Calculating variance in prolog

i have made a function in Prolog:-

mean(L, M) :-
    sum(L, S),
    length(L, N),
    M is S/N.

sum([],0).
sum([H|T],Y):-
    sum(T,X),
    Y is X + H.

variance([],0).
variance([H|T], M, VO):-
    variance(T,M,Y),
    VO is( Y + ((H-M)*(H-M))).

statsList(L, M, V1) :-
    sum(L, S),
    length(L, N),
    M is S/N,
    variance(L, M, VO),
 开发者_如何学编程   V1 is V0/N.

for some reason when I try to calculate the variance it always replies "false" as so: variance([1,2,3],2,VO) or statsList([1,2,3],M,VO)

However if I use this just to test it works:

variance([],0).
variance([H|T], VO):-
    variance(T,Y),
    VO is( Y + ((H-2)*(H-2))).

Can someone tell me where I am going wrong?

variance([],0).
variance([H|T], M, VO):-
    variance(T,M,Y),
    VO is( Y + ((H-M)*(H-M))).

The first clause defines a predicate variance/2 (two arguments) while the second defines variance/3. The latter predicate then calls itself recursively until it hits the empty list, which it cannot handle.

You should define a proper base case for variance/3. In Prolog, clauses with the same predicate name but different arity (number of arguments) define different predicates.

The error does not show up in your test code since there you define variance/2 with a base case and a recursive case.

In your first code you have defined two predicates variance/2 and variance/3 (one with 2 arguments and the other with 3 arguments). You have probably misspelled the first predicate. It should read

variance([], _, 0).