Does It Matter If I Declare A Variable Inside A Loop? - c

Question

My question is that does it matter if I declare a variable outside the loop and reinitialize every time inside the loop or declaring and initializing inside the loop? So basically is there any difference between these two syntaxes (Performance,standard etc)?
Method 1
int a,count=0;
while(count<10)
a=0;
Method 2
int count=0;
while(count<10)
int a=0;
Please assume that this is only a part of a bigger program and that the body inside loop requires that the variable a have a value of 0 every time. So, will there be any difference in the execution times in both the methods?

Answer 1

Yes, it does matter. In second case
int count=0;
while(count<10)
int a=0;
a can't be referenced out side of while loop. It has block scope; the portion of the program text in which the variable can be referenced.
Another thing that Jonathan Leffler pointed out in his answer is both of these loops are infinite loop. And second, the most important second snippet would not compile without {} (in C) because a variable definition/declaration is not a statement and cannot appear as the body of a loop.
int count =0;
while(count++ < 10)
{
int a=0;
}

Answer 2

This
void f1(void)
{
int a, count = 10;
while (count--)
a = 0;
}
void f2(void)
{
int count = 10;
while (count--)
{
int a = 0;
}
}
results in this (using non optimising gcc (Debian 4.4.5-8) 4.4.5):
.globl f1
.type f1, #function
f1:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
movq %rsp, %rbp
.cfi_offset 6, -16
.cfi_def_cfa_register 6
movl $10, -4(%rbp)
jmp .L2
.L3:
movl $0, -8(%rbp)
.L2:
cmpl $0, -4(%rbp)
setne %al
subl $1, -4(%rbp)
testb %al, %al
jne .L3
leave
ret
.cfi_endproc
.LFE0:
.size f1, .-f1
.globl f2
.type f2, #function
f2:
.LFB1:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
movq %rsp, %rbp
.cfi_offset 6, -16
.cfi_def_cfa_register 6
movl $10, -8(%rbp)
jmp .L6
.L7:
movl $0, -4(%rbp)
.L6:
cmpl $0, -8(%rbp)
setne %al
subl $1, -8(%rbp)
testb %al, %al
jne .L7
leave
ret
.cfi_endproc
.LFE1:
.size f2, .-f2
The assembly code looks quiet the same.

Answer 3

The first compiles; the second doesn't. The first runs for a very long time.
If the second was:
int count = 0;
while (count++ < 10)
{
int a = 0;
...do something with a...
}
and you also made similar changes and did something with a in the first loop, then the difference is that a is set to zero on each iteration of the loop in the second case, but it holds whatever value it is set to in the loop in the first case. Also, in the second case, the variable a does not exist outside the loop and cannot therefore be referenced after the loop.

Answer 4

yes in your 1st method it is zero for next of your bigger program.
while in your method 2 the same 'a' cannot be accessed in further statements. if so would produce an error as undefined reference to variable 'a'. it would be zero for that loop life but is nothing after the loop. you can again define variable 'a' after that while loop.