Question:C program to print hello world without using semicolon 

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

Question:C program to print hello world using while loop 

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

Question:C program to print hello world using switch 

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

Question:How do you write a program which produces its own source code as its output in c language? 

#include<stdio.h>

int main(){
    FILE *fp;
    char c;

    fp = fopen(__FILE__,"r");
 
    do{
         c= getc(fp);
         putchar(c);
    }
    while(c!=EOF);

    fclose(fp);
   
    return 0;
}

Question:Write a c program to reverse a given number 

#include<stdio.h>
int main(){
    int num,r,reverse=0;

    printf("Enter any number: ");
    scanf("%d",&num);

    while(num){
         r=num%10;
         reverse=reverse*10+r;
         num=num/10;
    }

    printf("Reversed of number: %d",reverse);
    return 0;
}
Sample output:
Enter any number: 12
Reversed of number: 21

Question:Reverse very large or big numbers beyond the range of long int 

#include<stdio.h>
#define MAX 1000

int main(){

    char num[MAX];
    int i=0,j,flag=0;

    printf("Enter any positive integer: ");
    scanf("%s",num);

    while(num[i]){
         if(num[i] < 48 || num[i] > 57){
             printf("Invalid integer number");
             return 0;
         }
         i++;
    }

    printf("Reverse: ");
    for(j=i-1;j>=0;j--)
         if(flag==0 &&  num[j] ==48){
         }
         else{
             printf("%c",num[j]);
             flag =1;
         }

    return 0;
Sample output:
Enter any positive integer: 234561000045645679001237800000000000
Reverse: 8732100976546540000165432

Question:C program to reverse a number using for loop 

#include<stdio.h>
int main(){
    int num,r,reverse=0;

    printf("Enter any number: ");
    scanf("%d",&num);

    for(;num!=0;num=num/10){
         r=num%10;
         reverse=reverse*10+r;
    }

    printf("Reversed of number: %d",reverse);
    return 0;
}
Sample output:
Enter any number: 123
Reversed of number: 321

Question:C program to reverse a number using recursion 

#include<stdio.h>
int main(){
    int num,reverse;

    printf("Enter any number: ");
    scanf("%d",&num);

    reverse=rev(num);
    printf("Reverse of number: %d",reverse);
    return 0;
}

int rev(int num){
    static sum,r;

    if(num){
         r=num%10;
         sum=sum*10+r;
         rev(num/10);
    }
    else
         return 0;

    return sum;
}
Sample output:
Enter any number: 456
Reverse of number: 654

Question:C program to calculate sum of digits 

#include<stdio.h>
int main(){
  int num,sum=0,r;
  printf("Enter a number: ");
  scanf("%d",&num);
  while(num){
      r=num%10;
      num=num/10;
      sum=sum+r;
  }
  printf("Sum of digits of number:  %d",sum);
  return 0;
}
Sample output:
Enter a number: 123
Sum of digits of number:  6

Question:Sum of digits of a number in c using for loop 

#include<stdio.h>
int main(){
  int num,sum=0,r;
  printf("Enter a number: ");
  scanf("%d",&num);

  for(;num!=0;num=num/10){
      r=num%10;
      sum=sum+r;
  }
  printf("Sum of digits of number:  %d",sum);
  return 0;
}
Sample output:
Enter a number: 567
Sum of digits of number:  18