why if we assign a variable 0 it does not work properly in if else statement

when i assign rat other than 0 its work properly if i assign rat=0 it's not working properly if rat is other than 0 my program work properly and give me proper prime number

#include<stdio.h>
int main()
{
    printf("\nenter a number");
    int num,rat;
    scanf("%d",&num);
    for(int i=2;i<num;i++)
    {
        if(num%i==0)
        {
            rat=0;
            break;
        }
        
    }
    if(rat==0)
    {
    printf("\nit is not a prime number");
    }
    else
    {
        printf("\nit is a prime number");
    }
  
}