pairs.pl -- Operations on key-value lists
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.
- 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.
- 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
?- group_pairs_by_key([a-2, a-1, b-4, a-3], X). X = [a-[2,1], b-, a-]
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.
- 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)
- 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),