Programming Perl, 4th Edition

map

map BLOCK LIST
map EXPR, LIST

This function evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and returns the list comprising the results of each such evaluation. It evaluates BLOCK or EXPR in list context, so each element of LIST may map to zero, one, or more elements in the returned value. These are all flattened into one list. For instance:

@words = map { split " " } @lines;

splits a list of lines into a list of words. But often there is a one-to-one mapping between input values and output values:

@chars = map chr, @nums;

translates a list of numbers to the corresponding characters. And here’s an example of a one-to-two mapping:

%hash = map { genkey($_) => $_ } @array;

which is just a funny functional way to write this:

%hash = ();
for my $_ (@array) {
    $hash{genkey($_)} = $_;
}

Because $_ is an alias (implicit reference) into the list’s values, this variable can be used to modify the elements of the array. This is useful and supported, although it can cause bizarre results if the LIST is not a named array. Using a regular foreach loop for this purpose may be clearer. See also grep; map differs from grep in that map returns a list consisting of the results of each successive evaluation of EXPR, whereas grep returns a list consisting of each value of LIST for which EXPR evaluates to true.