for loop code

[payam.a]

Hi dear friends;
I have a question about this code. please guide me.
what will happen if we don't write the first expression of the for loop.If we can write another things instead, what should we write?
please help me about this code.
thanks
payam

/* for_show.c */

#include <stdio.h>

int main(void)

{

    int num = 0;



    for (printf("Keep entering numbers!\n"); num != 6;  )

        scanf("%d", &num);

    printf("That's the one I want!\n");

    return 0;

}

this is it's output.





Keep entering numbers!

3

5

8

6

That's the one I want!

[YAPP]

You could rewrite the for loop using a while loop easily.

while(num!=6)
{
    printf("Keep entering numbers!\n");
    scanf("%d",&num);
}
printf("That's the one I want!");

[payam.a]

thanks dear friend;
now I am reading the features of for loop. as I wrote in my first post I want to know what can I write in the first expression of a for loop instead intializing a variable.
thanks.
payam

[Xav]

You mean initialising the variable within the for loop declaration, instead of having to create a separate variable?

[Aereshaa]

I think he means:
'In the expression for(A,B,C){D;}, what are the legal values of A, B, C, and D?'
The answer is that they can be any legal C statements. For example, for a countdown:

int i = 11;
for(printf("Countdown to liftoff:\n"); (i = i - 1) != 0; printf("%d\n", i))sleep(1);

for(A,B,C){D;} is functionally equivalent to

A;
start_of_loop:
if (!B) goto end_of_loop;
D;
C;
goto start_of_loop;
end_of_loop:

So you can put whatever you want!

[payam.a]

thanks dear friends;
as you know the first expression of a for loop shoyld be initializing a variable.
but in the code that I wrote, there is another thing instead of intializing I have used a printf function. I mean, is it correct to do these jobs? if it is correct whatelse can we write instead of the first expression in a for loop?
thanks
payam

[Aereshaa]

Whatever you want.

[dargueta]

You can also put multiple expressions or no expressions at all. For example:

//No initializing
    int i = SOMEVALUE;
    ...code...
    for(; i < 5; ++i){ ... }
//Multiple statements
    int i , j;
    for(i = 0, j = 2; (i < 6)&&(j < 10); ++i, j +=5) { ... }

[payam.a]

thanks dear friends for your excellent answers specially you dear Aresheaa;