Program 1:

#include<stdio.h>
void main(){
    if(printf("Hello world")){
    }
}

Result

Hello world

Program 2:

#include<stdio.h>
void main(){
    while(!printf("Hello world")){
    }
}

Result

Hello world

Program 3:

#include<stdio.h>
void main(){
    switch(printf("Hello world")){
    }
}

Result

Hello world