A.32 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 discussion 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),