What's the best way to break from nested (for)

问题:

What\'s the best way to break from nested loops in Javascript?

//Write the links to the page.
for (var x = 0; x < Args.length; x++)
{
   for (var Heading in Navigation.Headings)
   {
      for (var Item in Navigation.Headings[Heading])
      {
         if (Args[x] == Navigation.Headings[Heading][Item].Name)
         {
            document.write(\"<a href=\\\"\" 
               + Navigation.Headings[Heading][Item].URL + \"\\\">\" 
               + Navigation.Headings[Heading][Item].Name + \"</a> : \");
            break; // <---HERE, I need to break out of two loops.
         }
      }
   }
}

回答1:

Just like Perl,

loop1:
    for (var i in set1) {
loop2:
        for (var j in set2) {
loop3:
            for (var k in set3) {
                break loop2;  // breaks out of loop3 and loop2
            }
        }
    }

as defined in EMCA-262 section 12.12. [MDN Docs]

Unlike C, these labels can only be used for continue and break, as Javascript does not have goto.

回答2:

Wrap that up in a function and then just return.

回答3:

I\'m a little late to the party but the following is a language-agnostic approach which doesn\'t use GOTO/labels or function wrapping:

for (var x = Set1.length; x > 0; x--)
{
   for (var y = Set2.length; y > 0; y--)
   {
      for (var z = Set3.length; z > 0; z--)
      {
          z = y = -1; // terminates second loop
          // z = y = x = -1; // terminate first loop
      }
   }
}

On the upside it flows naturally which should please the non-GOTO crowd. On the downside, the inner loop needs to complete the current iteration before terminating so it might not be applicable in some scenarios.

回答4:

I realize this is a really old topic, but since my standard approach is not here yet, I thought I post it for the future googlers.

var a, b, abort = false;
for (a = 0; a < 10 && !abort; a++) {
    for (b = 0; b < 10 && !abort; b++) {
        if (condition) {
            doSomeThing();
            abort = true;
        }
    }
}

回答5:

var str = \"\";
for (var x = 0; x < 3; x++) {
    (function() {  // here\'s an anonymous function
        for (var y = 0; y < 3; y++) {
            for (var z = 0; z < 3; z++) {
                // you have access to \'x\' because of closures
                str += \"x=\" + x + \"  y=\" + y + \"  z=\" + z + \"<br />\";
                if (x == z && z == 2) {
                    return;
                }
            }
        }
    })();  // here, you execute your anonymous function
}

How\'s that? :)

回答6:

quite simple

var a=[1,2,3];
var b=[4,5,6];
var breakCheck1=false;

for (var i in a){
    for (var j in b){
        breakCheck1=true;
        break;
    }
    if (breakCheck1) {break;}
}

回答7:

How about using no breaks at all, no abort flags, and no extra condition checks. This version just blasts the loop variables (makes them Number.MAX_VALUE) when the condition is met and forces all the loops to terminate elegantly.

// No breaks needed
for (var i = 0; i < 10; i++) {
  for (var j = 0; j < 10; j++) {
    if (condition) {
      console.log(\"condition met\");
      i = j = Number.MAX_VALUE; // Blast the loop variables
    }
  }
}

There was a similar-ish answer for decrementing-type nested loops, but this works for incrementing-type nested loops without needing to consider each loop\'s termination value for simple loops.

Another example:

// No breaks needed
for (var i = 0; i < 89; i++) {
  for (var j = 0; j < 1002; j++) {
    for (var k = 0; k < 16; k++) {
      for (var l = 0; l < 2382; l++) {
        if (condition) {
          console.log(\"condition met\");
          i = j = k = l = Number.MAX_VALUE; // Blast the loop variables
        }
      }
    }
  }
}

回答8:

How about pushing loops to their end limits

    for(var a=0; a<data_a.length; a++){
       for(var b=0; b<data_b.length; b++){
           for(var c=0; c<data_c.length; c++){
              for(var d=0; d<data_d.length; d++){
                 a =  data_a.length;
                 b =  data_b.length;
                 c =  data_b.length;
                 d =  data_d.length;
            }
         }
       }
     }

回答9:

If you use Coffeescript, there is a convenient \"do\" keyword that makes it easier to define and immediately execute an anonymous function:

do ->
  for a in first_loop
    for b in second_loop
      if condition(...)
        return

...so you can simply use \"return\" to get out of the loops.

回答10:

I thought I\'d show a functional-programming approach. You can break out of nested Array.prototype.some() and/or Array.prototype.every() functions, as in my solutions. An added benefit of this approach is that Object.keys() enumerates only an object\'s own enumerable properties, whereas \"a for-in loop enumerates properties in the prototype chain as well\".

Close to the OP\'s solution:

    Args.forEach(function (arg) {
        // This guard is not necessary,
        // since writing an empty string to document would not change it.
        if (!getAnchorTag(arg))
            return;

        document.write(getAnchorTag(arg));
    });

    function getAnchorTag (name) {
        var res = \'\';

        Object.keys(Navigation.Headings).some(function (Heading) {
            return Object.keys(Navigation.Headings[Heading]).some(function (Item) {
                if (name == Navigation.Headings[Heading][Item].Name) {
                    res = (\"<a href=\\\"\"
                                 + Navigation.Headings[Heading][Item].URL + \"\\\">\"
                                 + Navigation.Headings[Heading][Item].Name + \"</a> : \");
                    return true;
                }
            });
        });

        return res;
    }

Solution that reduces iterating over the Headings/Items:

    var remainingArgs = Args.slice(0);

    Object.keys(Navigation.Headings).some(function (Heading) {
        return Object.keys(Navigation.Headings[Heading]).some(function (Item) {
            var i = remainingArgs.indexOf(Navigation.Headings[Heading][Item].Name);

            if (i === -1)
                return;

            document.write(\"<a href=\\\"\"
                                         + Navigation.Headings[Heading][Item].URL + \"\\\">\"
                                         + Navigation.Headings[Heading][Item].Name + \"</a> : \");
            remainingArgs.splice(i, 1);

            if (remainingArgs.length === 0)
                return true;
            }
        });
    });

回答11:

Already mentioned previously by swilliams, but with an example below (Javascript):

// Function wrapping inner for loop
function CriteriaMatch(record, criteria) {
  for (var k in criteria) {
    if (!(k in record))
      return false;

    if (record[k] != criteria[k])
      return false;
  }

  return true;
}

// Outer for loop implementing continue if inner for loop returns false
var result = [];

for (var i = 0; i < _table.length; i++) {
  var r = _table[i];

  if (!CriteriaMatch(r[i], criteria))
    continue;

  result.add(r);
}

回答12:

Here are five ways to break out of nested loops in JavaScript:

1) Set parent(s) loop to the end

for (i = 0; i < 5; i++)
{
    for (j = 0; j < 5; j++)
    {
        if (j === 2)
        {
            i = 5;
            break;
        }
    }
}

2) Use label

exit_loops:
for (i = 0; i < 5; i++)
{
    for (j = 0; j < 5; j++)
    {
        if (j === 2)
            break exit_loops;
    }
}

3) Use variable

var exit_loops = false;
for (i = 0; i < 5; i++)
{
    for (j = 0; j < 5; j++)
    {
        if (j === 2)
        {
            exit_loops = true;
            break;
        }
    }
    if (exit_loops)
        break;
}

4) Use self executing function

(function()
{
    for (i = 0; i < 5; i++)
    {
        for (j = 0; j < 5; j++)
        {
             if (j === 2)
                 return;
        }
    }
})();

5) Use regular function

function nested_loops()
{
    for (i = 0; i < 5; i++)
    {
        for (j = 0; j < 5; j++)
        {
             if (j === 2)
                 return;
        }
    }
}
nested_loops();

回答13:

XXX.Validation = function() {
    var ok = false;
loop:
    do {
        for (...) {
            while (...) {
                if (...) {
                    break loop; // Exist the outermost do-while loop
                }
                if (...) {
                    continue; // skips current iteration in the while loop
                }
            }
        }
        if (...) {
            break loop;
        }
        if (...) {
            break loop;
        }
        if (...) {
            break loop;
        }
        if (...) {
            break loop;
        }
        ok = true;
        break;
    } while(true);
    CleanupAndCallbackBeforeReturning(ok);
    return ok;
};

回答14:

the best way is -
1) Sort the both array which are used in first and second loop.
2) if item matched then break the inner loop and hold the index value.
3) when start next iteration start inner loop with hold index value.

回答15:

I know this was asked 8 years ago but in ES6 we got the for...of loop which enables the use of standard break functionality:

for (let item of items) {
  if (item.id === id) {
    //do something cool
    break;
  }
}