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Pack prolog_library_collection -- prolog/pair_ext.pl
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Extends the support for pairs in the SWI-Prolog standard library.

 change_keys(+Pairs1:list(pair(atom,term)), +Changes:ordset(pair(atom,atom)), -Pairs2:list(pair(atom,term))) is det
 compound_pair(+Compound:compound, +Pair:pair) is semidet
compound_pair(+Compound:compound, -Pair:pair) is semidet
compound_pair(-Compound:compound, +Pair:pair) is det
 group_values_by_key(+Pairs:ordset(pair(term,term)), -Groups:ordset(term)) is det
 merge_pairs(+New:list(pair), +Old:list(pair), -Merge:list(pair)) is det
 sum_value(+Pair1:pair(term,number), -Pair2:pair(term,number)) is det

Re-exported predicates

The following predicates are re-exported from other modules

 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.

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)
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),