How Can I simulate a while loop in Prolog with unchangeable conditions?

Question

So basically I am trying to simulate some C code in Prolog.

It is easy to simulate while loop in Prolog Here is the case:

C code:

int a = 1;
         


        

Answer 1

The most obvious way to do an infinite loop in Prolog is with repeat/0, and it looks like this:

while(1)
    do_something();

becomes

repeat,
do_something.

The real problem then becomes that there's no obvious analog to goto or break in Prolog. So I would be inclined to look for a pattern like this:

while(1) {
  if (test)
    break;
  do_something();
}

and convert it to Prolog like this:

loop_entry :-
  test,
  do_something,
  loop_entry.

You will of course need to add your local variables as parameters to loop_entry/0 and implement test, but this way when test fails the loop will end naturally.

Following your example with N and A leads to this kind of thing:

loop_entry(N, A) :-
  N > 0,
  succ(N0, N),
  succ(A, A1),
  loop_entry(N0, A1).

The "test" in this case is simply N > 0. If it isn't true, the predicate will simply fail and you can go on with life the Prolog way.

Edit #2. If you want the results (N and A) then add additional parameters for the values you want to return and add one more clause:

loop_entry(N, A, ResultN, ResultA) :-
  N > 0, !, 
  succ(N0, N),
  succ(A, A1),
  loop_entry(N0, A1, ResultN, ResultA).
loop_entry(N, A, N, A).

You can either add a cut after the condition or put the inverse condition in this new clause.