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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

- length(+, +) is semidet
- length(+, -) is det
- length(-, +) is det
- length(-, -) is multi

### 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