在C99中的嵌套for循环中声明计数器变量的最佳实践

Question

I'm asking which is the best practice between this two implementation:

for ( int i = 0; i < 5; i++ )
    for ( int j = 0; j < 5; j++ )
        ...some code here...

...other code...

for ( int i = 0; i < 5; i++ )
    for ( int j = 0; j < 5; j++ )
        ...some code here...

or this one:

beginning of function/main
int i,j;

...some code...

for ( i = 0; i < 5; i++ )
    for ( j = 0; j < 5; j++ )
        ...some code here...

...other code...

for ( i = 0; i < 5; i++ )
    for ( j = 0; j < 5; j++ )
        ...some code here...

In other words is it better:

1. declaring counter loops one time for all and defining them inside each loop
2. or declaring them every time?

Thank you

EDIT: My question is: Is it better to perform N declarations (and definitions) for 2 variables or 2 declarations and N definitions ?

EDIT2: Ok now I understand. I didn't know that declaration only affect the compilation and not the execution ( I saw assembly language of C99 compiled source file). So there's no difference and Lundin's answer shows the standard to use.

Answer 1

This is actually not opinion-based, but there exists a widely recognized industry standard:

Reduce the scope of a local variable as much as possible.

This is the very reason why C++ and C99 allow iterator declarations inside the loop itself. Meaning that the first version is better, period.

However , the second version has to be used if you need to know the iterator values after the loop has finished, or if you need C90 backwards-compatibility.

Answer 2

Always keep the scope of variables as tight as possible.

Your second option leaks i and j unnecessarily into the surrounding scope, so is not to be preferred.