Question

I want to append to a list a variable, N, that’s bound to a number.

N = 1.

append([N], [2,3,4], Z).

Z = [N,2,3,4]. //Wrong output!

I want to get Z = [1,2,3,4]

How do I append the number part of a variable, not the actual variable itself?

Answer 1

I’m afraid Prolog doesn’t have variable assignment like you’re used to, just variable binding. So the “statements”

N = 1.
append([N], [2,3,4], Z).

actually constitute two completely unrelated queries. Fortunately, the effect you desire can be achieved by combining your queries:

N = 1, append([N], [2,3,4], Z).

If you truly need a global variable, you can always use a fact or asserta/1 to define one dynamically.

Also note: in the future, you’ll probably want to make sure you use is instead of = when dealing with numbers.