I want to take each element in an array and do something to it with each other element of the same array. [...] what is this called?
Permutations (if ordered) or combinations (if unordered). The latter in this case.
For choosing two from the list, one could simply use:
for my $y (0..$#list) {
for my $x ($y+1..$#list) {
...
}
}
But that would be inefficient here. It's O(N**2), whereas the following is O(N):
my %counts;
++$counts{$_} for @list;
my $dups = grep { $_ > 1 } values(%counts);
die "Walrus Festival!" if $dups >= 3;
One can optimise the above to exit as soon as the answer is obvious:
my %counts;
my $dups;
for (@list) {
if (++$counts{$_} == 2) {
if (++$dups >= 3) {
die "Walrus Festival!";
}
}
}
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