Question

I have some code

tree1(tree(1,
            tree(2,
                tree(3,nil,nil),
                tree(4,nil,nil)),
            tree(5,
                tree(6,nil,nil),
                tree(7,nil,nil))
        )
    ).
rbt_count_nodes(e,0):-!.
rbt_count_nodes(t(_,L,R),N):-
    rbt_count_nodes(L,NL),
    rbt_count_nodes(R,NR),
    N=NL+NR+1.

?-tree1(T),rbt_count_nodes(T,N),write(N).

But goal always return No. Why ?

Answer 1

rbt_count_nodes(t(_,L,R),N)

should become

rbt_count_nodes(tree(_,L,R),N)

and

N=NL+NR+1

should become

N is NL + NR + 1

((=)/2 is used to perform unification, (is)/2 to perform arithmetic).

and

rbt_count_nodes(e,0):-!.

should become

rbt_count_nodes(nil,0).

After those three edits it should be fine, though I cannot test so I might be proven wrong.