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ADDED: nth0/3, relating indices to list elements

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Markus Triska
2020-04-28 17:42:12 +02:00
parent 2696fd1291
commit f7256c75d5
1 changed files with 24 additions and 1 deletions
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25
src/prolog/lib/lists.pl
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@@ -1,7 +1,7 @@
:- module(lists, [member/2, select/3, append/2, append/3, foldl/4, foldl/5,
memberchk/2, reverse/2, length/2, maplist/2,
maplist/3, maplist/4, maplist/5, maplist/6,
maplist/7, maplist/8, maplist/9, same_length/2,
maplist/7, maplist/8, maplist/9, same_length/2, nth0/3,
sum_list/2, transpose/2, list_to_set/2]).
@@ -177,3 +177,26 @@ unify_same(E-V, Prev-Var, E-V) :-
Var = V
; true
).
nth0(N, Es, E) :-
can_be(integer, N),
can_be(list, Es),
( integer(N) ->
nth0_index(N, Es, E)
; nth0_search(N, Es, E)
).
nth0_index(0, [E|_], E) :- !.
nth0_index(N, [_|Es], E) :-
N > 0,
N1 is N - 1,
nth0_index(N1, Es, E).
nth0_search(N, Es, E) :-
nth0_search(0, N, Es, E).
nth0_search(N, N, [E|_], E).
nth0_search(N0, N, [_|Es], E) :-
N1 is N0 + 1,
nth0_search(N1, N, Es, E).