Provided by: liblist-compare-perl_0.55-2_all 
NAME
List::Compare - Compare elements of two or more lists
VERSION
This document refers to version 0.55 of List::Compare. This version was released August 16 2020.
SYNOPSIS
The bare essentials:
@Llist = qw(abel abel baker camera delta edward fargo golfer);
@Rlist = qw(baker camera delta delta edward fargo golfer hilton);
$lc = List::Compare->new(\@Llist, \@Rlist);
@intersection = $lc->get_intersection;
@union = $lc->get_union;
... and so forth.
DISCUSSION: Modes and Methods
Regular Case: Compare Two Lists
• Constructor: "new()"
Create a List::Compare object. Put the two lists into arrays (named or anonymous) and pass
references to the arrays to the constructor.
@Llist = qw(abel abel baker camera delta edward fargo golfer);
@Rlist = qw(baker camera delta delta edward fargo golfer hilton);
$lc = List::Compare->new(\@Llist, \@Rlist);
By default, List::Compare's methods return lists which are sorted using Perl's default "sort" mode:
ASCII-betical sorting. Should you not need to have these lists sorted, you may achieve a speed boost
by constructing the List::Compare object with the unsorted option:
$lc = List::Compare->new('-u', \@Llist, \@Rlist);
or
$lc = List::Compare->new('--unsorted', \@Llist, \@Rlist);
• Alternative Constructor
If you prefer a more explicit delineation of the types of arguments passed to a function, you may use
this 'single hashref' kind of constructor to build a List::Compare object:
$lc = List::Compare->new( { lists => [\@Llist, \@Rlist] } );
or
$lc = List::Compare->new( {
lists => [\@Llist, \@Rlist],
unsorted => 1,
} );
• "get_intersection()"
Get those items which appear at least once in both lists (their intersection).
@intersection = $lc->get_intersection;
• "get_union()"
Get those items which appear at least once in either list (their union).
@union = $lc->get_union;
• "get_unique()"
Get those items which appear (at least once) only in the first list.
@Lonly = $lc->get_unique;
@Lonly = $lc->get_Lonly; # alias
• "get_complement()"
Get those items which appear (at least once) only in the second list.
@Ronly = $lc->get_complement;
@Ronly = $lc->get_Ronly; # alias
• "get_symmetric_difference()"
Get those items which appear at least once in either the first or the second list, but not both.
@LorRonly = $lc->get_symmetric_difference;
@LorRonly = $lc->get_symdiff; # alias
@LorRonly = $lc->get_LorRonly; # alias
• "get_bag()"
Make a bag of all those items in both lists. The bag differs from the union of the two lists in that
it holds as many copies of individual elements as appear in the original lists.
@bag = $lc->get_bag;
• Return references rather than lists
An alternative approach to the above methods: If you do not immediately require an array as the
return value of the method call, but simply need a reference to an (anonymous) array, use one of the
following parallel methods:
$intersection_ref = $lc->get_intersection_ref;
$union_ref = $lc->get_union_ref;
$Lonly_ref = $lc->get_unique_ref;
$Lonly_ref = $lc->get_Lonly_ref; # alias
$Ronly_ref = $lc->get_complement_ref;
$Ronly_ref = $lc->get_Ronly_ref; # alias
$LorRonly_ref = $lc->get_symmetric_difference_ref;
$LorRonly_ref = $lc->get_symdiff_ref; # alias
$LorRonly_ref = $lc->get_LorRonly_ref; # alias
$bag_ref = $lc->get_bag_ref;
• "is_LsubsetR()"
Return a true value if the first argument passed to the constructor ('L' for 'left') is a subset of
the second argument passed to the constructor ('R' for 'right').
$LR = $lc->is_LsubsetR;
Return a true value if R is a subset of L.
$RL = $lc->is_RsubsetL;
• "is_LequivalentR()"
Return a true value if the two lists passed to the constructor are equivalent, i.e. if every element
in the left-hand list ('L') appears at least once in the right-hand list ('R') and vice versa.
$eqv = $lc->is_LequivalentR;
$eqv = $lc->is_LeqvlntR; # alias
• "is_LdisjointR()"
Return a true value if the two lists passed to the constructor are disjoint, i.e. if the two lists
have zero elements in common (or, what is the same thing, if their intersection is an empty set).
$disj = $lc->is_LdisjointR;
• "print_subset_chart()"
Pretty-print a chart showing whether one list is a subset of the other.
$lc->print_subset_chart;
• "print_equivalence_chart()"
Pretty-print a chart showing whether the two lists are equivalent (same elements found at least once
in both).
$lc->print_equivalence_chart;
• "is_member_which()"
Determine in which (if any) of the lists passed to the constructor a given string can be found. In
list context, return a list of those indices in the constructor's argument list corresponding to
lists holding the string being tested.
@memb_arr = $lc->is_member_which('abel');
In the example above, @memb_arr will be:
( 0 )
because 'abel' is found only in @Al which holds position 0 in the list of arguments passed to
"new()".
In scalar context, the return value is the number of lists passed to the constructor in which a given
string is found.
As with other List::Compare methods which return a list, you may wish the above method returned a
(scalar) reference to an array holding the list:
$memb_arr_ref = $lc->is_member_which_ref('baker');
In the example above, $memb_arr_ref will be:
[ 0, 1 ]
because 'baker' is found in @Llist and @Rlist, which hold positions 0 and 1, respectively, in the
list of arguments passed to "new()".
Note: methods "is_member_which()" and "is_member_which_ref" test only one string at a time and hence
take only one argument. To test more than one string at a time see the next method,
"are_members_which()".
• "are_members_which()"
Determine in which (if any) of the lists passed to the constructor one or more given strings can be
found. The strings to be tested are placed in an array (named or anonymous); a reference to that
array is passed to the method.
$memb_hash_ref =
$lc->are_members_which([ qw| abel baker fargo hilton zebra | ]);
Note: In versions of List::Compare prior to 0.25 (April 2004), the strings to be tested could be
passed as a flat list. This is no longer possible; the argument must now be a reference to an array.
The return value is a reference to a hash of arrays. The key for each element in this hash is the
string being tested. Each element's value is a reference to an anonymous array whose elements are
those indices in the constructor's argument list corresponding to lists holding the strings being
tested. In the examples above, $memb_hash_ref will be:
{
abel => [ 0 ],
baker => [ 0, 1 ],
fargo => [ 0, 1 ],
hilton => [ 1 ],
zebra => [ ],
};
Note: "are_members_which()" can take more than one argument; "is_member_which()" and
"is_member_which_ref()" each take only one argument. Unlike those two methods, "are_members_which()"
returns a hash reference.
• "is_member_any()"
Determine whether a given string can be found in any of the lists passed as arguments to the
constructor. Return 1 if a specified string can be found in any of the lists and 0 if not.
$found = $lc->is_member_any('abel');
In the example above, $found will be 1 because 'abel' is found in one or more of the lists passed as
arguments to "new()".
• "are_members_any()"
Determine whether a specified string or strings can be found in any of the lists passed as arguments
to the constructor. The strings to be tested are placed in an array (named or anonymous); a
reference to that array is passed to "are_members_any".
$memb_hash_ref = $lc->are_members_any([ qw| abel baker fargo hilton zebra | ]);
Note: In versions of List::Compare prior to 0.25 (April 2004), the strings to be tested could be
passed as a flat list. This is no longer possible; the argument must now be a reference to an array.
The return value is a reference to a hash where an element's key is the string being tested and the
element's value is 1 if the string can be found in any of the lists and 0 if not. In the examples
above, $memb_hash_ref will be:
{
abel => 1,
baker => 1,
fargo => 1,
hilton => 1,
zebra => 0,
};
"zebra"'s value is 0 because "zebra" is not found in either of the lists passed as arguments to
"new()".
• "get_version()"
Return current List::Compare version number.
$vers = $lc->get_version;
Accelerated Case: When User Only Wants a Single Comparison
• Constructor "new()"
If you are certain that you will only want the results of a single comparison, computation may be
accelerated by passing '-a' or "'--accelerated" as the first argument to the constructor.
@Llist = qw(abel abel baker camera delta edward fargo golfer);
@Rlist = qw(baker camera delta delta edward fargo golfer hilton);
$lca = List::Compare->new('-a', \@Llist, \@Rlist);
or
$lca = List::Compare->new('--accelerated', \@Llist, \@Rlist);
As with List::Compare's Regular case, should you not need to have a sorted list returned by an
accelerated List::Compare method, you may achieve a speed boost by constructing the accelerated
List::Compare object with the unsorted option:
$lca = List::Compare->new('-u', '-a', \@Llist, \@Rlist);
or
$lca = List::Compare->new('--unsorted', '--accelerated', \@Llist, \@Rlist);
• Alternative Constructor
You may use the 'single hashref' constructor format to build a List::Compare object calling for the
Accelerated mode:
$lca = List::Compare->new( {
lists => [\@Llist, \@Rlist],
accelerated => 1,
} );
or
$lca = List::Compare->new( {
lists => [\@Llist, \@Rlist],
accelerated => 1,
unsorted => 1,
} );
• Methods
All the comparison methods available in the Regular case are available to you in the Accelerated case
as well.
@intersection = $lca->get_intersection;
@union = $lca->get_union;
@Lonly = $lca->get_unique;
@Ronly = $lca->get_complement;
@LorRonly = $lca->get_symmetric_difference;
@bag = $lca->get_bag;
$intersection_ref = $lca->get_intersection_ref;
$union_ref = $lca->get_union_ref;
$Lonly_ref = $lca->get_unique_ref;
$Ronly_ref = $lca->get_complement_ref;
$LorRonly_ref = $lca->get_symmetric_difference_ref;
$bag_ref = $lca->get_bag_ref;
$LR = $lca->is_LsubsetR;
$RL = $lca->is_RsubsetL;
$eqv = $lca->is_LequivalentR;
$disj = $lca->is_LdisjointR;
$lca->print_subset_chart;
$lca->print_equivalence_chart;
@memb_arr = $lca->is_member_which('abel');
$memb_arr_ref = $lca->is_member_which_ref('baker');
$memb_hash_ref = $lca->are_members_which(
[ qw| abel baker fargo hilton zebra | ]);
$found = $lca->is_member_any('abel');
$memb_hash_ref = $lca->are_members_any(
[ qw| abel baker fargo hilton zebra | ]);
$vers = $lca->get_version;
All the aliases for methods available in the Regular case are available to you in the Accelerated
case as well.
Multiple Case: Compare Three or More Lists
• Constructor "new()"
Create a List::Compare object. Put each list into an array and pass references to the arrays to the
constructor.
@Al = qw(abel abel baker camera delta edward fargo golfer);
@Bob = qw(baker camera delta delta edward fargo golfer hilton);
@Carmen = qw(fargo golfer hilton icon icon jerky kappa);
@Don = qw(fargo icon jerky);
@Ed = qw(fargo icon icon jerky);
$lcm = List::Compare->new(\@Al, \@Bob, \@Carmen, \@Don, \@Ed);
As with List::Compare's Regular case, should you not need to have a sorted list returned by a
List::Compare method, you may achieve a speed boost by constructing the object with the unsorted
option:
$lcm = List::Compare->new('-u', \@Al, \@Bob, \@Carmen, \@Don, \@Ed);
or
$lcm = List::Compare->new('--unsorted', \@Al, \@Bob, \@Carmen, \@Don, \@Ed);
• Alternative Constructor
You may use the 'single hashref' constructor format to build a List::Compare object to process three
or more lists at once:
$lcm = List::Compare->new( {
lists => [\@Al, \@Bob, \@Carmen, \@Don, \@Ed],
} );
or
$lcm = List::Compare->new( {
lists => [\@Al, \@Bob, \@Carmen, \@Don, \@Ed],
unsorted => 1,
} );
• Multiple Mode Methods Analogous to Regular and Accelerated Mode Methods
Each List::Compare method available in the Regular and Accelerated cases has an analogue in the
Multiple case. However, the results produced usually require more careful specification.
Note: Certain of the following methods available in List::Compare's Multiple mode take optional
numerical arguments where those numbers represent the index position of a particular list in the list
of arguments passed to the constructor. To specify this index position correctly,
• start the count at 0 (as is customary with Perl array indices); and
• do not count any unsorted option ('-u' or '--unsorted') preceding the array references in the
constructor's own argument list.
Example:
$lcmex = List::Compare->new('--unsorted', \@alpha, \@beta, \@gamma);
For the purpose of supplying a numerical argument to a method which optionally takes such an
argument, '--unsorted' is skipped, @alpha is 0, @beta is 1, and so forth.
• "get_intersection()"
Get those items found in each of the lists passed to the constructor (their intersection):
@intersection = $lcm->get_intersection;
• "get_union()"
Get those items found in any of the lists passed to the constructor (their union):
@union = $lcm->get_union;
• "get_unique()"
To get those items which appear only in one particular list, provide "get_unique()" with that
list's index position in the list of arguments passed to the constructor (not counting any '-u'
or '--unsorted' option).
Example: @Carmen has index position 2 in the constructor's @_. To get elements unique to
@Carmen:
@Lonly = $lcm->get_unique(2);
If no index position is passed to "get_unique()" it will default to 0 and report items unique to
the first list passed to the constructor.
• "get_complement()"
To get those items which appear in any list other than one particular list, provide
"get_complement()" with that list's index position in the list of arguments passed to the
constructor (not counting any '-u' or '--unsorted' option).
Example: @Don has index position 3 in the constructor's @_. To get elements not found in @Don:
@Ronly = $lcm->get_complement(3);
If no index position is passed to "get_complement()" it will default to 0 and report items found
in any list other than the first list passed to the constructor.
• "get_symmetric_difference()"
Get those items each of which appears in only one of the lists passed to the constructor (their
symmetric_difference);
@LorRonly = $lcm->get_symmetric_difference;
• "get_bag()"
Make a bag of all items found in any list. The bag differs from the lists' union in that it
holds as many copies of individual elements as appear in the original lists.
@bag = $lcm->get_bag;
• Return reference instead of list
An alternative approach to the above methods: If you do not immediately require an array as the
return value of the method call, but simply need a reference to an array, use one of the
following parallel methods:
$intersection_ref = $lcm->get_intersection_ref;
$union_ref = $lcm->get_union_ref;
$Lonly_ref = $lcm->get_unique_ref(2);
$Ronly_ref = $lcm->get_complement_ref(3);
$LorRonly_ref = $lcm->get_symmetric_difference_ref;
$bag_ref = $lcm->get_bag_ref;
• "is_LsubsetR()"
To determine whether one particular list is a subset of another list passed to the constructor,
provide "is_LsubsetR()" with the index position of the presumed subset (ignoring any unsorted
option), followed by the index position of the presumed superset.
Example: To determine whether @Ed is a subset of @Carmen, call:
$LR = $lcm->is_LsubsetR(4,2);
A true value (1) is returned if the left-hand list is a subset of the right-hand list; a false
value (0) is returned otherwise.
If no arguments are passed, "is_LsubsetR()" defaults to "(0,1)" and compares the first two lists
passed to the constructor.
• "is_LequivalentR()"
To determine whether any two particular lists are equivalent to each other, provide
"is_LequivalentR" with their index positions in the list of arguments passed to the constructor
(ignoring any unsorted option).
Example: To determine whether @Don and @Ed are equivalent, call:
$eqv = $lcm->is_LequivalentR(3,4);
A true value (1) is returned if the lists are equivalent; a false value (0) otherwise.
If no arguments are passed, "is_LequivalentR" defaults to "(0,1)" and compares the first two
lists passed to the constructor.
• "is_LdisjointR()"
To determine whether any two particular lists are disjoint from each other (i.e., have no members
in common), provide "is_LdisjointR" with their index positions in the list of arguments passed to
the constructor (ignoring any unsorted option).
Example: To determine whether @Don and @Ed are disjoint, call:
$disj = $lcm->is_LdisjointR(3,4);
A true value (1) is returned if the lists are equivalent; a false value (0) otherwise.
If no arguments are passed, "is_LdisjointR" defaults to "(0,1)" and compares the first two lists
passed to the constructor.
• "print_subset_chart()"
Pretty-print a chart showing the subset relationships among the various source lists:
$lcm->print_subset_chart;
• "print_equivalence_chart()"
Pretty-print a chart showing the equivalence relationships among the various source lists:
$lcm->print_equivalence_chart;
• "is_member_which()"
Determine in which (if any) of the lists passed to the constructor a given string can be found.
In list context, return a list of those indices in the constructor's argument list (ignoring any
unsorted option) corresponding to i lists holding the string being tested.
@memb_arr = $lcm->is_member_which('abel');
In the example above, @memb_arr will be:
( 0 )
because 'abel' is found only in @Al which holds position 0 in the list of arguments passed to
"new()".
• "is_member_which_ref()"
As with other List::Compare methods which return a list, you may wish the above method returned a
(scalar) reference to an array holding the list:
$memb_arr_ref = $lcm->is_member_which_ref('jerky');
In the example above, $memb_arr_ref will be:
[ 3, 4 ]
because 'jerky' is found in @Don and @Ed, which hold positions 3 and 4, respectively, in the list
of arguments passed to "new()".
Note: methods "is_member_which()" and "is_member_which_ref" test only one string at a time and
hence take only one argument. To test more than one string at a time see the next method,
"are_members_which()".
• "are_members_which()"
Determine in "which" (if any) of the lists passed to the constructor one or more given strings
can be found. The strings to be tested are placed in an anonymous array, a reference to which is
passed to the method.
$memb_hash_ref =
$lcm->are_members_which([ qw| abel baker fargo hilton zebra | ]);
Note: In versions of List::Compare prior to 0.25 (April 2004), the strings to be tested could be
passed as a flat list. This is no longer possible; the argument must now be a reference to an
anonymous array.
The return value is a reference to a hash of arrays. The key for each element in this hash is
the string being tested. Each element's value is a reference to an anonymous array whose
elements are those indices in the constructor's argument list corresponding to lists holding the
strings being tested.
In the two examples above, $memb_hash_ref will be:
{
abel => [ 0 ],
baker => [ 0, 1 ],
fargo => [ 0, 1, 2, 3, 4 ],
hilton => [ 1, 2 ],
zebra => [ ],
};
Note: "are_members_which()" can take more than one argument; "is_member_which()" and
"is_member_which_ref()" each take only one argument. "are_members_which()" returns a hash
reference; the other methods return either a list or a reference to an array holding that list,
depending on context.
• "is_member_any()"
Determine whether a given string can be found in any of the lists passed as arguments to the
constructor.
$found = $lcm->is_member_any('abel');
Return 1 if a specified string can be found in any of the lists and 0 if not.
In the example above, $found will be 1 because 'abel' is found in one or more of the lists passed
as arguments to "new()".
• "are_members_any()"
Determine whether a specified string or strings can be found in any of the lists passed as
arguments to the constructor. The strings to be tested are placed in an array (anonymous or
named), a reference to which is passed to the method.
$memb_hash_ref = $lcm->are_members_any([ qw| abel baker fargo hilton zebra | ]);
Note: In versions of List::Compare prior to 0.25 (April 2004), the strings to be tested could be
passed as a flat list. This is no longer possible; the argument must now be a reference to an
anonymous array.
The return value is a reference to a hash where an element's key is the string being tested and
the element's value is 1 if the string can be found in "any" of the lists and 0 if not. In the
two examples above, $memb_hash_ref will be:
{
abel => 1,
baker => 1,
fargo => 1,
hilton => 1,
zebra => 0,
};
"zebra"'s value will be 0 because "zebra" is not found in any of the lists passed as arguments to
"new()".
• "get_version()"
Return current List::Compare version number:
$vers = $lcm->get_version;
• Multiple Mode Methods Not Analogous to Regular and Accelerated Mode Methods
• "get_nonintersection()"
Get those items found in any of the lists passed to the constructor which do not appear in all of
the lists (i.e., all items except those found in the intersection of the lists):
@nonintersection = $lcm->get_nonintersection;
• "get_shared()"
Get those items which appear in more than one of the lists passed to the constructor (i.e., all
items except those found in their symmetric difference);
@shared = $lcm->get_shared;
• "get_nonintersection_ref()"
If you only need a reference to an array as a return value rather than a full array, use the
following alternative methods:
$nonintersection_ref = $lcm->get_nonintersection_ref;
$shared_ref = $lcm->get_shared_ref;
• "get_unique_all()"
Get a reference to an array of array references where each of the interior arrays holds the list
of those items unique to the list passed to the constructor with the same index position.
$unique_all_ref = $lcm->get_unique_all();
In the example above, $unique_all_ref will hold:
[
[ qw| abel | ],
[ ],
[ qw| jerky | ],
[ ],
[ ],
]
• "get_complement_all()"
Get a reference to an array of array references where each of the interior arrays holds the list
of those items in the complement to the list passed to the constructor with the same index
position.
$complement_all_ref = $lcm->get_complement_all();
In the example above, $complement_all_ref will hold:
[
[ qw| hilton icon jerky | ],
[ qw| abel icon jerky | ],
[ qw| abel baker camera delta edward | ],
[ qw| abel baker camera delta edward jerky | ],
[ qw| abel baker camera delta edward jerky | ],
]
Multiple Accelerated Case: Compare Three or More Lists but Request Only a Single Comparison among the Lists
• Constructor "new()"
If you are certain that you will only want the results of a single comparison among three or more
lists, computation may be accelerated by passing '-a' or "'--accelerated" as the first argument to
the constructor.
@Al = qw(abel abel baker camera delta edward fargo golfer);
@Bob = qw(baker camera delta delta edward fargo golfer hilton);
@Carmen = qw(fargo golfer hilton icon icon jerky kappa);
@Don = qw(fargo icon jerky);
@Ed = qw(fargo icon icon jerky);
$lcma = List::Compare->new('-a',
\@Al, \@Bob, \@Carmen, \@Don, \@Ed);
As with List::Compare's other cases, should you not need to have a sorted list returned by a
List::Compare method, you may achieve a speed boost by constructing the object with the unsorted
option:
$lcma = List::Compare->new('-u', '-a',
\@Al, \@Bob, \@Carmen, \@Don, \@Ed);
or
$lcma = List::Compare->new('--unsorted', '--accelerated',
\@Al, \@Bob, \@Carmen, \@Don, \@Ed);
As was the case with List::Compare's Multiple mode, do not count the unsorted option ('-u' or
'--unsorted') or the accelerated option ('-a' or '--accelerated') when determining the index position
of a particular list in the list of array references passed to the constructor.
Example:
$lcmaex = List::Compare->new('--unsorted', '--accelerated',
\@alpha, \@beta, \@gamma);
• Alternative Constructor
The 'single hashref' format may be used to construct a List::Compare object which calls for
accelerated processing of three or more lists at once:
$lcmaex = List::Compare->new( {
accelerated => 1,
lists => [\@alpha, \@beta, \@gamma],
} );
or
$lcmaex = List::Compare->new( {
unsorted => 1,
accelerated => 1,
lists => [\@alpha, \@beta, \@gamma],
} );
• Methods
For the purpose of supplying a numerical argument to a method which optionally takes such an
argument, '--unsorted' and "'--accelerated" are skipped, @alpha is 0, @beta is 1, and so forth. To
get a list of those items unique to @gamma, you would call:
@gamma_only = $lcmaex->get_unique(2);
Passing Seen-hashes to the Constructor Instead of Arrays
• When Seen-Hashes Are Already Available to You
Suppose that in a particular Perl program, you had to do extensive munging of data from an external
source and that, once you had correctly parsed a line of data, it was easier to assign that datum to
a hash than to an array. More specifically, suppose that you used each datum as the key to an
element of a lookup table in the form of a seen-hash:
my %Llist = (
abel => 2,
baker => 1,
camera => 1,
delta => 1,
edward => 1,
fargo => 1,
golfer => 1,
);
my %Rlist = (
baker => 1,
camera => 1,
delta => 2,
edward => 1,
fargo => 1,
golfer => 1,
hilton => 1,
);
In other words, suppose it was more convenient to compute a lookup table implying a list than to
compute that list explicitly.
Since in almost all cases List::Compare takes the elements in the arrays passed to its constructor
and internally assigns them to elements in a seen-hash, why shouldn't you be able to pass (references
to) seen-hashes directly to the constructor and avoid unnecessary array assignments before the
constructor is called?
• Constructor "new()"
You can now do so:
$lcsh = List::Compare->new(\%Llist, \%Rlist);
• Methods
All of List::Compare's output methods are supported without further modification when references to
seen-hashes are passed to the constructor.
@intersection = $lcsh->get_intersection;
@union = $lcsh->get_union;
@Lonly = $lcsh->get_unique;
@Ronly = $lcsh->get_complement;
@LorRonly = $lcsh->get_symmetric_difference;
@bag = $lcsh->get_bag;
$intersection_ref = $lcsh->get_intersection_ref;
$union_ref = $lcsh->get_union_ref;
$Lonly_ref = $lcsh->get_unique_ref;
$Ronly_ref = $lcsh->get_complement_ref;
$LorRonly_ref = $lcsh->get_symmetric_difference_ref;
$bag_ref = $lcsh->get_bag_ref;
$LR = $lcsh->is_LsubsetR;
$RL = $lcsh->is_RsubsetL;
$eqv = $lcsh->is_LequivalentR;
$disj = $lcsh->is_LdisjointR;
$lcsh->print_subset_chart;
$lcsh->print_equivalence_chart;
@memb_arr = $lsch->is_member_which('abel');
$memb_arr_ref = $lsch->is_member_which_ref('baker');
$memb_hash_ref = $lsch->are_members_which(
[ qw| abel baker fargo hilton zebra | ]);
$found = $lsch->is_member_any('abel');
$memb_hash_ref = $lsch->are_members_any(
[ qw| abel baker fargo hilton zebra | ]);
$vers = $lcsh->get_version;
$unique_all_ref = $lcsh->get_unique_all();
$complement_all_ref = $lcsh->get_complement_all();
• Accelerated Mode and Seen-Hashes
To accelerate processing when you want only a single comparison among two or more lists, you can pass
'-a' or "'--accelerated" to the constructor before passing references to seen-hashes.
$lcsha = List::Compare->new('-a', \%Llist, \%Rlist);
To compare three or more lists simultaneously, pass three or more references to seen-hashes. Thus,
$lcshm = List::Compare->new(\%Alpha, \%Beta, \%Gamma);
will generate meaningful comparisons of three or more lists simultaneously.
• Unsorted Results and Seen-Hashes
If you do not need sorted lists returned, pass '-u' or "--unsorted" to the constructor before passing
references to seen-hashes.
$lcshu = List::Compare->new('-u', \%Llist, \%Rlist);
$lcshau = List::Compare->new('-u', '-a', \%Llist, \%Rlist);
$lcshmu = List::Compare->new('--unsorted', \%Alpha, \%Beta, \%Gamma);
As was true when we were using List::Compare's Multiple and Multiple Accelerated modes, do not count
any unsorted or accelerated option when determining the array index of a particular seen-hash
reference passed to the constructor.
• Alternative Constructor
The 'single hashref' form of constructor is also available to build List::Compare objects where seen-
hashes are used as arguments:
$lcshu = List::Compare->new( {
unsorted => 1,
lists => [\%Llist, \%Rlist],
} );
$lcshau = List::Compare->new( {
unsorted => 1,
accelerated => 1,
lists => [\%Llist, \%Rlist],
} );
$lcshmu = List::Compare->new( {
unsorted => 1,
lists => [\%Alpha, \%Beta, \%Gamma],
} );
DISCUSSION: Principles
General Comments
List::Compare is an object-oriented implementation of very common Perl code (see "History, References and
Development" below) used to determine interesting relationships between two or more lists at a time. A
List::Compare object is created and automatically computes the values needed to supply List::Compare
methods with appropriate results. In the current implementation List::Compare methods will return new
lists containing the items found in any designated list alone (unique), any list other than a designated
list (complement), the intersection and union of all lists and so forth. List::Compare also has (a)
methods to return Boolean values indicating whether one list is a subset of another and whether any two
lists are equivalent to each other (b) methods to pretty-print very simple charts displaying the subset
and equivalence relationships among lists.
Except for List::Compare's "get_bag()" method, multiple instances of an element in a given list count
only once with respect to computing the intersection, union, etc. of the two lists. In particular,
List::Compare considers two lists as equivalent if each element of the first list can be found in the
second list and vice versa. 'Equivalence' in this usage takes no note of the frequency with which
elements occur in either list or their order within the lists. List::Compare asks the question: Did I
see this item in this list at all? Only when you use "List::Compare::get_bag()" to compute a bag holding
the two lists do you ask the question: How many times did this item occur in this list?
List::Compare Modes
In its current implementation List::Compare has four modes of operation.
• Regular Mode
List::Compare's Regular mode is based on List::Compare v0.11 -- the first version of List::Compare
released to CPAN (June 2002). It compares only two lists at a time. Internally, its initializer
does all computations needed to report any desired comparison and its constructor stores the results
of these computations. Its public methods merely report these results.
This approach has the advantage that if you need to examine more than one form of comparison between
two lists (e.g., the union, intersection and symmetric difference of two lists), the comparisons are
pre-calculated. This approach is efficient because certain types of comparison presuppose that other
types have already been calculated. For example, to calculate the symmetric difference of two lists,
one must first determine the items unique to each of the two lists.
• Accelerated Mode
The current implementation of List::Compare offers you the option of getting even faster results
provided that you only need the result from a single form of comparison between two lists. (e.g.,
only the union -- nothing else). In the Accelerated mode, List::Compare's initializer does no
computation and its constructor stores only references to the two source lists. All computation
needed to report results is deferred to the method calls.
The user selects this approach by passing the option flag '-a' to the constructor before passing
references to the two source lists. List::Compare notes the option flag and silently switches into
Accelerated mode. From the perspective of the user, there is no further difference in the code or in
the results.
Benchmarking suggests that List::Compare's Accelerated mode (a) is faster than its Regular mode when
only one comparison is requested; (b) is about as fast as Regular mode when two comparisons are
requested; and (c) becomes considerably slower than Regular mode as each additional comparison above
two is requested.
• Multiple Mode
List::Compare now offers the possibility of comparing three or more lists at a time. Simply store
the extra lists in arrays and pass references to those arrays to the constructor. List::Compare
detects that more than two lists have been passed to the constructor and silently switches into
Multiple mode.
As described in the Synopsis above, comparing more than two lists at a time offers you a wider, more
complex palette of comparison methods. Individual items may appear in just one source list, in all
the source lists, or in some number of lists between one and all. The meaning of 'union',
'intersection' and 'symmetric difference' is conceptually unchanged when you move to multiple lists
because these are properties of all the lists considered together. In contrast, the meaning of
'unique', 'complement', 'subset' and 'equivalent' changes because these are properties of one list
compared with another or with all the other lists combined.
List::Compare takes this complexity into account by allowing you to pass arguments to the public
methods requesting results with respect to a specific list (for "get_unique()" and
"get_complement()") or a specific pair of lists (for "is_LsubsetR()" and "is_LequivalentR()").
List::Compare further takes this complexity into account by offering the new methods "get_shared()"
and "get_nonintersection()" described in the Synopsis above.
• Multiple Accelerated Mode
Beginning with version 0.25, introduced in April 2004, List::Compare offers the possibility of
accelerated computation of a single comparison among three or more lists at a time. Simply store the
extra lists in arrays and pass references to those arrays to the constructor preceded by the '-a'
argument as was done with the simple (two lists only) accelerated mode. List::Compare detects that
more than two lists have been passed to the constructor and silently switches into Multiple
Accelerated mode.
• Unsorted Option
When List::Compare is used to return lists representing various comparisons of two or more lists
(e.g., the lists' union or intersection), the lists returned are, by default, sorted using Perl's
default "sort" mode: ASCII-betical sorting. Sorting produces results which are more easily human-
readable but may entail a performance cost.
Should you not need sorted results, you can avoid the potential performance cost by calling
List::Compare's constructor using the unsorted option. This is done by calling '-u' or '--unsorted'
as the first argument passed to the constructor, i.e., as an argument called before any references to
lists are passed to the constructor.
Note that if are calling List::Compare in the Accelerated or Multiple Accelerated mode and wish to
have the lists returned in unsorted order, you first pass the argument for the unsorted option ('-u'
or '--unsorted') and then pass the argument for the Accelerated mode ('-a' or '--accelerated').
Miscellaneous Methods
It would not really be appropriate to call "get_shared()" and "get_nonintersection()" in Regular or
Accelerated mode since they are conceptually based on the notion of comparing more than two lists at a
time. However, there is always the possibility that a user may be comparing only two lists (accelerated
or not) and may accidentally call one of those two methods. To prevent fatal run-time errors and to
caution you to use a more appropriate method, these two methods are defined for Regular and Accelerated
modes so as to return suitable results but also generate a carp message that advise you to re-code.
Similarly, the method "is_RsubsetL()" is appropriate for the Regular and Accelerated modes but is not
really appropriate for Multiple mode. As a defensive maneuver, it has been defined for Multiple mode so
as to return suitable results but also to generate a carp message that advises you to re-code.
In List::Compare v0.11 and earlier, the author provided aliases for various methods based on the
supposition that the source lists would be referred to as 'A' and 'B'. Now that you can compare more
than two lists at a time, the author feels that it would be more appropriate to refer to the elements of
two-argument lists as the left-hand and right-hand elements. Hence, we are discouraging the use of
methods such as "get_Aonly()", "get_Bonly()" and "get_AorBonly()" as aliases for "get_unique()",
"get_complement()" and "get_symmetric_difference()". However, to guarantee backwards compatibility for
the vast audience of Perl programmers using earlier versions of List::Compare (all 10e1 of you) these and
similar methods for subset relationships are still defined.
List::Compare::SeenHash Discontinued Beginning with Version 0.26
Prior to v0.26, introduced April 11, 2004, if a user wished to pass references to seen-hashes to
List::Compare's constructor rather than references to arrays, he or she had to call a different, parallel
module: List::Compare::SeenHash. The code for that looked like this:
use List::Compare::SeenHash;
my %Llist = (
abel => 2,
baker => 1,
camera => 1,
delta => 1,
edward => 1,
fargo => 1,
golfer => 1,
);
my %Rlist = (
baker => 1,
camera => 1,
delta => 2,
edward => 1,
fargo => 1,
golfer => 1,
hilton => 1,
);
my $lcsh = List::Compare::SeenHash->new(\%Llist, \%Rlist);
List::Compare::SeenHash is deprecated beginning with version 0.26. All its functionality (and more) has
been implemented in List::Compare itself, since a user can now pass either a series of array references
or a series of seen-hash references to List::Compare's constructor.
To simplify future maintenance of List::Compare, List::Compare::SeenHash.pm will no longer be distributed
with List::Compare, nor will the files in the test suite which tested List::Compare::SeenHash upon
installation be distributed.
Should you still need List::Compare::SeenHash, use version 0.25 from CPAN, or simply edit your Perl
programs which used List::Compare::SeenHash. Those scripts may be edited quickly with, for example, this
editing command in Unix text editor vi:
:1,$s/List::Compare::SeenHash/List::Compare/gc
A Non-Object-Oriented Interface: List::Compare::Functional
Version 0.21 of List::Compare introduced List::Compare::Functional, a functional (i.e., non-object-
oriented) interface to list comparison functions. List::Compare::Functional supports the same functions
currently supported by List::Compare. It works similar to List::Compare's Accelerated and Multiple
Accelerated modes (described above), bit it does not require use of the '-a' flag in the function call.
List::Compare::Functional will return unsorted comparisons of two lists by passing '-u' or '--unsorted'
as the first argument to the function. Please see the documentation for List::Compare::Functional to
learn how to import its functions into your main package.
ASSUMPTIONS AND QUALIFICATIONS
The program was created with Perl 5.6. The use of h2xs to prepare the module's template installed
"require 5.005_62;" at the top of the module. This has been commented out in the actual module as the
code appears to be compatible with earlier versions of Perl; how earlier the author cannot say. In
particular, the author would like the module to be installable on older versions of MacPerl. As is, the
author has successfully installed the module on Linux, Windows 9x and Windows 2000. See
<http://testers.cpan.org/show/List-Compare.html> for a list of other systems on which this version of
List::Compare has been tested and installed.
HISTORY, REFERENCES AND DEVELOPMENT
The Code Itself
List::Compare is based on code presented by Tom Christiansen & Nathan Torkington in Perl Cookbook
<http://www.oreilly.com/catalog/cookbook/> (a.k.a. the 'Ram' book), O'Reilly & Associates, 1998, Recipes
4.7 and 4.8. Similar code is presented in the Camel book: Programming Perl, by Larry Wall, Tom
Christiansen, Jon Orwant. <http://www.oreilly.com/catalog/pperl3/>, 3rd ed, O'Reilly & Associates, 2000.
The list comparison code is so basic and Perlish that I suspect it may have been written by Larry himself
at the dawn of Perl time. The "get_bag()" method was inspired by Jarkko Hietaniemi's Set::Bag module and
Daniel Berger's Set::Array module, both available on CPAN.
List::Compare's original objective was simply to put this code in a modular, object-oriented framework.
That framework, not surprisingly, is taken mostly from Damian Conway's Object Oriented Perl
<http://www.manning.com/Conway/index.html>, Manning Publications, 2000.
With the addition of the Accelerated, Multiple and Multiple Accelerated modes, List::Compare expands
considerably in both size and capabilities. Nonetheless, Tom and Nat's Cookbook code still lies at its
core: the use of hashes as look-up tables to record elements seen in lists. Please note: List::Compare
is not concerned with any concept of 'equality' among lists which hinges upon the frequency with which,
or the order in which, elements appear in the lists to be compared. If this does not meet your needs,
you should look elsewhere or write your own module.
The Inspiration
I realized the usefulness of putting the list comparison code into a module while preparing an
introductory level Perl course given at the New School University's Computer Instruction Center in April-
May 2002. I was comparing lists left and right. When I found myself writing very similar functions in
different scripts, I knew a module was lurking somewhere. I learned the truth of the mantra ''Repeated
Code is a Mistake'' from a 2001 talk by Mark-Jason Dominus <http://perl.plover.com/> to the New York
Perlmongers <http://ny.pm.org/>. See <http://www.perl.com/pub/a/2000/11/repair3.html>.
The first public presentation of this module took place at Perl Seminar New York
<http://groups.yahoo.com/group/perlsemny> on May 21, 2002. Comments and suggestions were provided there
and since by Glenn Maciag, Gary Benson, Josh Rabinowitz, Terrence Brannon and Dave Cross.
The placement in the installation tree of Test::ListCompareSpecial came as a result of a question
answered by Michael Graham in his talk ''Test::More to Test::Extreme'' given at Yet Another Perl
Conference::Canada in Ottawa, Ontario, on May 16, 2003.
In May-June 2003, Glenn Maciag made valuable suggestions which led to changes in method names and
documentation in v0.20.
Another presentation at Perl Seminar New York in October 2003 prompted me to begin planning
List::Compare::Functional.
In a November 2003 Perl Seminar New York presentation, Ben Holtzman discussed the performance costs
entailed in Perl's "sort" function. This led me to ask, ''Why should a user of List::Compare pay this
performance cost if he or she doesn't need a human-readable list as a result (as would be the case if the
list returned were used as the input into some other function)?'' This led to the development of
List::Compare's unsorted option.
An April 2004 offer by Kevin Carlson to write an article for The Perl Journal (<http://tpj.com>) led me
to re-think whether a separate module (the former List::Compare::SeenHash) was truly needed when a user
wanted to provide the constructor with references to seen-hashes rather than references to arrays. Since
I had already adapted List::Compare::Functional to accept both kinds of arguments, I adapted
List::Compare in the same manner. This meant that List::Compare::SeenHash and its related installation
tests could be deprecated and deleted from the CPAN distribution.
A remark by David H. Adler at a New York Perlmongers meeting in April 2004 led me to develop the 'single
hashref' alternative constructor format, introduced in version 0.29 the following month.
Presentations at two different editions of Yet Another Perl Conference (YAPC) inspired the development of
List::Compare versions 0.30 and 0.31. I was selected to give a talk on List::Compare at YAPC::NA::2004
in Buffalo. This spurred me to improve certain aspects of the documentation. Version 0.31 owes its
inspiration to one talk at the Buffalo YAPC and one earlier talk at YAPC::EU::2003 in Paris. In Paris I
heard Paul Johnson speak on his CPAN module Devel::Cover and on coverage analysis more generally. That
material was over my head at that time, but in Buffalo I heard Andy Lester discuss Devel::Cover as part
of his discussion of testing and of the Phalanx project (<http://qa.perl.org/phalanx>). This time I got
it, and when I returned from Buffalo I applied Devel::Cover to List::Compare and wrote additional tests
to improve its subroutine and statement coverage. In addition, I added two new methods, "get_unique_all"
and "get_complement_all". In writing these two methods, I followed a model of test-driven development
much more so than in earlier versions of List::Compare and my other CPAN modules. The result?
List::Compare's test suite grew by over 3300 tests to nearly 23,000 tests.
At the Second New York Perl Hackathon (May 02 2015), a project was created to request performance
improvements in certain List::Compare functions
(<https://github.com/nyperlmongers/nyperlhackathon2015/wiki/List-Compare-Performance-Improvements>).
Hackathon participant Michael Rawson submitted a pull request with changes to
List::Compare::Base::_Auxiliary. After these revisions were benchmarked, a patch embodying the pull
request was accepted, leading to CPAN version 0.53.
If You Like List::Compare, You'll Love ...
While preparing this module for distribution via CPAN, I had occasion to study a number of other modules
already available on CPAN. Each of these modules is more sophisticated than List::Compare -- which is
not surprising since all that List::Compare originally aspired to do was to avoid typing Cookbook code
repeatedly. Here is a brief description of the features of these modules. (Warning: The following
discussion is only valid as of June 2002. Some of these modules may have changed since then.)
• Algorithm::Diff - Compute 'intelligent' differences between two files/lists
(<http://search.cpan.org/dist/Algorithm-Diff/>)
Algorithm::Diff is a sophisticated module originally written by Mark-Jason Dominus, later maintained
by Ned Konz, now maintained by Tye McQueen. Think of the Unix "diff" utility and you're on the right
track. Algorithm::Diff exports methods such as "diff", which ''computes the smallest set of
additions and deletions necessary to turn the first sequence into the second, and returns a
description of these changes.'' Algorithm::Diff is mainly concerned with the sequence of elements
within two lists. It does not export functions for intersection, union, subset status, etc.
• Array::Compare - Perl extension for comparing arrays (<http://search.cpan.org/dist/Array-Compare/>)
Array::Compare, by Dave Cross, asks whether two arrays are the same or different by doing a "join" on
each string with a separator character and comparing the resulting strings. Like List::Compare, it
is an object-oriented module. A sophisticated feature of Array::Compare is that it allows you to
specify how 'whitespace' in an array (an element which is undefined, the empty string, or whitespace
within an element) should be evaluated for purpose of determining equality or difference. It does
not directly provide methods for intersection and union.
• Data::Compare - compare perl data structures (<http://search.cpan.org/dist/Data-Compare/>)
This library compares Perl data structures recursively. Comes recommended by Slaven Rezić!
• List::Util - A selection of general-utility list subroutines
(<http://search.cpan.org/dist/Scalar-List-Utils/>)
List::Util, by Graham Barr, exports a variety of simple, useful functions for operating on one list
at a time. The "min" function returns the lowest numerical value in a list; the "max" function
returns the highest value; and so forth. List::Compare differs from List::Util in that it is object-
oriented and that it works on two strings at a time rather than just one -- but it aims to be as
simple and useful as List::Util. List::Util will be included in the standard Perl distribution as of
Perl 5.8.0.
Lists::Util (<http://search.cpan.org/dist/List-MoreUtils/>), by Tassilo von Parseval, building on
code by Terrence Brannon, provides methods which extend List::Util's functionality.
• Quantum::Superpositions (<http://search.cpan.org/dist/Quantum-Superpositions/>), originally by Damian
Conway, now maintained by Steven Lembark is useful if, in addition to comparing lists, you need to
emulate quantum supercomputing as well. Not for the eigen-challenged.
• Set::Scalar - basic set operations (<http://search.cpan.org/dist/Set-Scalar/>)
Set::Bag - bag (multiset) class (<http://search.cpan.org/dist/Set-Bag/>)
Both of these modules are by Jarkko Hietaniemi. Set::Scalar has methods to return the intersection,
union, difference and symmetric difference of two sets, as well as methods to return items unique to
a first set and complementary to it in a second set. It has methods for reporting considerably more
variants on subset status than does List::Compare. However, benchmarking suggests that
List::Compare, at least in Regular mode, is considerably faster than Set::Scalar for those comparison
methods which List::Compare makes available.
Set::Bag enables one to deal more flexibly with the situation in which one has more than one instance
of an element in a list.
• Set::Array - Arrays as objects with lots of handy methods (including set comparisons) and support for
method chaining. (<http://search.cpan.org/dist/Set-Array/>)
Set::Array, by Daniel Berger, now maintained by Ron Savage, ''aims to provide built-in methods for
operations that people are always asking how to do,and which already exist in languages like Ruby.''
Among the many methods in this module are some for intersection, union, etc. To install Set::Array,
you must first install the Want module, also available on CPAN.
ADDITIONAL CONTRIBUTORS
• Syohei YOSHIDA
Pull request accepted May 22 2015.
• Paulo Custodio
Pull request accepted June 07 2015, correcting errors in "_subset_subengine()".
BUGS
There are no bug reports outstanding on List::Compare as of the most recent CPAN upload date of this
distribution.
SUPPORT
Please report any bugs by mail to "bug-List-Compare@rt.cpan.org" or through the web interface at
<http://rt.cpan.org>.
AUTHOR
James E. Keenan (jkeenan@cpan.org). When sending correspondence, please include 'List::Compare' or
'List-Compare' in your subject line.
Creation date: May 20, 2002. Last modification date: August 16 2020.
Development repository: <https://github.com/jkeenan/list-compare>
COPYRIGHT
Copyright (c) 2002-20 James E. Keenan. United States. All rights reserved. This is free software and
may be distributed under the same terms as Perl itself.
DISCLAIMER OF WARRANTY
BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE SOFTWARE, TO THE EXTENT
PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER
PARTIES PROVIDE THE SOFTWARE ''AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH YOU. SHOULD THE
SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR, OR CORRECTION.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY
OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE
TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING OUT OF
THE USE OR INABILITY TO USE THE SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE SOFTWARE TO OPERATE
WITH ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
DAMAGES.
perl v5.36.0 2022-10-22 List::Compare(3pm)