[ library(lists) | Reference Manual | Alphabetic Index ]

# 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.

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).

```