how to use recursion for nested 'for' loops

Question

I have a problem with nested for loops in java. My problem is that at the beginning I don't know exactly how many for loops I will need. It is set somewhere in the middle of my program. So let say my program creates an array. If the array has 3 elements then I create a three for loops like below.

for(int i = 0; i<tab[0].length() ; i++){
    for(int j = 0; j<tab[1].length() ; j++){
        for(int k = 0; k<tab[2].length() ; k++){
            System.out.println(i+" "+j+" "+k);
        }
    }
}

If my program created an array with 4 elements then it would be like this:

for(int i = 0; i<tab[0].length() ; i++){
    for(int j = 0; j<tab[1].length() ; j++){
        for(int k = 0; k<tab[2].length() ; k++){
             for(int h = 0; h<tab[3].length() ; h++){
                System.out.println(i+" "+j+" "+k+" "+h);
             }
        }
    }
}

Can any one tell me how to do this with recursion? I can have 2 nested loops but I can have 10 of them and always at the end I would like to print in the console numbers associated with all loops (i,j,k,h)

csander · Accepted Answer · 2017-08-02 20:39:19Z

3

Here is a solution. At each recursive call, previousTabs becomes 1 longer and tabs becomes 1 shorter.

public static void iterate(int[] previousValues, int[] tabs) {
    if (tabs.length == 0) {
        System.out.println(Arrays.toString(previousValues));
    }
    else {
        final int[] values = new int[previousValues.length + 1];
        for (int i = 0; i < previousValues.length; i++) {
            values[i] = previousValues[i];
        }
        final int[] nextTabs = new int[tabs.length - 1];
        for (int i = 0; i < nextTabs.length; i++) {
            nextTabs[i] = tabs[i + 1];
        }
        for (int i = 0; i < tabs[0]; i++) {
            values[values.length - 1] = i;
            iterate(values, nextTabs);
        }
    }
}
public static void iterate(int[] tabs) {
    iterate(new int[0], tabs);
}

answered Aug 2, 2017 at 20:39

csander

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