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- library(aggregate): Aggregation operators on backtrackable predicates
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- library(occurs): Finding and counting sub-terms
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- library(pairs): Operations on key-value lists
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## 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`usingmap_list_to_pairs(length, ListOfLists, Pairs),