Can I use break to exit multiple nested for loops?

问题:

Is it possible to use the break function to exit several nested for loops? If so, how would you go about doing this? Can you also control how many loops the break exits?

回答1:

AFAIK, C++ doesn\'t support naming loops, like Java and other languages do. You can use a goto, or create a flag value that you use. At the end of each loop check the flag value. If it is set to true, then you can break out of that iteration.

回答2:

No, don\'t spoil it with a break. This is the last remaining stronghold for the use of goto.

回答3:

Another approach to breaking out of a nested loop is to factor out both loops into a separate function, and return from that function when you want to exit.

Of course, this brings up the other argument of whether you should ever explicitly return from a function anywhere other than at the end.

回答4:

Just to add an explicit answer using lambdas:

  for (int i = 0; i < n1; ++i) {
    [&] {
      for (int j = 0; j < n2; ++j) {
        for (int k = 0; k < n3; ++k) {
          return; // yay we\'re breaking out of 2 loops here
        }
      }
    }();
  }

Of course this pattern has a certain limitations and obviously C++11 only but I think it\'s quite useful.

回答5:

break will exit only the innermost loop containing it.

You can use goto to break out of any number of loops.

Of course goto is often Considered Harmful.

is it proper to use the break function[...]?

Using break and goto can make it more difficult to reason about the correctness of a program. See here for a discussion on this: Dijkstra was not insane.

回答6:

Although this answear was already presented, i think a good approach is to do the following:

for(unsigned int z = 0; z < z_max; z++)
{
    bool gotoMainLoop = false;
    for(unsigned int y = 0; y < y_max && !gotoMainLoop; y++)
    {
        for(unsigned int x = 0; x < x_max && !gotoMainLoop; x++)
        {
                          //do your stuff
                          if(condition)
                            gotoMainLoop = true;
        }
    }

}

回答7:

How about this?

for(unsigned int i=0; i < 50; i++)
{
    for(unsigned int j=0; j < 50; j++)
    {
        for(unsigned int k=0; k < 50; k++)
        {
            //Some statement
            if (condition)
            {
                j=50;
                k=50;
            }
        }
    }
}

回答8:

One nice way to break out of several nested loops is to refactor your code into a function:

void foo()
{
    for(unsigned int i=0; i < 50; i++)
    {
        for(unsigned int j=0; j < 50; j++)
        {
            for(unsigned int k=0; k < 50; k++)
            {
                // If condition is true
                return;
            }
        }
    }
}

回答9:

A code example using goto and a label to break out of a nested loop:

for (;;)
  for (;;)
    goto theEnd;
theEnd:

回答10:

goto can be very helpful for breaking nested loops

for (i = 0; i < 1000; i++) {
    for (j = 0; j < 1000; j++) {
        for (k = 0; k < 1000; k++) {
             for (l = 0; l < 1000; l++){
                ....
                if (condition)
                    goto break_me_here;
                ....
            }
        }
    }
}

break_me_here:
// Statements to be executed after code breaks at if condition

回答11:

The break statement terminates the execution of the nearest enclosing do, for, switch, or while statement in which it appears. Control passes to the statement that follows the terminated statement.

from msdn.

回答12:

I do think a goto is valid in this circumstance:

To simulate a break/continue, you\'d want:

Break

for ( ;  ;  ) {
    for ( ;  ;  ) {
        /*Code here*/
        if (condition) {
            goto theEnd;
        }
    }
}
theEnd:

Continue

for ( ;  ; ) {
    for ( ;  ;  ) {
        /*Code here*/
        if (condition) {
            i++;
            goto multiCont;
        }
    }
    multiCont:
}

回答13:

Other languages such as PHP accept a parameter for break (i.e. break 2;) to specify the amount of nested loop levels you want to break out of, C++ however doesn\'t. You will have to work it out by using a boolean that you set to false prior to the loop, set to true in the loop if you want to break, plus a conditional break after the nested loop, checking if the boolean was set to true and break if yes.

回答14:

I know this is old post . But I would suggest a bit logical and simpler answer.

for(unsigned int i=0; i < 50; i++)
    {
        for(unsigned int j=0; j < conditionj; j++)
        {
            for(unsigned int k=0; k< conditionk ; k++)
            {
                // If condition is true

                j= conditionj;
               break;
            }
        }
    }

回答15:

  while (i<n) {
    bool shouldBreakOuter = false;
    for (int j=i + 1; j<n; ++j) {
      if (someCondition) {
          shouldBreakOuter = true;
      }
    }

    if (shouldBreakOuter == true)
      break;

  }

回答16:

Breaking out of a for-loop is a little strange to me, since the semantics of a for-loop typically indicate that it will execute a specified number of times. However, it\'s not bad in all cases; if you\'re searching for something in a collection and want to break after you find it, it\'s useful. Breaking out of nested loops, however, isn\'t possible in C++; it is in other languages through the use of a labeled break. You can use a label and a goto, but that might give you heartburn at night..? Seems like the best option though.

回答17:

You can use try...catch.

try {
    for(int i=0; i<10; ++i) {
        for(int j=0; j<10; ++j) {
            if(i*j == 42)
                throw 0; // this is something like \"break 2\"
        }
    }
}
catch(int e) {} // just do nothing
// just continue with other code

If you have to break out of several loops at once, it is often an exception anyways.