group_pairs_by

Documentation
- Reference manual
  - The SWI-Prolog library
    - library(pairs): Operations on key-value lists
- Packages

Availability::- use_module(library(pairs)).(can be autoloaded)

[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`.