declaring variables inside loops

This is a discussion on declaring variables inside loops within the C Programming forums, part of the General Programming Boards category; Ok so, I know that if you declare a variable inside an if statement, then the variable will exist only ...

  1. #1
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    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.

  2. #2
    Dr Dipshi++ mike_g's Avatar
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    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)

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    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?

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    Dr Dipshi++ mike_g's Avatar
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    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.

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    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.

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    cool thanks

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    >Is it efficient? I thought that maybe it could declare multiple times the variable and be inefficient actually, but I'm not sure.
    It should be as efficient as it would be declaring it at the top of the function. It only declares it once, when it enters the scope (the scope being what's between the left and right braces), and depending on the compiler, may actually delete the variable, once it exits the scope (though it may wait until the end of the function).

    >Thing is that I want to declare a variable that will only be used inside the for.
    For gcc, you can add -std=c99 to your compiler options. That would include Dev-C++ and code::blocks IDEs.

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

  9. #9
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    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.

  10. #10
    Dr Dipshi++ mike_g's Avatar
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    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?

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    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.

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    If you're not convinced of how the compiler handle those variables, this is quite easy:
    write two simple functions using the two cases (declaration outside and inside a loop)
    then compare the disassembled code, you'll see that the instructions generated
    are the same, you don't even have to understand what it does.

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    >yes, I use MinGW... what do I do, just write -std=c99 as argument?
    Yes. And of course there's -std=c89 for the C89 standard.
    >And, what's better? C99 or gcc's default?
    If you want to write portable code (works on any system) it's best to use the c89 switch (and also turn up the warning level (-Wall). gcc's default is best if you want to use any of gcc's extensions and don't care as much about portability. C99 is the latest standard. Use it if you need some of it's features (like specific sizes for int's or complex math, for example).

    I think gcc's default allows variable declarations anywhere.
    Last edited by swoopy; 04-29-2008 at 07:13 PM.

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    Code:
    #include <stdio.h>
    
    int main(void)
    {
    	int x;
    	int number;
    	
    	for(x=0; x<100; x++) {
    		number = 0;
    		number += 2;
    		printf("Number = %d\n",number);
    	}
    	
    	return 0;
    }
    Same assembler code as:
    Code:
    #include <stdio.h>
    
    int main(void)
    {
    	int x;
    	
    	for(x=0; x<100; x++) {
    		int number = 0;
    		number += 2;
    		printf("Number = %d\n",number);
    	}
    	
    	return 0;
    }
    Code:
    	.file	"test1.c"
    	.def	___main;	.scl	2;	.type	32;	.endef
    	.section .rdata,"dr"
    LC0:
    	.ascii "Number = %d\12\0"
    	.text
    .globl _main
    	.def	_main;	.scl	2;	.type	32;	.endef
    _main:
    	pushl	%ebp
    	movl	%esp, %ebp
    	subl	$24, %esp
    	andl	$-16, %esp
    	movl	$0, %eax
    	addl	$15, %eax
    	addl	$15, %eax
    	shrl	$4, %eax
    	sall	$4, %eax
    	movl	%eax, -12(%ebp)
    	movl	-12(%ebp), %eax
    	call	__alloca
    	call	___main
    	movl	$0, -4(%ebp)
    L2:
    	cmpl	$99, -4(%ebp)
    	jg	L3
    	movl	$0, -8(%ebp)
    	leal	-8(%ebp), %eax
    	addl	$2, (%eax)
    	movl	-8(%ebp), %eax
    	movl	%eax, 4(%esp)
    	movl	$LC0, (%esp)
    	call	_printf
    	leal	-4(%ebp), %eax
    	incl	(%eax)
    	jmp	L2
    L3:
    	movl	$0, %eax
    	leave
    	ret
    	.def	_printf;	.scl	2;	.type	32;	.endef
    The only thing that changes is the file name.

    And the only problem I have is that if you declare an initialized variable inside the for loop, it will be initialized in every single loop. So... its not good in some cases.

  15. #15
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    Unless the loop is iterating millions (or maybe even billions) of times, it probably won't make much of a speed difference either way; but to be sure the loop is optimal, I'd declare variables before the loop. Some compiler optimizers might not be as good as others, and in debug mode it won't optimize anything.

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