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pairs.pl -- Operations on key-value lists
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This module implements common operations on Key-Value lists, also known as Pairs. Pairs have great practical value, especially due to keysort/2 and the library assoc.pl.

This library is based on disussion in the SWI-Prolog mailinglist, including specifications from Quintus and a library proposal by Richard O'Keefe.

author
- Jan Wielemaker
See also
- keysort/2, library(assoc)
Source pairs_keys_values(?Pairs, ?Keys, ?Values) is det
True if Keys holds the keys of Pairs and Values the values.

Deterministic if any argument is instantiated to a finite list and the others are either free or finite lists. All three lists are in the same order.

See also
- pairs_values/2 and pairs_keys/2.
Source pairs_values(+Pairs, -Values) is det
Remove the keys from a list of Key-Value pairs. Same as pairs_keys_values(Pairs, _, Values)
Source pairs_keys(+Pairs, -Keys) is det
Remove the values from a list of Key-Value pairs. Same as pairs_keys_values(Pairs, Keys, _)
Source group_pairs_by_key(+Pairs, -Joined:list(Key-Values)) is det
Group values with equivalent (==/2) consecutive keys. For example:
?- group_pairs_by_key([a-2, a-1, b-4, a-3], X).

X = [a-[2,1], b-[4], a-[3]]

Sorting the list of pairs before grouping can be used to group all values associated with a key. For example, finding all values associated with the largest key:

?- sort(1, @>=, [a-1, b-2, c-3, a-4, a-5, c-6], Ps),
   group_pairs_by_key(Ps, [K-Vs|_]).
K = c,
Vs = [3, 6].

In this example, sorting by key only (first argument of sort/4 is 1) ensures that the order of the values in the original list of pairs is maintained.

Arguments:
Pairs- Key-Value list
Joined- List of Key-Group, where Group is the list of Values associated with equivalent consecutive Keys in the same order as they appear in Pairs.
Source transpose_pairs(+Pairs, -Transposed) is det
Swap Key-Value to Value-Key. The resulting list is sorted using keysort/2 on the new key.
Source map_list_to_pairs(:Function, +List, -Keyed)
Create a Key-Value list by mapping each element of List. For example, if we have a list of lists we can create a list of Length-List using
        map_list_to_pairs(length, ListOfLists, Pairs),