The Perl Advent Calendar needs more articles for 2022. Submit your idea today!


Array::Compare - Perl extension for comparing arrays.


  use Array::Compare;

  my $comp1 = Array::Compare->new;
  $comp->Skip({3 => 1, 4 => 1});

  my $comp2 = Array::Compare->new(Sep => '|', WhiteSpace => 0, Case => 1,
                                  Skip => {3 => 1, 4 => 1});

  my @arr1 = 0 .. 10;
  my @arr2 = 0 .. 10;

  $comp1->compare(\@arr1, \@arr2);
  $comp2->compare(\@arr1, \@arr2);


If you have two arrays and you want to know if they are the same or different, then Array::Compare will be useful to you.

All comparisons are carried out via a comparator object. In the simplest usage, you can create and use a comparator object like this:

  my @arr1 = 0 .. 10;
  my @arr2 = 0 .. 10;

  my $comp = Array::Compare->new;

  if ($comp->compare(\@arr1, \@arr2) {
    print "Arrays are the same\n";
  } else {
    print "Arrays are different\n";

Notice that you pass references to the two arrays to the comparision method.

Internally the comparator compares the two arrays by using join to turn both arrays into strings and comparing the strings using eq. In the joined strings, the elements of the original arrays are separated with the ^G character. This can cause problems if your array data contains ^G characters as it is possible that two different arrays can be converted to the same string.

To avoid this, it is possible to override the default separator character, either by passing and alternative to the new function

  my $comp = Array::Compare->new(Sep => '|');

or by changing the seperator for an existing comparator object


In general you should choose a separator character that won't appear in your data.

You can also control whether or not whitespace within the elements of the arrays should be considered significant when making the comparison. The default is that all whitespace is significant. The alternative is for all consecutive white space characters to be converted to a single space for the pruposes of the comparison. Again, this can be turned on when creating a comparator object:

  my $comp = Array::Compare->new(WhiteSpace => 0);

or by altering an existing object:


You can also control whether or not the case of the data is significant in the comparison. The default is that the case of data is taken into account. This can be changed in the standard ways when creating a new comparator object:

  my $comp = Array::Compare->new(Case => 0);

or by altering an existing object:


In addition to the simple comparison described above (which returns true if the arrays are the same and false if they're different) there is also a full comparison which returns a list of elements which are different. If the arrays are the same it returns an empty list. In scalar context the full comparison returns the length of this list (i.e. the number of elements that differ). You can access the full comparision in two ways. Firstly, there is a DefFull attribute. If this is true then a full comparison if carried out whenever the compare method is called.

  my $comp = Array::Compare->new(DefFull => 1);
  $comp->compare(\@arr1, \@arr2); # Full comparison

  $comp->compare(\@arr1, \@arr2); # Simple comparison

  $comp->compare(\@arr1, \@arr2); # Full comparison again

Secondly, you can access the full comparison method directly

  $comp->full_compare(\@arr1, \@arr2);

For symmetry, there is also a direct method to use to call the simple comparison.

  $comp->simple_compare(\@arr1, \@arr2);

The final complication is the ability to skip elements in the comparison. If you know that two arrays will always differ in a particular element but want to compare the arrays ignoring this element, you can do it with Array::Compare without taking array slices. To do this, a comparator object has an optional attribute called Skip which is a reference to a hash. The keys in this hash are the indexes of the array elements and the values should be any true value for elements that should be skipped.

For example, if you want to compare two arrays, ignoring the values in elements two and four, you can do something like this:

  my %skip = (2 => 1, 4 => 1);
  my @a = (0, 1, 2, 3, 4, 5);
  my @b = (0, 1, X, 3, X, 5);

  my $comp = Array::Compare->new(Skip => \%skip);

  $comp->compare(\@a, \@b);

This should return true, as we are explicitly ignoring the columns which differ.

Of course, having created a comparator object with no skip hash, it is possible to add one later:

  $comp->Skip({1 => 1, 2 => 1});


  my %skip = (1 => 1, 2 => 2);

To reset the comparator so that no longer skips elements, set the skip hash to an empty hash.


You can also check to see if one array is a permutation of another, i.e. they contain the same elements but in a different order.

  if ($comp->perm(\@a, \@b) {
    print "Arrays are perms\n";
  else {
    print "Nope. Arrays are completely different\n";

In this case the values of WhiteSpace and Case are still used, but Skip is ignored for, hopefully, obvious reasons.


Dave Cross <>