What's the difference between these for loops?

Question

What's the difference between these two for loops? I was told not to use the second example in my code. I'm baffled by this since It's not a problem in other languages.

So what's the difference/issue with this or is it only a preference?

//ex1
int i;
for(i = 0; i<whatever; i++){...code}

//ex2
for(int i=0; i<whatever;i++){...code}

[This](http://stackoverflow.com/questions/1287863/c-for-loop-int-initial-declaration) might answer your question at least partly: the second is not valid C89. — Matthias C. M. Troffaes, Nov 03 '13 at 14:48
For my curiosity, who told you not to use the second construct? This is really bad advice. — Jens Gustedt, Nov 03 '13 at 14:58

Charlie Burns · Answer 1 · 2013-11-03T14:51:19.010

17

The difference is in the scoping of i.

In the first example, i has scope inside AND after the loop.

In the second example, i has scope inside the loop.

The second example is C99 and later.

Use the second example when you can, the first when you need i after the loop, or when your compiler does not support C99.

edited Nov 03 '13 at 14:51

answered Nov 03 '13 at 14:45

Charlie Burns

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Sadique · Answer 2 · 2013-11-03T15:14:17.953

The second is supported by C99 but not by C89. The difference lies in scoping as well as initialization points. Scoping is determined by parenthesis'. To understand what i mean:

int main()
{
    int i=0;//-------------------------------------------------
    {//                                                       |
        int i=1;//------------------------------------------  |
        {//                                                |  |
            for(int i=2; i < 1; i++)//-----------------    |  |
            {//Declaring in loop works only in C99    |                                       |    |  |
                 {//-----------------                 |    |  |
                      int i=3;//    |                 |    |  |
                 }//-----------------                 |    |  |
            }//----------------------------------------    |  | 
        }//                                                |  |   
    }//-----------------------------------------------------  |
    //---------------------------------------------------------
}

The dotted lines show the scoping of the different i's in the blocks.

score 0 · Answer 3 · answered Nov 03 '13 at 14:47

0

The scope of i variable is different.

In the second case variable scope is for loop itself. In first case, i variable belongs to outer scope.

answered Nov 03 '13 at 14:47

knov

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score 0 · Answer 4 · answered Nov 03 '13 at 14:48

Rule of thumb: Variables should be declared in the tightest scope possible.

Why?
Consider a typo, that you insert a ; after your for loop:

int i; 
for(i = 0; i<whatever; i++);
{  }

You won't get an error if you declare i outside the scope of the loop and that's a way better than the previous case.

BUT

for(int i = 0; i<whatever; i++);
{  }

You'll get an error because i is not known in the scope of { }. So it's better to declare variables in the tightest scope when it's possible.

Jocke · Answer 5 · 2013-11-03T15:02:39.303

0

IIRC, the second example is invalid in C89 and/or C90, but is available in C99 and later. Any other differences than that are that i is only accessible within the loop in the second example, where the first example's i is in the outer-scope.

I always use the form in the first example because the program becomes more consistent and error resistant without me thinking too much.

edited Nov 03 '13 at 15:02

answered Nov 03 '13 at 14:51

Jocke

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What's the difference between these for loops?

5 Answers5