How to make a program to determine if an integer is prime or not?

Question

Hello everybody,
I have to make a program to determine if a supplied integer is prime or not. At the same time it has to be within the range of 1 and 999, otherwise it will ask the user to enter a number within that range. It should also not be considered in case 1 is entered as a prime number.
I have tried to do it but it always gives me an error that I do not understand where it comes from. Could you explain to me where I fail,
The program I did so far:
#include <stdio.h>
int main () {
int num, cont = 0, div = 1;
printf ("enter a number");
scanf ("% d", &num);
while (num> = 1 && num <= 999) {
if (num% div == 0) {
cont ++;
}
div ++;
}
if (cont == 2) {
printf ("The number is prime");
}
return 0;
}

Answer 1

Brian Ayvar Here's an efficient prime generator (especially when dealing with large numbers):
int is_prime(int a) {
if (a == 2) return 1;
if (a < 2 || !(a & 1)) return 0;
int l = a / 3;
for (int i=3;i<=l;i+=2) {
if (!(a % i)) return 0;
}
return 1;
}
// Hope this helps

Answer 2

You need to be really careful about spaces .
Compare the following two lines with yours.
scanf ("%d", &num);
while (num >= 1 && num <= 999) {
There should be no space between % and d here, otherwise it can change the whole meaning of what type of data compiler expects .
It should be >= not > = . Again no spaces .

Answer 3

hey Brian Ayvar
take loop from 2 (say counter) to less than number , see that counter completely divides that number or not , if it divides that mean that's not prime , exit !
>>>you can make efficient by taking loop from 2 to " half of number " !