What is a neat way of breaking out of many for loops at once?

Question

Suppose I need to break out of three or four nested for loops at once at the occurence of some event inside the innermost loop. What is a neat way of doing that?

what I do is use flags like this:

int i, j, k;
int flag1 = 0;
int flag2 = 0;

for (i = 0; i < 100; i++) {
    for (j = 0; j < 100; j++) {
        for (k = 0; k < 100; k++) {
            if (k == 50) {
                flag1 = 1;
                flag2 = 1;
                break;
            }
        }
        if (flag1 == 1)break;
    }
    if (flag2 == 1)break;
}

I don't think this is particularly neat.

How would you accomplish the same thing? (w/o using jumps)

Answer 1

use goto. it's clean and simple.

Answer 2

Put all the loops in a function and just return instead of break.

Answer 3

If you're using Java, you can associate labels with each for block and then reference the label after a continue statement. For example:

outerfor:
for (int i=0; i<5; i++) {
    innerfor:
    for (int j=0; j<5; j++) {
        if (i == 1 && j == 2) {
             continue outerfor;
        }
    }
}

Answer 4

How would you accomplish the same thing? (w/o using jumps)

Why? Nothing is universally evil, and every put-upon tool has its uses (except gets()). Using goto here makes your code look cleaner, and is one of the only choices we have (assuming C). Look:

int i, j, k;

for (i = 0; i < 100; i++) {
    for (j = 0; j < 100; j++) {
        for (k = 0; k < 100; k++) {
            if (k == 50) {
                goto END;
            }
        }
    }
}
END:

Much cleaner than all those flag variables, and it even shows more clearly what your code is doing.

Answer 5

Just a wee bit better.

int i, j, k;
int flag1 = 0;
int flag2 = 0;

for (i = 0; i < 100 && !flag2; i++) {
    for (j = 0; j < 100 && !flag1; j++) {
        for (k = 0; k < 100; k++) {
            if (k == 50) {
                flag1 = 1;
                flag2 = 1;
                break;
            }
        }
    }
}

But if you really need to have those loops, then it makes sense explicitly declaring in each loop what conditions must hold for it to continue, for readability.