Question

I have a program where I have two arrays throughout. They begin empty but through the program they get populated.

If this is my first function:

program():-
  populate_arrays([],[]),
  carry_on().

In order to pass my populated arrays into carry_on() must I have four parameters in populate_arrays()? Two to declare the arrays as empty upon starting and another two to pass into carry_on()?

So like:

program():-
  populate_arrays([],[],A,B),
  carry_on(A,B).

it seems awkward I have to double my parameters

Answer 1

Yes, this is what you do. And no, it is not awkward because these are not duplicates: the first pair is your input arguments, and the second pair is your output results.

This is a common use pattern, corresponding to state transformation: old in, new out. The state of your knowledge is changed by this predicate, populate_arrays. This is reflected in how you use it.

BTW, if the starting values are always [], you can provide a “wrapper” for your “working” predicate:

populate_arrays( A, B):-
   populate_arrays([], [], A, B).

populate_arrays( A, B, C, D):-
   % your implementation ...