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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
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| 5.11. Merging Hashes |  | 5.13. Hashing References |
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