Prime number checker

This is my first c program I have done totally by myself and prime point
of this program is to check if number(which user enter) is prime number or not.
I have done this kind of program before with Visual Basic but it's so slow language
so I decided to make one with c. Problem I have is that it always says that it's not
prime number. Here's the source:
************************CODE*******************
#include <iostream.h>
void main()
{
int a, i, o;
o=0;
cout << "Give number\n";
cin >> a;
for (i = 1; i<a; i++)
{
if ((a % i) == 0)
{
o=1;
break;
}
}
if (o==1)
cout << "Not prime number\n";
else
cout << "prime number\n";
}

********************END OF CODE***************

I can't figure out the reason, please help me.

This is happening because the loop goes from 1 to a. In the first iteration with i = 1 the if statement will read if (their_number % 1) == 0) which will always be true regardless of the value of their_number.

To avoid this problem run it from 2 instead of 1.

-KM-

Now it works thank you!