1

I can't understand why the following code outputs 10. What I understand is that !printf("0") means !0, which is TRUE. So why doesn't the code print "Sachin"

#include <stdio.h>

int main() {
    for (printf("1"); !printf("0"); printf("2"))
        printf("Sachin");
    return 0;
}

Output

10
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4
  • 2
    Perhaps you need a printf reference to see what it returns? Commented Sep 11, 2018 at 7:53
  • 2
    printf returns the number of characters printed. It can print 1, 10 or nothing at all, depending on after which character it does fail. Commented Sep 11, 2018 at 7:54
  • 1
    !printf("0") returns false. printf itself returns the number of chars printed if successful. check any c reference for details. Commented Sep 11, 2018 at 7:58
  • !printf("0") means !0... No. Given the result of printf it means !1 which is FALSE Commented Sep 11, 2018 at 8:37

3 Answers 3

Reset to default
8

let's analyze this side-effect loop statement:

for(printf("1"); !printf("0"); printf("2"))
  • The first statement is executed, always (init condition), yieiding 1
  • Then the condition is tested: !printf("0") prints 0, then since printf returns 1 because it just prints 1 character, the negation returns 0 and the loop is never entered because the condition is false right from the start. So neither 2 or Sachin are printed.

Of course, this code isn't practical, almost unreadable. So don't ever do things like this (puts("10"); is a good alternative for instance).

more on the return value of printf (that is often ignored):

Upon successful return, these functions return the number of characters printed (excluding the null byte used to end output to strings).

(from https://linux.die.net/man/3/printf)

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4 Comments

Meaning no offence sir, this code may not be practical but to me it's experimental which is important to understand the things better. However, I thank you for your assistance. 😊
no offence taken. I added that bit in my answer so everyone understands that if you want to print 10 you just do printf("10");
And only write puts("10"); if you also require a newline. ;-)
aaah right I wanted to avoid printf when no formatting was needed :)
3

If you look at the man printf reference on google, you'll see that this function returns the number of written bytes.

Here your condition is !printf("0"), in other words : "as long as the return of printf is not existing (or equal 0), do something. But you print the character '0' so printf actually return 1 so your condition is false.

Now WHY it prints 10 :

  • The first printf("1") prints 1.
  • Your condition is tested at least once, so the second printf("0") occurs one time (it prints 0)
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2 Comments

good first answer. Small piece of advice: don't tell people to "look on google". Google for them and include the link. It's better.
Thanks for the assistance.
0 
printf("1")

prints 1 and it return number of characters which is 1

printf("0")

prints 0 and it return number of characters which is 1

!1 means !(true) = false so execution will stop and you will see 10 as output.

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1 Comment

Understood. Thanks for the assistance.

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