C++ – How to break out of nested loops in C++?

breakc

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;'?
            }
        }
    }
}

Best Answer

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
}

Setting a bit

Use the bitwise OR operator (|) to set a bit.

number |= 1UL << n;

That will set the nth bit of number. n should be zero, if you want to set the 1st bit and so on upto n-1, if you want to set the nth bit.

Use 1ULL if number is wider than unsigned long; promotion of 1UL << n doesn't happen until after evaluating 1UL << n where it's undefined behaviour to shift by more than the width of a long. The same applies to all the rest of the examples.

Clearing a bit

Use the bitwise AND operator (&) to clear a bit.

number &= ~(1UL << n);

That will clear the nth bit of number. You must invert the bit string with the bitwise NOT operator (~), then AND it.

Toggling a bit

The XOR operator (^) can be used to toggle a bit.

number ^= 1UL << n;

That will toggle the nth bit of number.

Checking a bit

You didn't ask for this, but I might as well add it.

To check a bit, shift the number n to the right, then bitwise AND it:

bit = (number >> n) & 1U;

That will put the value of the nth bit of number into the variable bit.

Changing the nth bit to x

Setting the nth bit to either 1 or 0 can be achieved with the following on a 2's complement C++ implementation:

number ^= (-x ^ number) & (1UL << n);

Bit n will be set if x is 1, and cleared if x is 0. If x has some other value, you get garbage. x = !!x will booleanize it to 0 or 1.

To make this independent of 2's complement negation behaviour (where -1 has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation.

number ^= (-(unsigned long)x ^ number) & (1UL << n);

unsigned long newbit = !!x;    // Also booleanize to force 0 or 1
number ^= (-newbit ^ number) & (1UL << n);

It's generally a good idea to use unsigned types for portable bit manipulation.

number = (number & ~(1UL << n)) | (x << n);

(number & ~(1UL << n)) will clear the nth bit and (x << n) will set the nth bit to x.

It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.

Python – How to break out of multiple loops

My first instinct would be to refactor the nested loop into a function and use return to break out.