Question

Im trying to write a binary tree in prolog but getting the following error:

Syntax error: Operator expected

The error occurs here in the first call to addChildren:

addChildren(node(Left, Right, Cand, [(Name, Profit, Weight)|T])):-
getTotalWeight(Cand, 0, Total),
%if total weight is less than 20
((Total + Weight) < 20 -> %then
    New = [Cand | (Name, Profit, Weight)],
    addChildren(Left(_,_, New, T)), %error here
    addChildren(Right(_,_, Cand, T))
; %else
    %end).

I’m using the following node:

node(node, node, [], []).

Any and all help is appreciated.

Answer 1

Left(_,_, New, T)

is not a valid compound term, since the functor may not be a variable (uppercased identifier). Use

left(_, _, New, T)

instead, and similarly for Right.