Data::Cmp - Compare two data structures, return -1/0/1 like cmp


This document describes version 0.007 of Data::Cmp (from Perl distribution Data-Cmp), released on 2019-11-18.


 use Data::Cmp qw(cmp_data);

 cmp_data(["one", "two", "three"],
          ["one", "two", "three"]); # => 0

 cmp_data(["one", "two" , "three"],
          ["one", "two2", "three"]); # => -1

 cmp_data(["one", "two", "three"],
          ["one", "TWO", "three"]); # => 1

 # hash/array is not "comparable" with scalar
 cmp_data(["one", "two", {}],
          ["one", "two", "three"]); # => 2

Sort data structures (of similar structures):

 my @arrays = (["c"], ["b"], ["a", "b"], ["a"], ["a","c"]);
 my @sorted = sort { cmp_data($a, $b) } @arrays; # => (["a"], ["a","b"], ["a","c"], ["b"], ["c"])


This relatively lightweight (no non-core dependencies, under 100 lines of code) module offers the cmp_data function that, like Perl's cmp, returns -1/0/1 value. In addition to that, it can also return 2 if the two data structures differ but there is no sensible notion of which one is "greater than" the other.

This module can handle circular structure.

The following are the rules of comparison used by cmp_data():

  • Two undefs are the same

     cmp_data(undef, undef); # 0
  • A defined value is greater than undef

     cmp_data(undef, 0); # -1
  • Two non-reference scalars are compared string-wise using Perl's cmp

     cmp_data("a", "A"); # 1
     cmp_data(10, 9);    # -1
  • A reference and non-reference are different

     cmp_data([], 0); # 2
  • Two references that are of different types are different

     cmp_data([], {}); # 2
  • Blessed references that are blessed into different packages are different

     cmp_data(bless([], "foo"), bless([], "bar")); # 2
     cmp_data(bless([], "foo"), bless([], "foo")); # 0
  • Two array references are compared element by element (unless at least one of the arrayref has been seen, in which case see last rule)

     cmp_data(["a","b","c"], ["a","b","c"]); #  0
     cmp_data(["a","b","c"], ["a","b","d"]); # -1
     cmp_data(["a","d","c"], ["a","b","e"]); #  1
  • A longer arrayref is greater than its shorter subset

     cmp_data(["a","b"], ["a"]); # 1
  • Two hash references are compared key by key (unless at least one of the hashref has been seen, in which case see last rule)

     cmp_data({k1=>"a", k2=>"b", k3=>"c"}, {k1=>"a", k2=>"b", k3=>"c"}); # 0
     cmp_data({k1=>"a", k2=>"b", k3=>"c"}, {k1=>"a", k2=>"b", k3=>"d"}); # 1
  • When two hash references share a common subset of pairs but have non-common pairs, the greater hashref is the one that has more non-common pairs

    If the number of non-common pairs are the same, they are just different.

     cmp_data({k1=>"", k2=>"", k3=>""}, {k1=>"", k5=>""});                #  1
     cmp_data({k1=>"", k2=>"", k3=>""}, {k1=>"", k5=>"", k6=>"});         #  2
     cmp_data({k1=>"", k2=>"", k3=>""}, {k1=>"", k5=>"", k6=>", k7=>""}); # -1
  • All other types of references (i.e. non-hash, non-array) are the same only if their address is the same

     cmp_data(\1, \1); # 2
     my $ref = \1; cmp_data($ref, $ref); # 0
  • A seen (hash or array) reference is no longer recursed, it's compared by address (see previous rule)

     my $ary1 = [1]; push @$ary1, $ary1;
     my $ary2 = [1]; push @$ary2, $ary2;
     my $ary3 = [1]; push @$ary3, $ary1;
     cmp_data($ary1, $ary2); # 2
     cmp_data($ary1, $ary3); # 0




 cmp_data($d1, $d2) => -1/0/1/2


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Source repository is at


Please report any bugs or feature requests on the bugtracker website

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.


Data comparison

Other variants of Data::Cmp: Data::Cmp::Numeric, Data::Cmp::StrOrNumeric, Data::Cmp::Custom (allows custom actions and comparison routines), Data::Cmp::Diff (generates diff structure instead of just returning -1/0/1/2), Data::Cmp::Diff::Perl (generates diff in the form of Perl code).

Modules that just return boolean result ("same or different"): Data::Compare, Test::Deep::NoTest (offers flexibility or approximate or custom comparison).

Modules that return some kind of "diff" data: Data::Comparator, Data::Diff.

Of course, to check whether two structures are the same you can also serialize each one then compare the serialized strings/bytes. There are many modules for serialization: JSON, YAML, Sereal, Data::Dumper, Storable, Data::Dmp, just to name a few.

Test modules that do data structure comparison: Test::DataCmp (test module based on Data::Cmp::Custom), Test::More (is_deeply()), Test::Deep, Test2::Tools::Compare.


Scalar::Cmp which employs roughly the same rules as Data::Cmp but does not recurse into arrays/hashes and is meant to compare two scalar values.


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This software is copyright (c) 2019, 2018 by

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.