Yrurtj

Let's say I have an associative array in bash,

declare -A hash

hash=(

    ["foo"]=aa

    ["bar"]=bb

    ["baz"]=aa

    ["quux"]=bb

    ["wibble"]=cc

    ["wobble"]=aa

)

where both keys and values are unknown to me (the actual data is read from external sources).

How may I create an array of the keys corresponding to the same value, so that I may, in a loop over all unique values, do

printf 'Value "%s" is present with the following keys: %sn' "$value" "${keys[*]}"

and get the output (not necessarily in this order)

Value "aa" is present with the following keys: foo baz wobble

Value "bb" is present with the following keys: bar quux

Value "cc" is present with the following keys: wibble

The important bit is that the keys are stored as separate elements in the keys array and that they therefore do not need to be parsed out of a text string.

I could do something like

declare -A seen

seen=()

for value in "${hash[@]}"; do

    if [ -n "${seen[$value]}" ]; then

        continue

    fi



    keys=()

    for key in "${!hash[@]}"; do

        if [ "${hash[$key]}" = "$value" ]; then

            keys+=( "$key" )

        fi

    done



    printf 'Value "%s" is present with the following keys: %sn' 

        "$value" "${keys[*]}"



    seen[$value]=1

done

But it seems a bit inefficient with that double loop.

Is there a piece of array syntax that I've missed for bash?

Would doing this in e.g. zsh give me access to more powerful array manipulation tools?

In Perl, I would do

my %hash = (

    'foo'    => 'aa',

    'bar'    => 'bb',

    'baz'    => 'aa',

    'quux'   => 'bb',

    'wibble' => 'cc',

    'wobble' => 'aa'

);



my %keys;

while ( my ( $key, $value ) = each(%hash) ) {

    push( @{ $keys{$value} }, $key );

}



foreach my $value ( keys(%keys) ) {

    printf( "Value "%s" is present with the following keys: %sn",

        $value, join( " ", @{ $keys{$value} } ) );

}

But bash associative arrays can't hold arrays...

I'd also be interested in any old school solution possibly using some form of indirect indexing (building a set of index array(s) when reading the values that I said I had in hash above?). It feels like there ought to be a way to do this in linear time.