Question

I have been looking for a way to do the following action using the perl function map:
given a hash, I want to extract the pairs (key, value) in which the value equals or matches a specified parameter.

In my example I want to extract the (key, value) pairs where value = failed, but it could also be an expression (i.e. string starting with A, or a REGEX). That’s why I want as a result a hash and not only a table of the keys matching the value.

my %strings = (
    bla => "success",
    ble => "failed",
    bli => "failed",
    foo => "success",
    blo => "failed",
    bar => "failed",
    blu => "success"
);

my %failed_s = ();

while (my ($k, $v) = each %strings) {
    if ( $v eq 'failed' ) {$failed_s{$k} = $v};
};

I tried several ways of doing this, but without great results so I think I’m getting confused about references, affectation, results, etc.

my %failed_s = 
    map { { $_ => $strings{$_} } 
    if ( $strings{$_}./failed/ ) } 
    keys %strings;
my %failed_s = 
    map { ( $strings{$_} eq 'failed') &&
          ($_, $strings{$_}) } 
    keys %strings;

print "Dumper \%failed_s: \n" . Dumper(\%failed_s);

It might be not possible or not effective to use map, in this case, but it would help me (and probably others) to know why.

Answer 1

In your first map example, you are returning single element hash references, which is not what you want.

In your second, the && operator imposes scalar context on its arguments, so the list ($_, $strings{$_}) only returns the last item in the list.

What you are looking for is:

my %failed_s = map {
    $strings{$_} eq 'failed' ? ($_, $strings{$_}) : () 
} keys %strings;

Where the ? : operator returns the list if the condition is true, and an empty list if it is false.