In perl, checking an array for a value and removing it if exists for each value of another array

Question

Basically, I have an array, let's say @badvalues.

I have another array, let's say @values.

Basically, I want this:

For each element in @badvalues

• See if it is in @values
• If it is, delete it
• Ultimately, I should end up with either the array @values, containing no elements that are in the array @badvalues, or a new array, @goodvalues, containing every element of @values that is not an element of @badvalues.

I know it sounds simple, and maybe it's because I'm tired, but I can't seem to find a clear answer to this question when searching around.

Answer 1

# Get only bad values
my %values = map {$_=>1} @values;
my @new_badvalues = grep { !$values{$_} } @badvalues;

# Get only good values
my %badvalues = map {$_=>1} @badvalues;
my @goodvalues = grep { !$badvalues{$_} } @values;

# An alternative
@badvalues{@badvalues} = ();
foreach $item (@values) {
    push(@goodvalues, $item) unless exists $badvalues{$item};
}

For more complete reference, please see "Chapter 4.7. Finding Elements in One Array but Not Another" of "Perl Cookbook"

Answer 2

If you've got a modern Perl version, like say >= 5.10.1, then you can also do this:

my @final_good = grep { !($_ ~~ @badvalues ) } @values;

or for clearer precendence:

my @final_good = grep { not $_ ~~ @badvalues } @values;

This is using the smartmatch operator that was added in Perl 5.10.

Answer 3

Little bit faster and less memory consuming version of approach shown in DVK's answer

my %badvalues;
@badvalues{@badvalues} = ();
my @goodvalues = grep !exists $badvalues{$_}, @values;