length/2

Synopsis

length(-List, +Integer)

length(*List, *Integer)

Integer is the length of List. If List is instantiated to a proper list, the predicate is determinate, also when Integer is var.

Arguments

List list

a list

Integer integer

non-negative integer

Description

If List is a list of indefinite length (that is, either a variable or of the form [...|X]) and if Integer is bound to an integer, then List is made to be a list of length Integer with unique variables used to "pad" the list. If List cannot be made into a list of length Integer, the call fails.

     | ?-  List = [a,b|X], length(List, 4).
     
     List = [a,b,_3473,_3475],
     X = [_3473,_3475] ;
     
     | ?-
     

If Integer is unbound, then it is unified with all possible lengths for the list List.

If List is bound, and is not a list, length/2 simply fails.

Backtracking

If both List and Integer are variables, the system will backtrack, generating lists of increasing length whose elements are anonymous variables.

Exceptions

type_error

Integer integer

Examples

     
     | ?- length([1,2], 2).
     
     yes
     | ?- length([1,2], 0).
     
     no
     | ?- length([1,2], X).
     
     X = 2 ;
     
     no