Recipe 5.12 Finding Common or Different Keys in Two Hashes

5.12.1 Problem

You need to find keys in one hash that are or are not present in another hash.

5.12.2 Solution

Use keys to loop through the keys of one hash, checking whether each key is also in the other hash.

5.12.2.1 Find common keys

my @common = ( );
foreach (keys %hash1) {
        push(@common, $_) if exists $hash2{$_};
}
# @common now contains common keys

5.12.2.2 Find keys from one hash that aren't in both

my @this_not_that = ( );
foreach (keys %hash1) {
        push(@this_not_that, $_) unless exists $hash2{$_};
}

5.12.3 Discussion

Because we're finding common or different keys of the hashes, we can apply our earlier array recipes for finding common or different elements to arrays of the hashes' keys. For an explanation, see Recipe 4.9.

This code uses the difference technique to find non-citrus foods:

# %food_color per the introduction

# %citrus_color is a hash mapping citrus food name to its color.
%citrus_color = ( Lemon  => "yellow",
                  Orange => "orange",
                  Lime   => "green" );

# build up a list of non-citrus foods
@non_citrus = ( );

foreach (keys %food_color) {
    push (@non_citrus, $_) unless $citrus_color{$_};
}

5.12.4 See Also

The "Hashes" section of Chapter 2 of Programming Perl; the each function in perlfunc(1) and in Chapter 29 of Programming Perl