Prime Numbers Help

[DELB]

Hi,

#include <iostream>
// note the use of function prototypes

bool isDivisible (int number, int divisor);
bool isPrime (int number);

using namespace std;

int main ()
{
    for ( int i = 0; i < 100; i++ )
    {
        if ( isPrime( i ) )
        {
            cout << i << endl;
        }
    }
}

bool isPrime (int number)
{
for ( int i = 2; i < number; i++)
{
    if ( isDivisible( number, i ) )
     {
        return false;
     }
}
    return true;
}

bool isDivisible (int number, int divisor)
{
    return number % divisor == 0;
}

I'm not sure why the author is doing a for loop from i = 2 to i < number and I'm not sure what the isDivisible function does exactly.

Could someone explain it clearer? Thanks.

[keskiverto]

What is the definition of a prime number? Yes, math proves that one does not have to test them all, but that loop is the standard naive solution.


What you can tell about the isDivisible?

[SamuelAdams]

The loop in another view would look like this

while i < 100;
do something
i=i+1;

If your asking why i=2, well we don't need to check if 0 or 1 is prime....
0 cant' be divided and 1 can only be divided by itself.

[easycppcoder]

see this simple program to find prime numbers: http://www.easycppcodes.com/2015/01/c-program-to-find-weather-given-number.html