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;

while(N)
{
  N--;
  a++; 
}

Prolog code:

prolog_while(0) : !
prolog_while(N, A) :
   N1 is N -1,
   A1 is A + 1,
prolog_while(N1, A1).

One problem is how to simulate a while loop in Prolog with unchangeable conditions?

Here is the case:

int n = 1;
int a = 1;

while(1)
{
   if (N==0)
     goto while_end;
   else
    {
        N--; A++;
    }       
}

Or

while(1)
{
   if (N==0)
     break;
   else
    {
        N--; A++;
    }       
}

I know it is kinda of weird but basically these kind of C code is automatically generated by a source code analysis tool, so I have to deal with it...

Then basically how can I simulate in Prolog? Is it doable?

Could anyone give me some help?

===================update============

I tried to write some Prolog code in this way, but basically I still don't know how to handle test clause.

main :- loop_entry(2, 1), write(N), nl, write(A), nl.

do(A, N, A1, N1) :- A1 is (A + 1), N1 is (N - 1).
test(N) :- ...                 <----- # How to write this part?
loop_entry(N, A) :-
    test(N),
    do(A, N, A1, N1),
    loop_entry(N1,A1).

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.