I've got a HoH data structure for which I'm looking to keep only certain entries. The entries I'm looking for are contained in a list and I'd like to eliminate the entries which are not contained in that list ( I've substituted a straight hash in place of my HoH for simplicity's sake):

#! /usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

# Data Structure
my %h = (
    'a' => 'z',
    'b' => 'y',
    'c' => 'x',
    'd' => 'w',
);

my @to_keep = qw{ a c };

my %keepers = map { $_ => 1 } @to_keep;

foreach ( keys %h ) {
    if ( !exists $keepers{$_} ) {
        delete $h{$_};
    }
}

$Data::Dumper::Varname = 'h';
print Dumper( \%h );

$h1 = {
          'c' => 'x',
          'a' => 'z'
      };
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I'm creating the %keepers hash out of the items in @to_keep to compare against all of the keys in %h. Since I can create the hash this way:

my @to_keep = qw{ a c };
my %keepers;
foreach ( @to_keep ) {
    $keepers{$_}++;
}

$Data::Dumper::Varname = 'keepers';
print Dumper( \%keepers );

keepers1 = {
          'c' => 1,
          'a' => 1
        };
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I thought I could do the same thing using map:

my @to_keep = qw{ a c };
my %keepers;
%keepers = map { $_++ } @to_keep;

$Data::Dumper::Varname = 'keepers';
print Dumper( \%keepers );

$keepers1 = {
          'a' => 'c'
        };
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But it doesn't seem to work.

njcodewarrior

my @to_keep = qw{ a c };
my %keepers;
foreach ( @to_keep ) {
    $keepers{$_}++;
}
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... I thought I could do the same thing using map ...

The way to do that with map would be like this:

my %keepers = map { $_ => 1 } @to_keep;
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The point is that, in this case, you want each iteration in map to return a key/value pair, not just a single value, and the "fat comma" (=>) does that for you.