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length(?List, ?N)

Succeeds if the length of list List is N.
?List
List or variable.
?N
Integer or variable.

Description

Unifies N with the length of list List. length/2 can be used to create a list List of length N. The definition of this Prolog library predicate is:
length(List, Length) :-
        ( var(Length) ->
          length(List, 0, Length)
        ;
          Length >= 0,
          length1(List, Length) ).

length([], Length, Length).
length([_|L], N, Length) :-
        N1 is N+1,
        length(L, N1, Length).

length1([], 0) :- !.
length1([_|L], Length) :-
        N1 is Length-1,
        length1(L, N1).
This predicate does not perform any type testing functions.

Modes and Determinism

Fail Conditions

Fails if the length of list List does not unify with N.

Resatisfiable

Yes.

Examples

Success:
  length([1,2,3],N).   (gives N=3).
  length([1,2,1,X],N). (gives X=_g84; N=4).
  length(L,2).         (gives L=[_g62,_g72]). % creates list
Fail:
  length([1,2,3],2).


See Also

append / 3