The length(X,Y) goal succeeds if the number of elements in the list X matches the integer value Y.
Examples
?- length([],X).
X = 0
yes
?- length([a],X).
X = 1
yes
?- length([a,b],X).
X = 2
yes
?- length([a,b,c],X).
X = 3
yes
?- length([a,b],1).
no
?- length([a,b],3).
no
?- length(X,0).
X = []
yes
?- length(X,1).
X = [E0]
yes
?- length(X,3).
X = [E0,E1,E2]
yes
?- length(X,Y).
X = []
Y = 0
yes;
X = [E0]
Y = 1
yes;
X = [E0,E1]
Y = 2
yes;
X = [E0,E1,E2]
Y = 3
yes;
X = [E0,E1,E2,E3]
Y = 4
yes;
X = [E0,E1,E2,E3,E4]
Y = 5
yes;
X = [E0,E1,E2,E3,E4,E5]
Y = 6
yes;
X = [E0,E1,E2,E3,E4,E5,E6]
Y = 7
yes
?- length([a,b|X],Y).
X = []
Y = 2
yes;
X = [E0]
Y = 3
yes;
X = [E0,E1]
Y = 4
yes;
X = [E0,E1,E2]
Y = 5
yes;
X = [E0,E1,E2,E3]
Y = 6
yes;
X = [E0,E1,E2,E3,E4]
Y = 7
yes;
X = [E0,E1,E2,E3,E4,E5]
Y = 8
yes;
X = [E0,E1,E2,E3,E4,E5,E6]
Y = 9
yesTODO fix documentation generator to handle QUIT
?- length([a,b|X],8).
X = [E0,E1,E2,E3,E4,E5]
yes
?- length([a,b|X],3).
X = [E0]
yes
?- length([a,b|X],2).
X = []
yes
?- length([a,b|X],1).
no
?- length([a,b,c],a).
no
?- length(X,X).
no
?- length([a,b,c|X],X).
no
?- length(abc,X).
Expected list but got: ATOM with value: abc
?- length([a,b|c],X).
Expected list but got: LIST with value: .(a, .(b, c))
?- length([a,b|X],z).
Expected Numeric but got: ATOM with value: z