Confused with "for loop" output

Question

I have a very basic C program, and confused with its output:

#include<stdio.h>

int main() {
        int i;
        char s[] = "K";

        for(i=0; s[i]; i++); {
                printf("%d ", i);
                printf("%c ", s[i]);
        }
}

It outputs i value as 1, but as per one of the answer from this thread: Difference between pre-increment and post-increment in a loop?

    int i = 0; // Initialization

    loopStart:
    if (i < 5) // Condition
    {
       Output(i);

       i++ or ++i; // Increment

       goto loopStart;
     }

Increment happens after the Output(i) (equivalent to printf("%d ", i);), Then how i's value comes as 1 but not 0?

I think, I overlooked the semicolon but it actually lead me to see this thread. https://stackoverflow.com/questions/13421395/effect-of-semicolon-after-for-loop, to get clear idea, what is going on. — Milan, Sep 12 '21 at 19:16

score 4 · Accepted Answer · edited Sep 12 '21 at 19:04

4

It's because of ; after for loop

for(i=0; s[i]; i++);

Remove semicolon and it's fine.

for(i=0; s[i]; i++)

It's because the semicolon will be considered as an end of the statement and your printf is not part of the loop and will be executed once the loop execution gets completed.

edited Sep 12 '21 at 19:04

Weather Vane

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answered Sep 12 '21 at 19:00

Kaushal panchal

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Andreas Wenzel · Answer 2 · 2021-09-12T19:27:34.600

Your code block

for(i=0; s[i]; i++); {
    printf("%d ", i);
    printf("%c ", s[i]);
}

has misleading indentation. With correct indentation, it would look like this:

for(i=0; s[i]; i++)
    ;

{
    printf("%d ", i);
    printf("%c ", s[i]);
}

As you can see, the for loop is empty, and the code block with the printf statements is not part of the for statement (which means that it is not part of the loop). What you want is probably the following:

for(i=0; s[i]; i++) {
    printf("%d ", i);
    printf("%c ", s[i]);
}

Note that this code is identical to your code, except that a semicolon was removed.

score 0 · Answer 3 · answered Sep 12 '21 at 19:21

When you add semicolon at the end of for loop then execution will terminated after checking the condition in for loop. If you want the output : 0 And : K

Then try this

int main()
{
     int i = 0;
     char s[] = 'K';
     for(i = 0; s[]; i++)
     {
          printf("%d", i);
          printf("%c",s[i]);
     }
     return 0;
}

Confused with "for loop" output

3 Answers3