[c#] Breaking out of a nested loop


Answers

Don't know if it works in C#, but in C I often do this:

    for (int i = 0; i < 100; i++)
    {
        for (int j = 0; j < 100; j++)
        {
            if (exit_condition)
            {
                // cause the outer loop to break:
                i = INT_MAX;
                Console.WriteLine("Hi");
                // break the inner loop
                break;
            }
        }
    }
Question

If I have a for loop which is nested within another, how can I efficiently come out of both loops (inner and outer) in the quickest possible way?

I don't want to have to use a boolean and then have to say go to another method, but rather just to execute the first line of code after the outer loop.

What is a quick and nice way of going about this?

Thanks


I was thinking that exceptions aren't cheap/should only be thrown in a truly exceptional condition etc. Hence I don't think this solution would be good from a performance perspective.

I don't feel it it is right to take advantage of the newer features in .NET (anon methods) to do something which is pretty fundamental.

Because of that, tvon (sorry can't spell full username!) has a nice solution.

Marc: Nice use of anon methods, and this too is great but because I could be in a job where we don't use a version of .NET/C# that supports anon methods, I need to know a traditional approach too.




I remember from my student days that it was said it's mathematically provable that you can do anything in code without a goto (i.e. there is no situation where goto is the only answer). So, I never use goto's (just my personal preference, not suggesting that i'm right or wrong)

Anyways, to break out of nested loops I do something like this:

var isDone = false;
for (var x in collectionX) {
    for (var y in collectionY) {
        for (var z in collectionZ) {
            if (conditionMet) {
                // some code
                isDone = true;
            }
            if (isDone)
                break;
        }
        if (isDone) 
            break;
    }
    if (isDone)
        break;
}

... i hope that helps for those who like me are anti-goto "fanboys" :)




Is it possible to refactor the nested for loop into a private method? That way you could simply 'return' out of the method to exit the loop.




Did you even look at the break keyword? O.o

This is just pseudo-code, but you should be able to see what I mean:

<?php
for(...) {
    while(...) {
        foreach(...) {
            break 3;
        }
    }
}

If you think about break being a function like break(), then it's parameter would be the number of loops to break out of. As we are in the third loop in the code here, we can break out of all three.

Manual: http://php.net/break




Sometimes nice to abstract the code into it's own function and than use an early return - early returns are evil though : )

public void GetIndexOf(Transform transform, out int outX, out int outY)
{
    outX = -1;
    outY = -1;

    for (int x = 0; x < Columns.Length; x++)
    {
        var column = Columns[x];

        for (int y = 0; y < column.Transforms.Length; y++)
        {
            if(column.Transforms[y] == transform)
            {
                outX = x;
                outY = y;

                return;
            }
        }
    }
}



As i see you accepted the answer in which the person refers you goto statement, where in modern programming and in expert opinion goto is a killer, we called it a killer in programming which have some certain reasons, which i will not discuss it over here at this point, but the solution of your question is very simple, you can use a Boolean flag in this kind of scenario like i will demonstrate it in my example:

            for (; j < 10; j++)
            {
                //solution
                bool breakme = false;
                for (int k = 1; k < 10; k++)
                {
                   //place the condition where you want to stop it
                    if ()
                    {
                        breakme = true;
                        break;
                    }
                }

                if(breakme)
                    break;
               }

simple and plain. :)




You asked for a combination of quick, nice, no use of a boolean, no use of goto, and C#. You've ruled out all possible ways of doing what you want.

The most quick and least ugly way is to use a goto.




Use a suitable guard in the outer loop. Set the guard in the inner loop before you break.

bool exitedInner = false;

for (int i = 0; i < N && !exitedInner; ++i) {

    .... some outer loop stuff

    for (int j = 0; j < M; ++j) {

        if (sometest) {
            exitedInner = true;
            break;
        }
    }
    if (!exitedInner) {
       ... more outer loop stuff
    }
}

Or better yet, abstract the inner loop into a method and exit the outer loop when it returns false.

for (int i = 0; i < N; ++i) {

    .... some outer loop stuff

    if (!doInner(i, N, M)) {
       break;
    }

    ... more outer loop stuff
}



I've seen a lot of examples that use "break" but none that use "continue".

It still would require a flag of some sort in the inner loop:

while( some_condition )
{
    // outer loop stuff
    ...

    bool get_out = false;
    for(...)
    {
        // inner loop stuff
        ...

        get_out = true;
        break;
    }

    if( get_out )
    {
        some_condition=false;
        continue;
    }

    // more out loop stuff
    ...

}



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