program to find prime number after a given number

Program to find a prime number after a given number:
I used the theory that prime numbers are divisible by only 2 numbers 1 and itself.

#include <stdio.h>
#include <conio.h>
int main(int argc, char const *argv[])
{
   int num,a=0,i,j;
   printf("Enter a number: ");
   scanf("%d",&num);
   for(i=num+1;i>0;i++){
       for(j=1;j<=i;j++){
           if(i%j==0){
              a=a+1;
           }
       }
       if(a==2){
           printf("%d",i);
           break;
       }
   }
   return 0;
}

This code is working only for a single increment like if user gives the input as 4 it is returning the output as 5. But it wont return output if input is given as 7 (it should return 11)

Then I modified the code as below :

#include <stdio.h>
#include <conio.h>
void main(int argc, char const *argv[])
{
   int num,a=0,i,j;
   printf("Enter a number: ");
   scanf("%d",&num);
   for(i=num+1;i>0;i++){
        a=0;
        for(j=1;j<=i;j++){
            if(i%j==0){
               a=a+1;
            }
        }
        if(a==2){
            printf("%d",i);
            break;
        }
    }
}

This one runs well.
I am confused why the program gives the output if i declared the variable after the for loop but fails to give it if declared before?

>Solution :

Notice the a variable is not changed through the bigger for loop.

Meaning, after the first iteration we’ll have a>=2 (since every number has at least 2 divisors, while primes have precisely 2), and at the next iteration, this value does not reset.

You can try and print the value of a in every iteration from the first code snippet to make sure 🙂