Dynamically change the number of nested 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 a way to achieve this without recursion? Another question: what is the use of Multiple Dispatch in Java? I am trying to encode something in ONE METHOD, and it should fire different events in different cases of the parameter. NO IF REPORTS / TERNARY OPERATORS / CASES.

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

Answer 1

Think about how many times you go through this cycle. 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 on what exactly you are trying to do. Recursion can always be replaced with iteration (see this post for examples, using Stack to store state).

But perhaps the modulo (%) operator could work here? those. have one loop that increments the value of variable ( i ), and then other variables are computed using modulo ( i % 3 , etc.). You can use Map to store variable values ​​indirectly if there is a variable number of variables.

Answer 3

You need to create an array of loop counts 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)) your own code.

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

Answer 4

I think you need a backtracking algorithm. But then you replace your nested loops with recursion.

I do not want to post links here, as moderators do not like it.

Look at the “eight queen puzzle” (you can do it on Google), you get my idea.

I know this idea works, since I asked this very question (which you have) for me in many cases, and I have applied it several times successfully.

Here is a small example (I modified it since the previous one was a bit complicated).

 public class Test001 { public static void main(String[] args) { loop(0, 5, 10); } /** * max_level - the max count of nesting loops * size - the size of the collection * * 0 - top most level * level 1 - nested into 0 * level 2 - nested into 1 * ... * and so on. */ private static void loop(int level, int max_level, int size){ if (level > max_level) return; for (int i=0; i<size-level; i++){ System.out.println("Now at level: " + level + " counter = " + i); loop(level + 1, max_level, size); } } } 

But it still uses recursion.

Answer 5

This is a bit confusing, but: here is a way to do it without recursion, in one function and without if s.

 public static void no_ifs_no_recursion(int n){ int[] helper = new int[n-1]; int[] pointers = new int[n]; //helper for printing the results int totalsize = 1; for (int loops = 2; loops <= n; loops++){ helper[n - loops] = totalsize; totalsize*=loops; } for (int i=0; i<totalsize; i++){ int carry = i; for (int loops = 0; loops < n-1; loops++){ pointers[loops] = carry/helper[loops]; carry = carry - (pointers[loops]*helper[loops]); } System.out.println(Arrays.toString(pointers)); //or do something else with `i` -> `pointers[0]`, `j` -> `pointers[1]`, `k` -> `pointers[2]` etc.. } }