Dynamically change the number of nested for loops

Question

I don't know if this is a stupid question, but I need to dynamically change the number of for-loops without using recursion.

For example, if n=3, I need 3 nested for-loops.

for(int i=0; i<size; i++){
   for(int j=0; j<size-1; j++){
       for(int k=0; k<size-2; k++){
          //do something
       }
   }
}

If n=5:

for(int i=0; i<size; i++){
   for(int j=0; j<size-1; j++){
       for(int k=0; k<size-2; k++){
          for(int l=0; l<size-3; l++){
              for(int m=0; m<size-4; m++){
                  //do something
              }
          }
       }
   }
}

Is there any way to achieve this without recursion? Another question: what is the use of Multiple Dispatch in Java? I'm trying to code something in ONE METHOD, and it should run different events in different cases of the parameter. NO IF STATEMENTS / TERNARY OPERATORS / CASES.

NOTE: I can ONLY have one method (part of the problem), and cannot use recursion. Sorry.

Answer 1

Think about how many times you run through this loop. It looks like (size!) / (size - n)!:

int numLoops = 1;
for (int i = 0; i < n; i++) {
    numLoops*= (size - i);
}

for (int i = 0; i < numLoops; i++) {
    //do something
}

Answer 2

It depends what exactly you're trying to do. Recursion can always be replaced with iteration (see this post for examples using a Stack to store state).

But perhaps the modulo (%) operator could work here? i.e. Have a single loop that increments a variable (i) and then the other variables are calculated using modulo (i % 3 etc). You could use a Map to store the values of the variables indirectly, if there are a varying number of variables.

Answer 3

You have to create array of loop counters and increment it manually.

Quick and dirty example:

public static void nestedFors(int n, int size) {
  assert n > size;
  assert size > 0;

  int[] i = new int[n];
  int l = n - 1;
  while(l >= 0) {
    if(l == n - 1) {
      System.out.println(Arrays.toString(i));
    }
    i[l]++;
    if(i[l] == size - l) {
      i[l] = 0;
      l--;
    } else if(l < n - 1) {
      l++;
    }
  }
}

Replace System.out.println(Arrays.toString(i)) with your own code.

You can check it here: http://ideone.com/IKbDUV