Indexing starts at 0 when using nth0. Indexing starts at 1 when using nth1.
Examples
?- nth0(0, [a,b,c], a).
yes
?- nth1(1, [a,b,c], a).
yes
?- nth0(1, [a,b,c], b).
yes
?- nth1(2, [a,b,c], b).
yes
?- nth0(2, [a,b,c], c).
yes
?- nth1(3, [a,b,c], c).
yes
?- nth0(-1, [a,b,c], a).
no
?- nth0(1, [a,b,c], a).
no
?- nth0(5, [a,b,c], a).
no
?- nth0(0, [a,b,c], X).
X = a
yes
?- nth0(1, [a,b,c], X).
X = b
yes
?- nth0(2, [a,b,c], X).
X = c
yes
?- nth0(-1, [a,b,c], X).
no
?- nth0(3, [a,b,c], X).
no
?- nth0(X, [h,e,l,l,o], e).
X = 1
yes;
no
?- nth0(X, [h,e,l,l,o], l).
X = 2
yes;
X = 3
yes;
no
?- nth0(X, [h,e,l,l,o], z).
no
?- nth0(X, [h,e,l,l,o], Y).
X = 0
Y = h
yes;
X = 1
Y = e
yes;
X = 2
Y = l
yes;
X = 3
Y = l
yes;
X = 4
Y = o
yes
?- nth1(0, [a,b,c], a).
no
?- nth1(2, [a,b,c], a).
no
?- nth1(4, [a,b,c], a).
no
?- nth1(1, [a,b,c], X).
X = a
yes
?- nth1(2, [a,b,c], X).
X = b
yes
?- nth1(3, [a,b,c], X).
X = c
yes
?- nth1(-1, [a,b,c], X).
no
?- nth1(0, [a,b,c], X).
no
?- nth1(4, [a,b,c], X).
no
?- nth1(X, [h,e,l,l,o], e).
X = 2
yes;
no
?- nth1(X, [h,e,l,l,o], l).
X = 3
yes;
X = 4
yes;
no
?- nth1(X, [h,e,l,l,o], z).
no
?- nth1(X, [h,e,l,l,o], Y).
X = 1
Y = h
yes;
X = 2
Y = e
yes;
X = 3
Y = l
yes;
X = 4
Y = l
yes;
X = 5
Y = o
yesNote: "nth" is a synonym for "nth1".
?- nth(2, [a,b,c], b).
yes
?- nth0(1, [h,e,l,l,o|Y], l).
no
?- nth1(1, [h,e,l,l,o|Y], l).
no
?- nth0(X, [h,e,l,l,o|Y], l).
X = 2
Y = UNINSTANTIATED VARIABLE
yes;
X = 3
Y = UNINSTANTIATED VARIABLE
yes;
X = 5
Y = [l|_5]
yes;
X = 6
Y = [_6,l|_5]
yes;
X = 7
Y = [_7,_6,l|_5]
yes
?- nth1(X, [h,e,l,l,o|Y], l).
X = 3
Y = UNINSTANTIATED VARIABLE
yes;
X = 4
Y = UNINSTANTIATED VARIABLE
yes;
X = 6
Y = [l|_6]
yes;
X = 7
Y = [_7,l|_6]
yes;
X = 8
Y = [_8,_7,l|_6]
yes
?- nth0(8,[a,b,c|X],Y).
X = [E4,E3,E2,E1,E0,Y|T]
Y = UNINSTANTIATED VARIABLE
yes
?- nth1(8,[a,b,c|X],Y).
X = [E3,E2,E1,E0,Y|T]
Y = UNINSTANTIATED VARIABLE
yes
?- nth0(X,[a,b,c|Y],Z).
X = 0
Y = UNINSTANTIATED VARIABLE
Z = a
yes;
X = 1
Y = UNINSTANTIATED VARIABLE
Z = b
yes;
X = 2
Y = UNINSTANTIATED VARIABLE
Z = c
yes;
X = 3
Y = [Z|_3]
Z = UNINSTANTIATED VARIABLE
yes;
X = 4
Y = [_4,Z|_3]
Z = UNINSTANTIATED VARIABLE
yes;
X = 5
Y = [_5,_4,Z|_3]
Z = UNINSTANTIATED VARIABLE
yes
?- nth1(X,[a,b,c|Y],Z).
X = 1
Y = UNINSTANTIATED VARIABLE
Z = a
yes;
X = 2
Y = UNINSTANTIATED VARIABLE
Z = b
yes;
X = 3
Y = UNINSTANTIATED VARIABLE
Z = c
yes;
X = 4
Y = [Z|_4]
Z = UNINSTANTIATED VARIABLE
yes;
X = 5
Y = [_5,Z|_4]
Z = UNINSTANTIATED VARIABLE
yes;
X = 6
Y = [_6,_5,Z|_4]
Z = UNINSTANTIATED VARIABLE
yes
?- nth0(X,Y,Z).
X = 0
Y = [Z|_0]
Z = UNINSTANTIATED VARIABLE
yes;
X = 1
Y = [_1,Z|_0]
Z = UNINSTANTIATED VARIABLE
yes;
X = 2
Y = [_2,_1,Z|_0]
Z = UNINSTANTIATED VARIABLE
yes
?- nth1(X,Y,Z).
X = 1
Y = [Z|_1]
Z = UNINSTANTIATED VARIABLE
yes;
X = 2
Y = [_2,Z|_1]
Z = UNINSTANTIATED VARIABLE
yes;
X = 3
Y = [_3,_2,Z|_1]
Z = UNINSTANTIATED VARIABLE
yes