- Documentation
- Reference manual
- The SWI-Prolog library
- library(aggregate): Aggregation operators on backtrackable predicates
- library(ansi_term): Print decorated text to ANSI consoles
- library(apply): Apply predicates on a list
- library(assoc): Association lists
- library(broadcast): Broadcast and receive event notifications
- library(charsio): I/O on Lists of Character Codes
- library(check): Consistency checking
- library(clpb): CLP(B): Constraint Logic Programming over Boolean Variables
- library(clpfd): CLP(FD): Constraint Logic Programming over Finite Domains
- library(clpqr): Constraint Logic Programming over Rationals and Reals
- library(csv): Process CSV (Comma-Separated Values) data
- library(dcg/basics): Various general DCG utilities
- library(dcg/high_order): High order grammar operations
- library(debug): Print debug messages and test assertions
- library(dicts): Dict utilities
- library(error): Error generating support
- library(gensym): Generate unique identifiers
- library(intercept): Intercept and signal interface
- library(iostream): Utilities to deal with streams
- library(lists): List Manipulation
- library(main): Provide entry point for scripts
- library(nb_set): Non-backtrackable set
- library(www_browser): Activating your Web-browser
- library(occurs): Finding and counting sub-terms
- library(option): Option list processing
- library(optparse): command line parsing
- library(ordsets): Ordered set manipulation
- library(pairs): Operations on key-value lists
- library(persistency): Provide persistent dynamic predicates
- library(pio): Pure I/O
- library(predicate_options): Declare option-processing of predicates
- library(prolog_pack): A package manager for Prolog
- library(prolog_xref): Prolog cross-referencer data collection
- library(quasi_quotations): Define Quasi Quotation syntax
- library(random): Random numbers
- library(readutil): Read utilities
- library(record): Access named fields in a term
- library(registry): Manipulating the Windows registry
- library(settings): Setting management
- library(simplex): Solve linear programming problems
- library(solution_sequences): Modify solution sequences
- library(thread_pool): Resource bounded thread management
- library(ugraphs): Unweighted Graphs
- library(url): Analysing and constructing URL
- library(varnumbers): Utilities for numbered terms
- library(yall): Lambda expressions

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## A.28 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`usingmap_list_to_pairs(length, ListOfLists, Pairs),