Thread: declaring variables inside loops

Ok so, I know that if you declare a variable inside an if statement, then the variable will exist only inside the if block. But what about in loops? If I do something like:

Code:

for(whatever;bla;etc.) {
    int x;
    bla bla bla...;
}

Is it efficient? I thought that maybe it could declare multiple times the variable and be inefficient actually, but I'm not sure.

Thing is that I want to declare a variable that will only be used inside the for. I have seen the declaration inside the for parentheses:

Code:

for(int x; bla; bla)

But the compiler gives me a warning, something about not C99.

Thanks.

Yeah, you can only declare them like that in C when compiling as C99. I think most compilers are set to C98 mode by default. (they are different C standards btw)

well is the first way (the one that the declaration is inside the for loop) efficient or does it make a new variable every time the loop turns?

well is the first way (the one that the declaration is inside the for loop) efficient or does it make a new variable every time the loop turns?

No, it is not reallocated for each loop, the only difference for the compiler is to reduce
the scope of the variable.

cool thanks

yes, I use MinGW... what do I do, just write -std=c99 as argument? And, what's better? C99 or gcc's default?

Unless C handles loops significantly differently than C++, the following should be true for this code:

Code:

int main()
{
	for ( int i = 0; i < 5; ++i )
	{
		double d;
	}

	return 0;
}

i is created once, and d is created 5 times.

Someone (I think it was Bubba) told me that the compiler would likely optimise it so that the variable declaration is done out of the loop. It also wouldent need to be switched in and out of scope (as theres only one scope for it here). I dont know if there would be any overhead for dealing with different scopes?

hmmm.. now I'm not sure. I thought swoopy said that d would be allocated only one time, that the scope would be the only difference if it is declared inside the loop. Now, in C89 I can't declare i like that, I have to set my compiler to C99.

I'll leave the declarations outside the loop just in case.

I don't know if the compiler would optimise it or not, but if its setting a new one each time then it would be less efficient. You could always time it and see.