Perl Cookbook

Finding Elements in One Array but Not Another

Problem

You want to find elements that are in one array but not another.

Solution

You want to find elements in @A that aren’t in @B. Build a hash of the keys of @B to use as a lookup table. Then check each element in @A to see if it is in @B.

Straightforward implementation

# assume @A and @B are already loaded
%seen = (); 	                 # lookup table to test membership of B
@aonly = ();	                 # answer

# build lookup table
foreach $item (@B) { $seen{$item} = 1 }

# find only elements in @A and not in @B
foreach $item (@A) {
    unless ($seen{$item}) {
        # it's not in %seen, so add to @aonly
        push(@aonly, $item);
    }
}

More idiomatic version

my %seen; 	# lookup table
my @aonly;	# answer

# build lookup table
@seen{@B} = ();

foreach $item (@A) {
    push(@aonly, $item) unless exists $seen{$item};
}

Discussion

As with nearly any problem in Perl that asks whether a scalar is in one list or another, this one uses a hash. First, process @B so that the %seen hash records each element from @B by setting its value to 1. Then process @A one element at a time, checking whether that particular element had been in @B by consulting the %seen hash.

The given code retains duplicate elements in @A. This can be easily fixed by adding the elements of @A to %seen as they are processed:

foreach $item (@A) {
    push(@aonly, $item) unless $seen{$item};
    $seen{$item} = 1;                       # mark as seen
}

The two solutions differ mainly in how they build the hash. The first iterates through @B. The second uses a hash slice ...