SWI-Prolog 7.4.2 Reference Manual: Section A.21

## A.21 library(pairs): Operations on key-value lists

author
  Jan Wielemaker
See also
  keysort/2, `library(assoc)`

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.

[det]pairs_keys_values(?Pairs, ?Keys, ?Values)
  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.
[det]pairs_values(+Pairs, -Values)
  Remove the keys from a list of Key-Value pairs. Same as `pairs_keys_values(Pairs, _, Values)`
[det]pairs_keys(+Pairs, -Keys)
  Remove the values from a list of Key-Value pairs. Same as `pairs_keys_values(Pairs, Keys, _)`
[det]group_pairs_by_key(+Pairs, -Joined:list(Key-Values))
  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.

  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.

[det]transpose_pairs(+Pairs, -Transposed)
  Swap Key-Value to Value-Key. The resulting list is sorted using keysort/2 on the new key.
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),