How do you break out of nested loops in C++?

Question

I am a beginner in C++, and I am wondering how to break out of nested loops. Is there a break(2)?

#include <iostream>

using namespace std;

int main() {
    for (int x = 5; x < 10; x++) {
        for (int j = 6; j < 9; j++) {
            for (int b = 7; b < 12; b++) {
                // Some statements
                // Is break(2) right or wrong
                // or can I use 'break; break;'?
            }
        }
    }
}

This is about the only place a `goto` is acceptable. A function is preferred (imho). A `goto` is actually better than `break(2)` would be if it were legal. — Galik, Aug 17 '16 at 14:53
I'm tempted to reverse-dupe-close the other question, as this one looks far better. — Quentin, Aug 17 '16 at 15:04

score 10 · Answer 1 · edited Aug 17 '16 at 14:50

10

You can use goto. It's essentially the same function

#include <iostream>

using namespace std;

int main() {
    for(int x = 5; x < 10; x++) {
        for(int j = 6; j < 9; j++) {
            for(int b = 7; b < 12; b++) {
                if (condition)
                    goto endOfLoop;
            }
        }
    }

    endOfLoop:
    // Do stuff here
}

edited Aug 17 '16 at 14:50

Quentin

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answered Aug 17 '16 at 14:48

Dillydill123

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Can you use goto in the in the for or not? – Ian Aug 17 '16 at 14:50
@Ian you can use it everywhere. Don't reach for it first thing, though. – StoryTeller - Unslander Monica Aug 17 '16 at 14:51
1

@Ian: you can safely `goto` *out* of the loop, but beware `goto` *into* the loop. – Dmitry Bychenko Aug 17 '16 at 14:52
thanks I get it now. :) – Ian Aug 17 '16 at 14:53
Just keep in mind that a goto kills all optimizations by waiting for the CPU's cycles to flush what's still in its pipe by forcing the compiler to add nop ASM opcodes before the goto. In short: it is a performance killer. – Jacques Jan 29 '23 at 01:56

John Zwinck · Answer 2 · 2016-08-17T14:53:30.610

4

No, there is no break(2) unfortunately (or perhaps fortunately, depending on your views of deep nesting of scopes).

There are two main ways to solve this:

Set a flag before you break which tells the outer loop to stop.
Put some of your nested loops into functions, so that they can do break but also return to jump out. For example:

// returns true if should be called again, false if not
bool foo() {
    for(int j = 6; j < 9; j++) {
        for(int b = 7; b < 12; b++) {
            if (something) {
                break; // one level
            }
            if (whatever) {
                return true; // two levels
            }
            if (another) {
                return false; // three levels
            }
        }
    }
}

int main() {
    for(int x = 5; x < 10; x++) {
        if (!foo()) {
            break;
        }
    }
}

edited Aug 17 '16 at 14:53

answered Aug 17 '16 at 14:48

John Zwinck

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5

Abusing functions or stuffing additional flags inside iteration logic to perform flow control are two good ways to make a mess of your code. – Quentin Aug 17 '16 at 14:55
3

As opposed to `goto` or even exceptions? – LogicStuff Aug 17 '16 at 14:56
1

@Quentin: The approaches I have outlined are very common in C and C++ and would be recognized by most anyone as normal practice. What, in your opinion, is the better approach? – John Zwinck Aug 17 '16 at 14:57
1

@LogicStuff, exceptions as a flow of control mechanism!? Better not. – StoryTeller - Unslander Monica Aug 17 '16 at 14:58
1

@LogicStuff as opposed to `goto`, which is the last flow control primitive when the other ones (here `break`) aren't enough. Exceptions should not in any way be involved in flow control. – Quentin Aug 17 '16 at 14:58
2

(I listed two, maybe only one scorned alternatives, It was not a suggestion) – LogicStuff Aug 17 '16 at 15:00
2

@JohnZwinck Dillydill123's answer is pretty much the canonical structure of a nested loop break. – Quentin Aug 17 '16 at 15:06

How do you break out of nested loops in C++?

2 Answers2