The C for() loop - A strange observation

Applications Programming
1

Hello,

I am optimizing my low level C coding style.

I have run into an strange C implementation fact:
----------------------------------------------------

unsigned int X;

for ( X = 0; X < 10; X++ ) printf("%u\n",X);

Produces:
0
1
2
3
4
5
6
7
8
9
----------------------------------------------------

unsigned int X;

for ( X = 10; X-- > 0; ) printf("%u\n",X);

Produces:
9
8
7
6
5
4
3
2
1
0
---------------------------------------------------

In the first case, the X++ operation is completed after the body of the loop, so the initial value of X is run through the loop.

In the second case, the X-- operation is completed before the body of the loop, so the initial value of X is skipped in the loop.

Why?

Is the final expression in the for() loop tied to the {} braces, whereas the loop condition is taking place distinctly before the {} braces?

All the Best,

Heavy J

  • Will the second loop really execute faster? or does compiler optimization take care of that problem even when the condition and initial value are not constants?
2

Not very strange. Your second example relates to a controlling expression which is always evaluated before each execution of the for loop. See ISO C99 6.8.5.3.

The statement for ( clause-1 ; expression-2 ; expression-3 ) statement
behaves as follows: The expression expression-2 is the controlling expression that is
evaluated before each execution of the loop body. The expression expression-3 is
evaluated as a void expression after each execution of the loop body. If clause-1 is a
declaration, the scope of any variables it declares is the remainder of the declaration and
the entire loop, including the other two expressions; it is reached in the order of execution
before the rst evaluation of the controlling expression. If clause-1 is an expression, it is
evaluated as a void expression before the rst evaluation of the controlling expression.
Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a
nonzero constant.

3

Thank you for the answer...

Is there any difference between these two loops?

for ( X = 0; X < 10; X++ ){
use X;
}

for ( X = 0; X < 10; ++X ){
use X;
}

4

No.. expression-3 is something that is executed after the body of the for statement. Then its back to the expression-2 evaluation.