Turning a recursion into an iteration in Java?

Question

I get a stack overflow error due to my recursion creating an infinite loop. Turning the method into an iteration would stop this, but I have no idea how!

Can anyone guide me in turning my recursion into a loop?

private int findEmpty(int startPos, int stepNum, String key) {
    if (arr[startPos] == null) {
        return startPos;
    }
    return findEmpty(getNextLocation(startPos, ++stepNum, key), stepNum, key);
}

It is specifically return findEmpty(getNextLocation(startPos, ++stepNum, key), stepNum, key); that causes the error!

If your recursive code recurses infinitely, so will an iterative version of it. — Nick
– Nick, Commented Nov 29, 2020 at 23:12
I suspect the problem is actually with getNextLocation, so we can't diagnose the problem without seeing that code. I suspect the following is not your problem, but it also looks like you're using ++stepNum in a way you don't expect; the way you have it, stepNum is the same at every iteration and is stepNum+1 inside of getNextLocation. — dantiston
– dantiston, Commented Nov 30, 2020 at 0:09
You asked an almost-identical question yesterday. It isn't even closed. Why did you abandon that question and ask this one? — NomadMaker
– NomadMaker, Commented Nov 30, 2020 at 17:15
edits must not invalidate existing answers. ask new, followup question with link to this one if needed! specifically, your revision 3 is a great candidate; it would attract specific answers no doubt. the newer revisions ask about completely different code, so merit yet another separate post. :) — Will Ness
– Will Ness, Commented Dec 1, 2020 at 14:35

Will Ness · Accepted Answer · 2020-11-29 23:09:51Z

3

Your recursive call is in tail position. Thus it already describes a loop. Just need to make it explicit, syntactically:

private int findEmpty(int startPos, int stepNum, String key) {
  while( True ) 
  {
    if (arr[startPos] == null) {
        return startPos;
    }
    // return findEmpty(getNextLocation(startPos, ++stepNum, key), stepNum, key);
    ++stepNum;
    int nextPos = getNextLocation(startPos, stepNum, key);
    // return findEmpty(nextPos, stepNum, key);
    startPos = nextPos;
  }
}

I don't code in Java. If the above code is non-compliant in any way, please regard it as a pseudocode and change into something suitable.

answered Nov 29, 2020 at 23:09

Will Ness

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1 Comment

Ingo Over a year ago

It's okay, though it is non-idiomatic to mutate the arguments of a function. However, this could easily circumvented by moving the values into auxiliary variables before the loop starts and then mutating those.

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