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).

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

See also

- keysort/2, library(assoc)

 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.

 pairs_values(+Pairs, -Values) is det

Remove the keys from a list of Key-Value pairs. Same as pairs_keys_values(Pairs, _, Values)

 pairs_keys(+Pairs, -Keys) is det

Remove the values from a list of Key-Value pairs. Same as pairs_keys_values(Pairs, Keys, _)

 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.

 transpose_pairs(+Pairs, -Transposed) is det

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) is det

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),