1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2006-2015, University of Amsterdam
    7                              VU University Amsterdam
    8    All rights reserved.
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   35
   36:- module(pairs,
   37          [ pairs_keys_values/3,
   38            pairs_values/2,
   39            pairs_keys/2,
   40            group_pairs_by_key/2,
   41            transpose_pairs/2,
   42            map_list_to_pairs/3
   43          ]).

 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.

   69pairs_keys_values(Pairs, Keys, Values) :-
   70    (   nonvar(Pairs) ->
   71        pairs_keys_values_(Pairs, Keys, Values)
   72    ;   nonvar(Keys) ->
   73        keys_values_pairs(Keys, Values, Pairs)
   74    ;   values_keys_pairs(Values, Keys, Pairs)
   75    ).
   76
   77pairs_keys_values_([], [], []).
   78pairs_keys_values_([K-V|Pairs], [K|Keys], [V|Values]) :-
   79    pairs_keys_values_(Pairs, Keys, Values).
   80
   81keys_values_pairs([], [], []).
   82keys_values_pairs([K|Ks], [V|Vs], [K-V|Pairs]) :-
   83    keys_values_pairs(Ks, Vs, Pairs).
   84
   85values_keys_pairs([], [], []).
   86values_keys_pairs([V|Vs], [K|Ks], [K-V|Pairs]) :-
   87    values_keys_pairs(Vs, Ks, Pairs).

 pairs_values(+Pairs, -Values) is det

Remove the keys from a list of Key-Value pairs. Same as pairs_keys_values(Pairs, _, Values)

   94pairs_values([], []).
   95pairs_values([_-V|T0], [V|T]) :-
   96    pairs_values(T0, T).

 pairs_keys(+Pairs, -Keys) is det

Remove the values from a list of Key-Value pairs. Same as pairs_keys_values(Pairs, Keys, _)

  104pairs_keys([], []).
  105pairs_keys([K-_|T0], [K|T]) :-
  106    pairs_keys(T0, T).

 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.

Arguments:

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.

  141group_pairs_by_key([], []).
  142group_pairs_by_key([M-N|T0], [M-[N|TN]|T]) :-
  143    same_key(M, T0, TN, T1),
  144    group_pairs_by_key(T1, T).
  145
  146same_key(M0, [M-N|T0], [N|TN], T) :-
  147    M0 == M,
  148    !,
  149    same_key(M, T0, TN, T).
  150same_key(_, L, [], L).

 transpose_pairs(+Pairs, -Transposed) is det

Swap Key-Value to Value-Key. The resulting list is sorted using keysort/2 on the new key.

  158transpose_pairs(Pairs, Transposed) :-
  159    flip_pairs(Pairs, Flipped),
  160    keysort(Flipped, Transposed).
  161
  162flip_pairs([], []).
  163flip_pairs([Key-Val|Pairs], [Val-Key|Flipped]) :-
  164    flip_pairs(Pairs, Flipped).

 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),

  177:- meta_predicate
  178    map_list_to_pairs(2, +, -).  179
  180map_list_to_pairs(Function, List, Pairs) :-
  181    map_list_to_pairs2(List, Function, Pairs).
  182
  183map_list_to_pairs2([], _, []).
  184map_list_to_pairs2([H|T0], Pred, [K-H|T]) :-
  185    call(Pred, H, K),
  186    map_list_to_pairs2(T0, Pred, T)