declaring

Question

if(x <= 1977571 && x != 0){
for(int i=2;i<=x;i++){
if(x%i==0){
cout<<x<<" is not prime.";
}
}
if( i == x){
cout<<x<<" is prime!";
}
}

Answer 1

you can use following data type for large numbers:
long int x;
long long int x;
and if number is only expected to be positive :
unsigned long int x;
unsigned long long int x;
Hope that helps.Thanks.
Suggestion:
you can make your loop efficient by checking if the given number is even or not(divisible by 2). If not then run a loop which jumps by two by all odd numbers which makes your code twice efficient then this one.
Hope that helps.Thanks.