This should trade lower memory usage for potentially longer run times before accounting for swapping effects.

This version assumes one can modify @a for even a bit more memory savings: 

my @a = ( 42, 42, 43, 43, 43, 44, 45, 46 );
my @b = ( 43, 45 );

for my $i ( 0 .. $#b ) {
    for my $j ( 0 .. $#a ) {
        next unless defined $a[ $j ];
        $a[ $j ] = undef, last if $a[ $j ] == $b[ $i ];
    }
}
@a = grep { defined } @a;
print join ', ', @a; print "\n";

[download]
outputs: 42, 42, 43, 43, 44, 46

This obviously trivial modification is more conservative and assumes one can't modify @a: 

my @a = ( 42, 42, 43, 43, 43, 44, 45, 46 );
my @b = ( 43, 45 );
my @c = @a;

for my $i ( 0 .. $#b  ) {
    for my $j ( 0 .. $#c ) {
        next unless defined $c[ $j ];
        $c[ $j ] = undef, last if $c[ $j ] == $b[ $i ];
    }
}
@c = grep { defined } @c;
print join ', ', @c; print "\n";

[download]

The second outputs the same as the first. Neither cares if the arrays are presorted, because it's O(m*n) and checking each against each already.

In reply to Re: Difference arrays. by mr_mischief
in thread Difference arrays. by BrowserUk

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