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Use keys to loop through the keys of one hash, checking whether each key is also in the other hash.
my @common = ( );
foreach (keys %hash1) {
push(@common, $_) if exists $hash2{$_};
}
# @common now contains common keysmy @this_not_that = ( );
foreach (keys %hash1) {
push(@this_not_that, $_) unless exists $hash2{$_};
}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 $citrus_color{$_};
}The "Hashes" section of Chapter 2 of Programming Perl; the each function in perlfunc(1) and in Chapter 29 of Programming Perl
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| 5.11. Merging Hashes |  | 5.13. Hashing References |
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