    


              What Does a Double Arrow () Operator Do in Tarjan's Language?

              Well, almost all variables in Tarjan's Language are pointers.
              <--> is a swap operation. The following prints true



              x_old := x
              y_old := y
              x <--> y
              print(x == y_old) # prints true
              print(y == x_old) # prints true


              After performing x <--> y, x points to the object which y used to point to and y points to the object which x used to point to.



              Below is a Tarjan statement using the <--> operator:



              x := [1, 2, 3]
              y := [4, 5, 6]
              x <--> y


              Below is a translation from the Tarjan code above to alternative pseudocode:



              Pointer X     = address of array [1, 2, 3];
              Pointer Y = address of array [4, 5, 6];
              Pointer X_OLD = address of whatever X points to;
              X = address of whatever Y points to;
              Y = address of whatever X_OLD points to;


              Alternatively, we could have:



              void operator_double_arrow(Array** lhs, Array** rhs) {

              // swap lhs and rhs

              int** old_lhs = 0;
              old_lhs = lhs;
              *lhs = *rhs;
              *rhs = *old_lhs;
              return;
              }

              int main() {

              Array* lhs = new Array<int>(1, 2, 3);
              Array* rhs = new Array<int>(4, 5, 6);
              operator_double_arrow(&lhs, &rhs);
              delete lhs;
              delete rhs;
              return 0;
              }


              Below is an example of one of Tarjan's functions using the operator:



              heap function mesh (heap nodes h1, h2);
              if key(h1) > key(h2) → h1 ⟷ h2 fi;
              right (h1) := if right(h1) = null → h2
              |right(h1) ≠ null → mesh (right(h1), h2)
              fi;

              if rank (left (h1)) < rank (right (h1))
              → left(h1) ⟷ right(h1)
              fi;

              rank (h1) := rank(right(h1)) + 1;
              return h1;
              end mesh;


              Below is a translation of Tarjan's mesh function into pseudo-code which is not C, but looks more like C (relatively speaking). The purpose of this is to illustrate how Tarjan's operator works.



              node pointer function mesh(node pointers h1, h2) {

              if (h1.key) > h2.key) {

              // swap h1 and h2
              node pointer temp;
              temp = h1;
              h1 = h2;
              h2 = temp;
              }

              // Now, h2.key <= h1.key

              if (h1.right == null) {
              h1.right = h2;

              } else // h1.key != null {
              h1.right = mesh(h1.right, h2);
              }



              if (h1.left.rank < h1.right.rank ) {
              // swap h1.left and h1.right

              node pointer temp;
              temp = h1;
              h1 = h2;
              h2 = temp;
              }

              h1.rank = h1.right.rank + 1;
              return h1;
              }


              (8) Tarjan's do-loops are like C/Java while-loops #



              Tarjan's language if and for constructs are familiar for C/Java programmers. However, the Tarjan keyword for a while-loop is do. All do-loops end with the keyword od, which is the backwards spelling of do. Below is an example:



              sum := 0
              do sum < 50 → sum := sum + 1


              In C-style pseudocode, we have:



              sum = 0;
              while(sum < 50) {
              sum = sum + 1;
              }


              The above is actually not quite right. A Tarjan do-loop is really a C/Java while(true) with an if-else block nested inside. A more literal translation of the Tarjan code is as follows:



              sum = 0;
              while(true) {
              if (sum < 50) {
              sum = sum + 1;
              continue;
              // This `continue` statement is questionable
              }
              break;
              }


              Below, we have a more complicated Tarjan do-loop:



              sum := 0
              do sum < 50 → sum := sum + 1 | sum < 99 → sum := sum + 5


              C/Java-style pseudocode for the complicated Tarjan do-loop is as follows:



              sum = 0;
              while(true) {

              if (sum < 50) {
              sum = sum + 1;
              continue;
              }
              else if (sum < 99) {
              sum = sum + 5;
              continue;
              }
              break;
              }


              (9) Tarjan's Conditional-assignment operator with all false conditions



              Although the lengthy explanation above covers most things, a few matters are still left unresolved.
              I hope that someone else will someday write a new-improved answer based on mine which answers these quandries.



              Notably, when the conditional assignment operator := if is used, and no condition is true, I am not what value is assigned to the variable.



              x  := if (False --> 1| False --> 2 | (99 < 2) --> 3) fi


              I am not sure, but it is possible that no assignment is made to x:



              x = 0;
              if (false) {
              x = 1;
              }
              else if (false) {
              x = 2;
              }
              else if (99 < 2) {
              x = 3;
              }
              // At this point (x == 0)


              You could require that the left-hand-side variable seen in an := if statement be previously declared. In that case, even if all conditions are false, the variable will still have a value.



              Alternatively, perhaps all-false conditions represents a runtime error. Another alternative is to return a special null value, and store null in the left-hand argument of the assignment.






              share|cite|improve this answer








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              Sam Muldoon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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              $endgroup$


















                2












                $begingroup$

                Table of Contents #



                I will divide my explanation of Tarjan's pseudocode into the following sections:



                (1) Tarjan's If-else Blocks (the -> & | operators)



                (2) Assignment and Equality Tests (:= and =)



                (3) There is else if, but no else construct



                (4) Tarjan's Conditional Assignment Operator := if



                (4.5) Tarjan Arrays (or Lists)



                (5) Additional Examples of Tarjan's if and := if



                (6) Summary of Operators



                (7) Tarjan's Double-pointed Arrow Operator ()



                **(8) Tarjan's do-loops are like C/Java while-loops **



                **(9) Tarjan's Conditional-assignment operator with all false conditions **



                1) Tarjan's If-else Blocks #



                (the operators and | )



                The if-else construct is perhaps the most fundamental control structure in Tarjan's language. In addition to C-like if-blocks, if-else behavior is very nearly built-into in Tarjan's assignments and Tarjan's while loops. Tarjan's arrow operator -> (or →) is a delimiter between the condition of a if-statement and the execution block of an if-statement.



                For example, in Tarjan's language we might have:



                # Example One
                if a = 4 → x := 9 fi


                If we partially translate the line of Tarjan code above into C or Java, we get the following:



                if (a = 4)
                x := 9
                fi



                Instead of a right curly braces (as in C and Java) Tarjan ends an if-block with a backwards spelling of the key-word: fi



                If we continue translating our above example, we get:



                if (a = 4) {
                x := 9
                }


                (2) Assignment and Equality Tests (:= and =)#



                Tarjan uses = for equality tests, not assignments.



                Tarjan's = is like Java ==



                Tarjan uses := for assignment.



                Tarjan's := is like Java =



                Thus, if we continue translating our example, we have:



                if (a == 4) {
                x = 9
                }


                A vertical bar (or "pipe" or |) in Tarjan's language is equivalent to the else if keyword in C or Java.

                For example, in Tarjan's language we might have:



                # Example Two
                if a = 4 → x := 9 | a > 4 → y := 11 fi


                The Tarjan-code above translates to:



                if (a == 4) {
                x = 9
                }
                else if (a > 4) {
                y = 11
                }


                (3) else if only and no else construct



                Earlier, I covered the basics of if-statements without describing the nuances. However, we will not discuss a small detail. The last clause in a Tarjan-ian if-else block must always contain an arrow () operator. As such, there is no else in Tarjan's language, only else if. The closest thing to an else-block in Tarjan's language is to make the rightmost test-condition true.



                if a = 4 → x := 9 |  a > 4  → y := 11 | true  → z := 99  fi


                In C/Java, we would have:



                if (a == 4) {
                x = 9
                }

                else if (a > 4) {
                y = 11
                }
                else { // else if (true)
                z = 99
                }


                Examples are easier to understand than general descriptions. However, now that we have some examples under our belt, know that the general formal of a Tarjan's if-else construct is as follows:



                if condition
                → stuff to do
                | condition
                → stuff to do
                [...]
                | condition
                → stuff to do
                fi


                The character | is like if else



                The character separates the test-condition from the stuff-to-do.



                (4) Tarjan's Conditional Assignment Operator := if#



                Tarjan's if can be used two very different ways. So far, we have only described one of the uses of the Tarjanian if. Somewhat confusingly, Tarjan still uses the notation/syntax if for the second type of if-construct. Which if is being used is based on context. Analyzing the context is actually very easy to do as the second type of Tarjan-if is always pre-fixed by an assignment operator.



                For example, we might have the following Tarjan code:



                # Example Three
                x := if a = 4 → 9 fi


                Begin Digression###



                After working with Tarjan code for awhile, you get used to the order of operations. If we parenthesize test condition in the example above, we obtain:



                x := if (a = 4) → 9 fi



                a = 4 is not an assignment operation. a = 4 is like a == 4 -- it returns true or false.



                End Digression###



                It can help to think of := if as syntax for a single operator, distinct from := and if In fact, we will refer to the := if operator as the "conditional assignment" operator.



                For if we list (condition → action). For := if we list (condition → value) where value is teh right-hand-side value we might assign to the left-hand-side lhs



                # Tarjan Example Four
                lhs := if (a = 4) → rhs fi



                in C or Java might look like:



                # Example Four
                if (a == 4) {
                lhs = rhs
                }


                Consider the following example of "conditional assignment" in Tarjanian code:



                # Tarjan Instantiation of Example Five
                x := a = 4 → 9 | a > 4 → 11 | true → 99 fi



                In C/Java, we would have:



                // C/Java Instantiation of Example Five
                if (a == 4) {
                x = 9
                }
                else if (a > 4) {
                x = 11
                }
                else if (true) { // else
                x = 99
                }


                (5) Summary of Operators:



                So far, we have:




                • := ...... Assignment operator (C/Java=)


                • = ...... Equality test (C/Java ==)


                • ...... Delimiter between test-condition of an if-block and the body of an if-block


                • | ..... C/Java else-if


                • if ... fi ..... if-else block


                • := if... fi ..... Conditional assignment based on an if-else block



                (5.5) Tarjan Lists/Arrays:



                Tarjan's Language has built-in array-like containers. The syntax for Tarjan arrays is much more intuitive than the notation for Tarjan if else statements.



                list1  := ['lion', 'witch', 'wardrobe'];
                list2a := [1, 2, 3, 4, 5];
                list2b := [1, 2];
                list3 := ["a", "b", "c", "d"];
                list4 := [ ]; # an empty array


                Tarjan array elementa are accessed with parentheses (), not square-brackets



                Indexing begins at 1. Thus,



                list3  := ["a", "b", "c", "d"]
                # list3(1) == "a" returns true
                # list3(2) == "b" return true


                Below shows how to create a new array containing the 1st and 5th elements of [1, 2, 3, 4, 5, 6, 7]



                nums := [1, 2, 3, 4, 5, 6, 7]
                new_arr := [nums(1), nums(5)]


                The equality operator is defined for arrays. The following code prints true



                x := false
                if [1, 2] = [1, 2, 3, 4, 5] --> x := true
                print(x)


                Tarjan's way to test if an array is empty is to compare it to an empty array



                arr := [1, 2]
                print(arr = [ ])
                # `=` is equality test, not assignment


                One can create a view (not copy) of a sub-array, by providing multiple indices to operator () combined with ..



                list3  := ["a", "b", "c", "d"]

                beg := list3(.. 2)
                # beg == ["a", "b"]
                # beg(1) == "a"

                end := list3(3..)
                # end == ["c", "d"]
                # end(1) == "c"

                mid := list3(2..3)
                # mid == ["b", "c"]
                # mid(2) == "c"

                # `list3(4)` is valid, but `mid(4)` is not


                (6) Additional Examples of Tarjan's if and := if#



                The following is another examples of an Tarjan conditional assignment (:= if):



                # Tarjan Example Six
                a := (false --> a | true --> b | false --> c1 + c2 | (2 + 3 < 99) --> d)


                (true --> b) is the leftmost (cond --> action) clause having a true condition.
                Thus, the original assignment Example Six has the same assignment-behavior as a := b



                Below is our most complicated example of Tarjan code thus far:



                # Tarjan Example -- merge two sorted lists

                list function merge (list s, t);

                return if s = --> t
                | t = [ ] --> s
                | s != [ ] and t != and s(l) <= t(1) -->
                [s(1)]& merge(s[2..], t)
                | s != [ ]and t != [ ] and s(1) > r(l) -->
                [t(1)] & merge (s,t(2..))
                fi
                end merge;


                The following is a translation of Tarjan's code for merging two sorted lists. The following is not exactly C or Java, but it is much closer to C/Java than the Tarjan version.



                list merge (list s, list t) {



                if (s is empty) {
                return t;
                }
                else if (t is empty){
                return s;
                }
                else if (s[1] <= t[1]) {
                return CONCATENATE([s[1]], merge(s[2...], t));
                else { // else if (s[1] > t[1])
                return CONCATENATE ([t[1]], merge(s,t[2..]);
                }


                }



                Below is yet another example of Tarjan-code and a translation in something similar to C or Java:



                heap function meld (heap h1, h2);

                return if h1 = null --> h2
                | h2 = null --> h1
                | h1 not null and h2 not null --> mesh (h1, h2)
                fi
                end meld;


                Below is the C/Java translation:



                HeapNode meld (HeapNode h1, HeapNode h2) {

                if (h1 == null) {
                return h2;
                }
                else if (h2 == null) {
                return h1;
                } else {
                mesh(h1, h2)
                }
                } // end function


                (7) Tarjan's Double-pointed Arrow Operator (<-->)#



                Below is an example of Tarjan code:



                x <--> y    


                What Does a Double Arrow () Operator Do in Tarjan's Language?

                Well, almost all variables in Tarjan's Language are pointers.
                <--> is a swap operation. The following prints true



                x_old := x
                y_old := y
                x <--> y
                print(x == y_old) # prints true
                print(y == x_old) # prints true


                After performing x <--> y, x points to the object which y used to point to and y points to the object which x used to point to.



                Below is a Tarjan statement using the <--> operator:



                x := [1, 2, 3]
                y := [4, 5, 6]
                x <--> y


                Below is a translation from the Tarjan code above to alternative pseudocode:



                Pointer X     = address of array [1, 2, 3];
                Pointer Y = address of array [4, 5, 6];
                Pointer X_OLD = address of whatever X points to;
                X = address of whatever Y points to;
                Y = address of whatever X_OLD points to;


                Alternatively, we could have:



                void operator_double_arrow(Array** lhs, Array** rhs) {

                // swap lhs and rhs

                int** old_lhs = 0;
                old_lhs = lhs;
                *lhs = *rhs;
                *rhs = *old_lhs;
                return;
                }

                int main() {

                Array* lhs = new Array<int>(1, 2, 3);
                Array* rhs = new Array<int>(4, 5, 6);
                operator_double_arrow(&lhs, &rhs);
                delete lhs;
                delete rhs;
                return 0;
                }


                Below is an example of one of Tarjan's functions using the operator:



                heap function mesh (heap nodes h1, h2);
                if key(h1) > key(h2) → h1 ⟷ h2 fi;
                right (h1) := if right(h1) = null → h2
                |right(h1) ≠ null → mesh (right(h1), h2)
                fi;

                if rank (left (h1)) < rank (right (h1))
                → left(h1) ⟷ right(h1)
                fi;

                rank (h1) := rank(right(h1)) + 1;
                return h1;
                end mesh;


                Below is a translation of Tarjan's mesh function into pseudo-code which is not C, but looks more like C (relatively speaking). The purpose of this is to illustrate how Tarjan's operator works.



                node pointer function mesh(node pointers h1, h2) {

                if (h1.key) > h2.key) {

                // swap h1 and h2
                node pointer temp;
                temp = h1;
                h1 = h2;
                h2 = temp;
                }

                // Now, h2.key <= h1.key

                if (h1.right == null) {
                h1.right = h2;

                } else // h1.key != null {
                h1.right = mesh(h1.right, h2);
                }



                if (h1.left.rank < h1.right.rank ) {
                // swap h1.left and h1.right

                node pointer temp;
                temp = h1;
                h1 = h2;
                h2 = temp;
                }

                h1.rank = h1.right.rank + 1;
                return h1;
                }


                (8) Tarjan's do-loops are like C/Java while-loops #



                Tarjan's language if and for constructs are familiar for C/Java programmers. However, the Tarjan keyword for a while-loop is do. All do-loops end with the keyword od, which is the backwards spelling of do. Below is an example:



                sum := 0
                do sum < 50 → sum := sum + 1


                In C-style pseudocode, we have:



                sum = 0;
                while(sum < 50) {
                sum = sum + 1;
                }


                The above is actually not quite right. A Tarjan do-loop is really a C/Java while(true) with an if-else block nested inside. A more literal translation of the Tarjan code is as follows:



                sum = 0;
                while(true) {
                if (sum < 50) {
                sum = sum + 1;
                continue;
                // This `continue` statement is questionable
                }
                break;
                }


                Below, we have a more complicated Tarjan do-loop:



                sum := 0
                do sum < 50 → sum := sum + 1 | sum < 99 → sum := sum + 5


                C/Java-style pseudocode for the complicated Tarjan do-loop is as follows:



                sum = 0;
                while(true) {

                if (sum < 50) {
                sum = sum + 1;
                continue;
                }
                else if (sum < 99) {
                sum = sum + 5;
                continue;
                }
                break;
                }


                (9) Tarjan's Conditional-assignment operator with all false conditions



                Although the lengthy explanation above covers most things, a few matters are still left unresolved.
                I hope that someone else will someday write a new-improved answer based on mine which answers these quandries.



                Notably, when the conditional assignment operator := if is used, and no condition is true, I am not what value is assigned to the variable.



                x  := if (False --> 1| False --> 2 | (99 < 2) --> 3) fi


                I am not sure, but it is possible that no assignment is made to x:



                x = 0;
                if (false) {
                x = 1;
                }
                else if (false) {
                x = 2;
                }
                else if (99 < 2) {
                x = 3;
                }
                // At this point (x == 0)


                You could require that the left-hand-side variable seen in an := if statement be previously declared. In that case, even if all conditions are false, the variable will still have a value.



                Alternatively, perhaps all-false conditions represents a runtime error. Another alternative is to return a special null value, and store null in the left-hand argument of the assignment.






                share|cite|improve this answer








                New contributor




                Sam Muldoon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.






                $endgroup$
















                  2












                  2








                  2





                  $begingroup$

                  Table of Contents #



                  I will divide my explanation of Tarjan's pseudocode into the following sections:



                  (1) Tarjan's If-else Blocks (the -> & | operators)



                  (2) Assignment and Equality Tests (:= and =)



                  (3) There is else if, but no else construct



                  (4) Tarjan's Conditional Assignment Operator := if



                  (4.5) Tarjan Arrays (or Lists)



                  (5) Additional Examples of Tarjan's if and := if



                  (6) Summary of Operators



                  (7) Tarjan's Double-pointed Arrow Operator ()



                  **(8) Tarjan's do-loops are like C/Java while-loops **



                  **(9) Tarjan's Conditional-assignment operator with all false conditions **



                  1) Tarjan's If-else Blocks #



                  (the operators and | )



                  The if-else construct is perhaps the most fundamental control structure in Tarjan's language. In addition to C-like if-blocks, if-else behavior is very nearly built-into in Tarjan's assignments and Tarjan's while loops. Tarjan's arrow operator -> (or →) is a delimiter between the condition of a if-statement and the execution block of an if-statement.



                  For example, in Tarjan's language we might have:



                  # Example One
                  if a = 4 → x := 9 fi


                  If we partially translate the line of Tarjan code above into C or Java, we get the following:



                  if (a = 4)
                  x := 9
                  fi



                  Instead of a right curly braces (as in C and Java) Tarjan ends an if-block with a backwards spelling of the key-word: fi



                  If we continue translating our above example, we get:



                  if (a = 4) {
                  x := 9
                  }


                  (2) Assignment and Equality Tests (:= and =)#



                  Tarjan uses = for equality tests, not assignments.



                  Tarjan's = is like Java ==



                  Tarjan uses := for assignment.



                  Tarjan's := is like Java =



                  Thus, if we continue translating our example, we have:



                  if (a == 4) {
                  x = 9
                  }


                  A vertical bar (or "pipe" or |) in Tarjan's language is equivalent to the else if keyword in C or Java.

                  For example, in Tarjan's language we might have:



                  # Example Two
                  if a = 4 → x := 9 | a > 4 → y := 11 fi


                  The Tarjan-code above translates to:



                  if (a == 4) {
                  x = 9
                  }
                  else if (a > 4) {
                  y = 11
                  }


                  (3) else if only and no else construct



                  Earlier, I covered the basics of if-statements without describing the nuances. However, we will not discuss a small detail. The last clause in a Tarjan-ian if-else block must always contain an arrow () operator. As such, there is no else in Tarjan's language, only else if. The closest thing to an else-block in Tarjan's language is to make the rightmost test-condition true.



                  if a = 4 → x := 9 |  a > 4  → y := 11 | true  → z := 99  fi


                  In C/Java, we would have:



                  if (a == 4) {
                  x = 9
                  }

                  else if (a > 4) {
                  y = 11
                  }
                  else { // else if (true)
                  z = 99
                  }


                  Examples are easier to understand than general descriptions. However, now that we have some examples under our belt, know that the general formal of a Tarjan's if-else construct is as follows:



                  if condition
                  → stuff to do
                  | condition
                  → stuff to do
                  [...]
                  | condition
                  → stuff to do
                  fi


                  The character | is like if else



                  The character separates the test-condition from the stuff-to-do.



                  (4) Tarjan's Conditional Assignment Operator := if#



                  Tarjan's if can be used two very different ways. So far, we have only described one of the uses of the Tarjanian if. Somewhat confusingly, Tarjan still uses the notation/syntax if for the second type of if-construct. Which if is being used is based on context. Analyzing the context is actually very easy to do as the second type of Tarjan-if is always pre-fixed by an assignment operator.



                  For example, we might have the following Tarjan code:



                  # Example Three
                  x := if a = 4 → 9 fi


                  Begin Digression###



                  After working with Tarjan code for awhile, you get used to the order of operations. If we parenthesize test condition in the example above, we obtain:



                  x := if (a = 4) → 9 fi



                  a = 4 is not an assignment operation. a = 4 is like a == 4 -- it returns true or false.



                  End Digression###



                  It can help to think of := if as syntax for a single operator, distinct from := and if In fact, we will refer to the := if operator as the "conditional assignment" operator.



                  For if we list (condition → action). For := if we list (condition → value) where value is teh right-hand-side value we might assign to the left-hand-side lhs



                  # Tarjan Example Four
                  lhs := if (a = 4) → rhs fi



                  in C or Java might look like:



                  # Example Four
                  if (a == 4) {
                  lhs = rhs
                  }


                  Consider the following example of "conditional assignment" in Tarjanian code:



                  # Tarjan Instantiation of Example Five
                  x := a = 4 → 9 | a > 4 → 11 | true → 99 fi



                  In C/Java, we would have:



                  // C/Java Instantiation of Example Five
                  if (a == 4) {
                  x = 9
                  }
                  else if (a > 4) {
                  x = 11
                  }
                  else if (true) { // else
                  x = 99
                  }


                  (5) Summary of Operators:



                  So far, we have:




                  • := ...... Assignment operator (C/Java=)


                  • = ...... Equality test (C/Java ==)


                  • ...... Delimiter between test-condition of an if-block and the body of an if-block


                  • | ..... C/Java else-if


                  • if ... fi ..... if-else block


                  • := if... fi ..... Conditional assignment based on an if-else block



                  (5.5) Tarjan Lists/Arrays:



                  Tarjan's Language has built-in array-like containers. The syntax for Tarjan arrays is much more intuitive than the notation for Tarjan if else statements.



                  list1  := ['lion', 'witch', 'wardrobe'];
                  list2a := [1, 2, 3, 4, 5];
                  list2b := [1, 2];
                  list3 := ["a", "b", "c", "d"];
                  list4 := [ ]; # an empty array


                  Tarjan array elementa are accessed with parentheses (), not square-brackets



                  Indexing begins at 1. Thus,



                  list3  := ["a", "b", "c", "d"]
                  # list3(1) == "a" returns true
                  # list3(2) == "b" return true


                  Below shows how to create a new array containing the 1st and 5th elements of [1, 2, 3, 4, 5, 6, 7]



                  nums := [1, 2, 3, 4, 5, 6, 7]
                  new_arr := [nums(1), nums(5)]


                  The equality operator is defined for arrays. The following code prints true



                  x := false
                  if [1, 2] = [1, 2, 3, 4, 5] --> x := true
                  print(x)


                  Tarjan's way to test if an array is empty is to compare it to an empty array



                  arr := [1, 2]
                  print(arr = [ ])
                  # `=` is equality test, not assignment


                  One can create a view (not copy) of a sub-array, by providing multiple indices to operator () combined with ..



                  list3  := ["a", "b", "c", "d"]

                  beg := list3(.. 2)
                  # beg == ["a", "b"]
                  # beg(1) == "a"

                  end := list3(3..)
                  # end == ["c", "d"]
                  # end(1) == "c"

                  mid := list3(2..3)
                  # mid == ["b", "c"]
                  # mid(2) == "c"

                  # `list3(4)` is valid, but `mid(4)` is not


                  (6) Additional Examples of Tarjan's if and := if#



                  The following is another examples of an Tarjan conditional assignment (:= if):



                  # Tarjan Example Six
                  a := (false --> a | true --> b | false --> c1 + c2 | (2 + 3 < 99) --> d)


                  (true --> b) is the leftmost (cond --> action) clause having a true condition.
                  Thus, the original assignment Example Six has the same assignment-behavior as a := b



                  Below is our most complicated example of Tarjan code thus far:



                  # Tarjan Example -- merge two sorted lists

                  list function merge (list s, t);

                  return if s = --> t
                  | t = [ ] --> s
                  | s != [ ] and t != and s(l) <= t(1) -->
                  [s(1)]& merge(s[2..], t)
                  | s != [ ]and t != [ ] and s(1) > r(l) -->
                  [t(1)] & merge (s,t(2..))
                  fi
                  end merge;


                  The following is a translation of Tarjan's code for merging two sorted lists. The following is not exactly C or Java, but it is much closer to C/Java than the Tarjan version.



                  list merge (list s, list t) {



                  if (s is empty) {
                  return t;
                  }
                  else if (t is empty){
                  return s;
                  }
                  else if (s[1] <= t[1]) {
                  return CONCATENATE([s[1]], merge(s[2...], t));
                  else { // else if (s[1] > t[1])
                  return CONCATENATE ([t[1]], merge(s,t[2..]);
                  }


                  }



                  Below is yet another example of Tarjan-code and a translation in something similar to C or Java:



                  heap function meld (heap h1, h2);

                  return if h1 = null --> h2
                  | h2 = null --> h1
                  | h1 not null and h2 not null --> mesh (h1, h2)
                  fi
                  end meld;


                  Below is the C/Java translation:



                  HeapNode meld (HeapNode h1, HeapNode h2) {

                  if (h1 == null) {
                  return h2;
                  }
                  else if (h2 == null) {
                  return h1;
                  } else {
                  mesh(h1, h2)
                  }
                  } // end function


                  (7) Tarjan's Double-pointed Arrow Operator (<-->)#



                  Below is an example of Tarjan code:



                  x <--> y    


                  What Does a Double Arrow () Operator Do in Tarjan's Language?

                  Well, almost all variables in Tarjan's Language are pointers.
                  <--> is a swap operation. The following prints true



                  x_old := x
                  y_old := y
                  x <--> y
                  print(x == y_old) # prints true
                  print(y == x_old) # prints true


                  After performing x <--> y, x points to the object which y used to point to and y points to the object which x used to point to.



                  Below is a Tarjan statement using the <--> operator:



                  x := [1, 2, 3]
                  y := [4, 5, 6]
                  x <--> y


                  Below is a translation from the Tarjan code above to alternative pseudocode:



                  Pointer X     = address of array [1, 2, 3];
                  Pointer Y = address of array [4, 5, 6];
                  Pointer X_OLD = address of whatever X points to;
                  X = address of whatever Y points to;
                  Y = address of whatever X_OLD points to;


                  Alternatively, we could have:



                  void operator_double_arrow(Array** lhs, Array** rhs) {

                  // swap lhs and rhs

                  int** old_lhs = 0;
                  old_lhs = lhs;
                  *lhs = *rhs;
                  *rhs = *old_lhs;
                  return;
                  }

                  int main() {

                  Array* lhs = new Array<int>(1, 2, 3);
                  Array* rhs = new Array<int>(4, 5, 6);
                  operator_double_arrow(&lhs, &rhs);
                  delete lhs;
                  delete rhs;
                  return 0;
                  }


                  Below is an example of one of Tarjan's functions using the operator:



                  heap function mesh (heap nodes h1, h2);
                  if key(h1) > key(h2) → h1 ⟷ h2 fi;
                  right (h1) := if right(h1) = null → h2
                  |right(h1) ≠ null → mesh (right(h1), h2)
                  fi;

                  if rank (left (h1)) < rank (right (h1))
                  → left(h1) ⟷ right(h1)
                  fi;

                  rank (h1) := rank(right(h1)) + 1;
                  return h1;
                  end mesh;


                  Below is a translation of Tarjan's mesh function into pseudo-code which is not C, but looks more like C (relatively speaking). The purpose of this is to illustrate how Tarjan's operator works.



                  node pointer function mesh(node pointers h1, h2) {

                  if (h1.key) > h2.key) {

                  // swap h1 and h2
                  node pointer temp;
                  temp = h1;
                  h1 = h2;
                  h2 = temp;
                  }

                  // Now, h2.key <= h1.key

                  if (h1.right == null) {
                  h1.right = h2;

                  } else // h1.key != null {
                  h1.right = mesh(h1.right, h2);
                  }



                  if (h1.left.rank < h1.right.rank ) {
                  // swap h1.left and h1.right

                  node pointer temp;
                  temp = h1;
                  h1 = h2;
                  h2 = temp;
                  }

                  h1.rank = h1.right.rank + 1;
                  return h1;
                  }


                  (8) Tarjan's do-loops are like C/Java while-loops #



                  Tarjan's language if and for constructs are familiar for C/Java programmers. However, the Tarjan keyword for a while-loop is do. All do-loops end with the keyword od, which is the backwards spelling of do. Below is an example:



                  sum := 0
                  do sum < 50 → sum := sum + 1


                  In C-style pseudocode, we have:



                  sum = 0;
                  while(sum < 50) {
                  sum = sum + 1;
                  }


                  The above is actually not quite right. A Tarjan do-loop is really a C/Java while(true) with an if-else block nested inside. A more literal translation of the Tarjan code is as follows:



                  sum = 0;
                  while(true) {
                  if (sum < 50) {
                  sum = sum + 1;
                  continue;
                  // This `continue` statement is questionable
                  }
                  break;
                  }


                  Below, we have a more complicated Tarjan do-loop:



                  sum := 0
                  do sum < 50 → sum := sum + 1 | sum < 99 → sum := sum + 5


                  C/Java-style pseudocode for the complicated Tarjan do-loop is as follows:



                  sum = 0;
                  while(true) {

                  if (sum < 50) {
                  sum = sum + 1;
                  continue;
                  }
                  else if (sum < 99) {
                  sum = sum + 5;
                  continue;
                  }
                  break;
                  }


                  (9) Tarjan's Conditional-assignment operator with all false conditions



                  Although the lengthy explanation above covers most things, a few matters are still left unresolved.
                  I hope that someone else will someday write a new-improved answer based on mine which answers these quandries.



                  Notably, when the conditional assignment operator := if is used, and no condition is true, I am not what value is assigned to the variable.



                  x  := if (False --> 1| False --> 2 | (99 < 2) --> 3) fi


                  I am not sure, but it is possible that no assignment is made to x:



                  x = 0;
                  if (false) {
                  x = 1;
                  }
                  else if (false) {
                  x = 2;
                  }
                  else if (99 < 2) {
                  x = 3;
                  }
                  // At this point (x == 0)


                  You could require that the left-hand-side variable seen in an := if statement be previously declared. In that case, even if all conditions are false, the variable will still have a value.



                  Alternatively, perhaps all-false conditions represents a runtime error. Another alternative is to return a special null value, and store null in the left-hand argument of the assignment.






                  share|cite|improve this answer








                  New contributor




                  Sam Muldoon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                  Check out our Code of Conduct.






                  $endgroup$



                  Table of Contents #



                  I will divide my explanation of Tarjan's pseudocode into the following sections:



                  (1) Tarjan's If-else Blocks (the -> & | operators)



                  (2) Assignment and Equality Tests (:= and =)



                  (3) There is else if, but no else construct



                  (4) Tarjan's Conditional Assignment Operator := if



                  (4.5) Tarjan Arrays (or Lists)



                  (5) Additional Examples of Tarjan's if and := if



                  (6) Summary of Operators



                  (7) Tarjan's Double-pointed Arrow Operator ()



                  **(8) Tarjan's do-loops are like C/Java while-loops **



                  **(9) Tarjan's Conditional-assignment operator with all false conditions **



                  1) Tarjan's If-else Blocks #



                  (the operators and | )



                  The if-else construct is perhaps the most fundamental control structure in Tarjan's language. In addition to C-like if-blocks, if-else behavior is very nearly built-into in Tarjan's assignments and Tarjan's while loops. Tarjan's arrow operator -> (or →) is a delimiter between the condition of a if-statement and the execution block of an if-statement.



                  For example, in Tarjan's language we might have:



                  # Example One
                  if a = 4 → x := 9 fi


                  If we partially translate the line of Tarjan code above into C or Java, we get the following:



                  if (a = 4)
                  x := 9
                  fi



                  Instead of a right curly braces (as in C and Java) Tarjan ends an if-block with a backwards spelling of the key-word: fi



                  If we continue translating our above example, we get:



                  if (a = 4) {
                  x := 9
                  }


                  (2) Assignment and Equality Tests (:= and =)#



                  Tarjan uses = for equality tests, not assignments.



                  Tarjan's = is like Java ==



                  Tarjan uses := for assignment.



                  Tarjan's := is like Java =



                  Thus, if we continue translating our example, we have:



                  if (a == 4) {
                  x = 9
                  }


                  A vertical bar (or "pipe" or |) in Tarjan's language is equivalent to the else if keyword in C or Java.

                  For example, in Tarjan's language we might have:



                  # Example Two
                  if a = 4 → x := 9 | a > 4 → y := 11 fi


                  The Tarjan-code above translates to:



                  if (a == 4) {
                  x = 9
                  }
                  else if (a > 4) {
                  y = 11
                  }


                  (3) else if only and no else construct



                  Earlier, I covered the basics of if-statements without describing the nuances. However, we will not discuss a small detail. The last clause in a Tarjan-ian if-else block must always contain an arrow () operator. As such, there is no else in Tarjan's language, only else if. The closest thing to an else-block in Tarjan's language is to make the rightmost test-condition true.



                  if a = 4 → x := 9 |  a > 4  → y := 11 | true  → z := 99  fi


                  In C/Java, we would have:



                  if (a == 4) {
                  x = 9
                  }

                  else if (a > 4) {
                  y = 11
                  }
                  else { // else if (true)
                  z = 99
                  }


                  Examples are easier to understand than general descriptions. However, now that we have some examples under our belt, know that the general formal of a Tarjan's if-else construct is as follows:



                  if condition
                  → stuff to do
                  | condition
                  → stuff to do
                  [...]
                  | condition
                  → stuff to do
                  fi


                  The character | is like if else



                  The character separates the test-condition from the stuff-to-do.



                  (4) Tarjan's Conditional Assignment Operator := if#



                  Tarjan's if can be used two very different ways. So far, we have only described one of the uses of the Tarjanian if. Somewhat confusingly, Tarjan still uses the notation/syntax if for the second type of if-construct. Which if is being used is based on context. Analyzing the context is actually very easy to do as the second type of Tarjan-if is always pre-fixed by an assignment operator.



                  For example, we might have the following Tarjan code:



                  # Example Three
                  x := if a = 4 → 9 fi


                  Begin Digression###



                  After working with Tarjan code for awhile, you get used to the order of operations. If we parenthesize test condition in the example above, we obtain:



                  x := if (a = 4) → 9 fi



                  a = 4 is not an assignment operation. a = 4 is like a == 4 -- it returns true or false.



                  End Digression###



                  It can help to think of := if as syntax for a single operator, distinct from := and if In fact, we will refer to the := if operator as the "conditional assignment" operator.



                  For if we list (condition → action). For := if we list (condition → value) where value is teh right-hand-side value we might assign to the left-hand-side lhs



                  # Tarjan Example Four
                  lhs := if (a = 4) → rhs fi



                  in C or Java might look like:



                  # Example Four
                  if (a == 4) {
                  lhs = rhs
                  }


                  Consider the following example of "conditional assignment" in Tarjanian code:



                  # Tarjan Instantiation of Example Five
                  x := a = 4 → 9 | a > 4 → 11 | true → 99 fi



                  In C/Java, we would have:



                  // C/Java Instantiation of Example Five
                  if (a == 4) {
                  x = 9
                  }
                  else if (a > 4) {
                  x = 11
                  }
                  else if (true) { // else
                  x = 99
                  }


                  (5) Summary of Operators:



                  So far, we have:




                  • := ...... Assignment operator (C/Java=)


                  • = ...... Equality test (C/Java ==)


                  • ...... Delimiter between test-condition of an if-block and the body of an if-block


                  • | ..... C/Java else-if


                  • if ... fi ..... if-else block


                  • := if... fi ..... Conditional assignment based on an if-else block



                  (5.5) Tarjan Lists/Arrays:



                  Tarjan's Language has built-in array-like containers. The syntax for Tarjan arrays is much more intuitive than the notation for Tarjan if else statements.



                  list1  := ['lion', 'witch', 'wardrobe'];
                  list2a := [1, 2, 3, 4, 5];
                  list2b := [1, 2];
                  list3 := ["a", "b", "c", "d"];
                  list4 := [ ]; # an empty array


                  Tarjan array elementa are accessed with parentheses (), not square-brackets



                  Indexing begins at 1. Thus,



                  list3  := ["a", "b", "c", "d"]
                  # list3(1) == "a" returns true
                  # list3(2) == "b" return true


                  Below shows how to create a new array containing the 1st and 5th elements of [1, 2, 3, 4, 5, 6, 7]



                  nums := [1, 2, 3, 4, 5, 6, 7]
                  new_arr := [nums(1), nums(5)]


                  The equality operator is defined for arrays. The following code prints true



                  x := false
                  if [1, 2] = [1, 2, 3, 4, 5] --> x := true
                  print(x)


                  Tarjan's way to test if an array is empty is to compare it to an empty array



                  arr := [1, 2]
                  print(arr = [ ])
                  # `=` is equality test, not assignment


                  One can create a view (not copy) of a sub-array, by providing multiple indices to operator () combined with ..



                  list3  := ["a", "b", "c", "d"]

                  beg := list3(.. 2)
                  # beg == ["a", "b"]
                  # beg(1) == "a"

                  end := list3(3..)
                  # end == ["c", "d"]
                  # end(1) == "c"

                  mid := list3(2..3)
                  # mid == ["b", "c"]
                  # mid(2) == "c"

                  # `list3(4)` is valid, but `mid(4)` is not


                  (6) Additional Examples of Tarjan's if and := if#



                  The following is another examples of an Tarjan conditional assignment (:= if):



                  # Tarjan Example Six
                  a := (false --> a | true --> b | false --> c1 + c2 | (2 + 3 < 99) --> d)


                  (true --> b) is the leftmost (cond --> action) clause having a true condition.
                  Thus, the original assignment Example Six has the same assignment-behavior as a := b



                  Below is our most complicated example of Tarjan code thus far:



                  # Tarjan Example -- merge two sorted lists

                  list function merge (list s, t);

                  return if s = --> t
                  | t = [ ] --> s
                  | s != [ ] and t != and s(l) <= t(1) -->
                  [s(1)]& merge(s[2..], t)
                  | s != [ ]and t != [ ] and s(1) > r(l) -->
                  [t(1)] & merge (s,t(2..))
                  fi
                  end merge;


                  The following is a translation of Tarjan's code for merging two sorted lists. The following is not exactly C or Java, but it is much closer to C/Java than the Tarjan version.



                  list merge (list s, list t) {



                  if (s is empty) {
                  return t;
                  }
                  else if (t is empty){
                  return s;
                  }
                  else if (s[1] <= t[1]) {
                  return CONCATENATE([s[1]], merge(s[2...], t));
                  else { // else if (s[1] > t[1])
                  return CONCATENATE ([t[1]], merge(s,t[2..]);
                  }


                  }



                  Below is yet another example of Tarjan-code and a translation in something similar to C or Java:



                  heap function meld (heap h1, h2);

                  return if h1 = null --> h2
                  | h2 = null --> h1
                  | h1 not null and h2 not null --> mesh (h1, h2)
                  fi
                  end meld;


                  Below is the C/Java translation:



                  HeapNode meld (HeapNode h1, HeapNode h2) {

                  if (h1 == null) {
                  return h2;
                  }
                  else if (h2 == null) {
                  return h1;
                  } else {
                  mesh(h1, h2)
                  }
                  } // end function


                  (7) Tarjan's Double-pointed Arrow Operator (<-->)#



                  Below is an example of Tarjan code:



                  x <--> y    


                  What Does a Double Arrow () Operator Do in Tarjan's Language?

                  Well, almost all variables in Tarjan's Language are pointers.
                  <--> is a swap operation. The following prints true



                  x_old := x
                  y_old := y
                  x <--> y
                  print(x == y_old) # prints true
                  print(y == x_old) # prints true


                  After performing x <--> y, x points to the object which y used to point to and y points to the object which x used to point to.



                  Below is a Tarjan statement using the <--> operator:



                  x := [1, 2, 3]
                  y := [4, 5, 6]
                  x <--> y


                  Below is a translation from the Tarjan code above to alternative pseudocode:



                  Pointer X     = address of array [1, 2, 3];
                  Pointer Y = address of array [4, 5, 6];
                  Pointer X_OLD = address of whatever X points to;
                  X = address of whatever Y points to;
                  Y = address of whatever X_OLD points to;


                  Alternatively, we could have:



                  void operator_double_arrow(Array** lhs, Array** rhs) {

                  // swap lhs and rhs

                  int** old_lhs = 0;
                  old_lhs = lhs;
                  *lhs = *rhs;
                  *rhs = *old_lhs;
                  return;
                  }

                  int main() {

                  Array* lhs = new Array<int>(1, 2, 3);
                  Array* rhs = new Array<int>(4, 5, 6);
                  operator_double_arrow(&lhs, &rhs);
                  delete lhs;
                  delete rhs;
                  return 0;
                  }


                  Below is an example of one of Tarjan's functions using the operator:



                  heap function mesh (heap nodes h1, h2);
                  if key(h1) > key(h2) → h1 ⟷ h2 fi;
                  right (h1) := if right(h1) = null → h2
                  |right(h1) ≠ null → mesh (right(h1), h2)
                  fi;

                  if rank (left (h1)) < rank (right (h1))
                  → left(h1) ⟷ right(h1)
                  fi;

                  rank (h1) := rank(right(h1)) + 1;
                  return h1;
                  end mesh;


                  Below is a translation of Tarjan's mesh function into pseudo-code which is not C, but looks more like C (relatively speaking). The purpose of this is to illustrate how Tarjan's operator works.



                  node pointer function mesh(node pointers h1, h2) {

                  if (h1.key) > h2.key) {

                  // swap h1 and h2
                  node pointer temp;
                  temp = h1;
                  h1 = h2;
                  h2 = temp;
                  }

                  // Now, h2.key <= h1.key

                  if (h1.right == null) {
                  h1.right = h2;

                  } else // h1.key != null {
                  h1.right = mesh(h1.right, h2);
                  }



                  if (h1.left.rank < h1.right.rank ) {
                  // swap h1.left and h1.right

                  node pointer temp;
                  temp = h1;
                  h1 = h2;
                  h2 = temp;
                  }

                  h1.rank = h1.right.rank + 1;
                  return h1;
                  }


                  (8) Tarjan's do-loops are like C/Java while-loops #



                  Tarjan's language if and for constructs are familiar for C/Java programmers. However, the Tarjan keyword for a while-loop is do. All do-loops end with the keyword od, which is the backwards spelling of do. Below is an example:



                  sum := 0
                  do sum < 50 → sum := sum + 1


                  In C-style pseudocode, we have:



                  sum = 0;
                  while(sum < 50) {
                  sum = sum + 1;
                  }


                  The above is actually not quite right. A Tarjan do-loop is really a C/Java while(true) with an if-else block nested inside. A more literal translation of the Tarjan code is as follows:



                  sum = 0;
                  while(true) {
                  if (sum < 50) {
                  sum = sum + 1;
                  continue;
                  // This `continue` statement is questionable
                  }
                  break;
                  }


                  Below, we have a more complicated Tarjan do-loop:



                  sum := 0
                  do sum < 50 → sum := sum + 1 | sum < 99 → sum := sum + 5


                  C/Java-style pseudocode for the complicated Tarjan do-loop is as follows:



                  sum = 0;
                  while(true) {

                  if (sum < 50) {
                  sum = sum + 1;
                  continue;
                  }
                  else if (sum < 99) {
                  sum = sum + 5;
                  continue;
                  }
                  break;
                  }


                  (9) Tarjan's Conditional-assignment operator with all false conditions



                  Although the lengthy explanation above covers most things, a few matters are still left unresolved.
                  I hope that someone else will someday write a new-improved answer based on mine which answers these quandries.



                  Notably, when the conditional assignment operator := if is used, and no condition is true, I am not what value is assigned to the variable.



                  x  := if (False --> 1| False --> 2 | (99 < 2) --> 3) fi


                  I am not sure, but it is possible that no assignment is made to x:



                  x = 0;
                  if (false) {
                  x = 1;
                  }
                  else if (false) {
                  x = 2;
                  }
                  else if (99 < 2) {
                  x = 3;
                  }
                  // At this point (x == 0)


                  You could require that the left-hand-side variable seen in an := if statement be previously declared. In that case, even if all conditions are false, the variable will still have a value.



                  Alternatively, perhaps all-false conditions represents a runtime error. Another alternative is to return a special null value, and store null in the left-hand argument of the assignment.







                  share|cite|improve this answer








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                  answered 1 hour ago









                  Sam MuldoonSam Muldoon

                  313




                  313




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                  New contributor





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                      1












                      $begingroup$

                      Never seen this before but I think I can infer what's meant from context.. Presumably the must be a swap operation, and if G1 -> S1 | G2 - >S2 | ... fi is an if/then/else-type construct that also returns a value, like the ternary ?: operator in C and Java.



                      With that in hand we could write the above function in a Java-like language like so:



                      HeapNode mesh(HeapNode h1, HeapNode h2)
                      {
                      if(h1.key > h2.key)
                      {
                      // swap h1 and h2

                      HeapNode t = h1;
                      h1 = h2;
                      h2 = t;
                      }

                      // One of the two cases has to hold in this case so we won't get to the
                      // exception, but it'd be an exception if none of the cases were satisified
                      // since this needs to return *something*.

                      h1.right = (h1.right == null) ? h2
                      : (h1.right != null) ? mesh(h1.right, h2)
                      : throw new Exception();

                      if(h1.left.rank < h1.right.rank)
                      {
                      // swap h1.left and h1.right

                      HeapNode t = h1.left;
                      h1.left = h1.right;
                      h1.right = t;
                      }

                      h1.rank = h1.right.rank + 1;

                      return h1;
                      }





                      share|cite|improve this answer









                      $endgroup$


















                        1












                        $begingroup$

                        Never seen this before but I think I can infer what's meant from context.. Presumably the must be a swap operation, and if G1 -> S1 | G2 - >S2 | ... fi is an if/then/else-type construct that also returns a value, like the ternary ?: operator in C and Java.



                        With that in hand we could write the above function in a Java-like language like so:



                        HeapNode mesh(HeapNode h1, HeapNode h2)
                        {
                        if(h1.key > h2.key)
                        {
                        // swap h1 and h2

                        HeapNode t = h1;
                        h1 = h2;
                        h2 = t;
                        }

                        // One of the two cases has to hold in this case so we won't get to the
                        // exception, but it'd be an exception if none of the cases were satisified
                        // since this needs to return *something*.

                        h1.right = (h1.right == null) ? h2
                        : (h1.right != null) ? mesh(h1.right, h2)
                        : throw new Exception();

                        if(h1.left.rank < h1.right.rank)
                        {
                        // swap h1.left and h1.right

                        HeapNode t = h1.left;
                        h1.left = h1.right;
                        h1.right = t;
                        }

                        h1.rank = h1.right.rank + 1;

                        return h1;
                        }





                        share|cite|improve this answer









                        $endgroup$
















                          1












                          1








                          1





                          $begingroup$

                          Never seen this before but I think I can infer what's meant from context.. Presumably the must be a swap operation, and if G1 -> S1 | G2 - >S2 | ... fi is an if/then/else-type construct that also returns a value, like the ternary ?: operator in C and Java.



                          With that in hand we could write the above function in a Java-like language like so:



                          HeapNode mesh(HeapNode h1, HeapNode h2)
                          {
                          if(h1.key > h2.key)
                          {
                          // swap h1 and h2

                          HeapNode t = h1;
                          h1 = h2;
                          h2 = t;
                          }

                          // One of the two cases has to hold in this case so we won't get to the
                          // exception, but it'd be an exception if none of the cases were satisified
                          // since this needs to return *something*.

                          h1.right = (h1.right == null) ? h2
                          : (h1.right != null) ? mesh(h1.right, h2)
                          : throw new Exception();

                          if(h1.left.rank < h1.right.rank)
                          {
                          // swap h1.left and h1.right

                          HeapNode t = h1.left;
                          h1.left = h1.right;
                          h1.right = t;
                          }

                          h1.rank = h1.right.rank + 1;

                          return h1;
                          }





                          share|cite|improve this answer









                          $endgroup$



                          Never seen this before but I think I can infer what's meant from context.. Presumably the must be a swap operation, and if G1 -> S1 | G2 - >S2 | ... fi is an if/then/else-type construct that also returns a value, like the ternary ?: operator in C and Java.



                          With that in hand we could write the above function in a Java-like language like so:



                          HeapNode mesh(HeapNode h1, HeapNode h2)
                          {
                          if(h1.key > h2.key)
                          {
                          // swap h1 and h2

                          HeapNode t = h1;
                          h1 = h2;
                          h2 = t;
                          }

                          // One of the two cases has to hold in this case so we won't get to the
                          // exception, but it'd be an exception if none of the cases were satisified
                          // since this needs to return *something*.

                          h1.right = (h1.right == null) ? h2
                          : (h1.right != null) ? mesh(h1.right, h2)
                          : throw new Exception();

                          if(h1.left.rank < h1.right.rank)
                          {
                          // swap h1.left and h1.right

                          HeapNode t = h1.left;
                          h1.left = h1.right;
                          h1.right = t;
                          }

                          h1.rank = h1.right.rank + 1;

                          return h1;
                          }






                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered 1 hour ago









                          Daniel McLauryDaniel McLaury

                          27216




                          27216






















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