Why printf() in while() as a condition prints different output

Question

First code

#include<stdio.h>
int main()
{
    while(printf("Hello"))
    return 0;
}

Produces only Hello as a output

Second code

#include<stdio.h>
int main()
{
    while(printf("Hello"));
    return 0;
}

Second code prints Hello for infinite times.

Third code

#include<stdio.h>
int main()
{
    while(printf("Hello"))
    {}
    return 0;
}

Third code also prints Hello for infinite times.

Compiler used - GCC 9.0.1

why this is happening?

Answer 1

while takes a statement after the closing ).

6.8.6 Iteration statements

 iteration-statement:
                while ( expression ) statement

 ....

In

while(printf("Hello"))
    return 0;

that statement (which is basically while's argument) is return 0; (6.8.6)

In

while(printf("Hello"));

the statement is ; (an empty (null)/expression statement (6.8.3)).

In

while(printf("Hello")){}

it's an empty compound statement ({}, 6.8.2), which is semantically equivalent to ;.

Your code snippets are examples of misleading whitespace—where the whitespace makes humans understand things differently from a compiler.

Less misleading renderings would be:

while(printf("Hello"))
    return 0;

,

while(printf("Hello"))
    ; //or perhaps a {} instead of the null statement

and

while(printf("Hello"))
    {}

Answer 2

printf returns number of characters printed (which is 5). Any non zero number evaluates to true. So the loop is an infinite loop.

The rest depends on what happens withing the loop. In the second and third cases, the loops are empty (contain no statements) so they keep executing

In the first case, return 0 is executed within the loop. Return breaks the control flow out of the loop causing the loop (and in this case the program) to stop executing