Finding Prime Numbers -Help Needed-

[bradltr95]

Hey guys, I'm trying to write a program that loops through numbers 1 to 100 and adds all the prime numbers to a vector list of doubles. The problem Im having is that it gets some prime numbers right but doesn't always add the proper ones. When I output the vector list at the end of the program I get a bunch of numbers that shouldn't be there

[b][OUTPUT][\b]
3
5
7
9
11
13
15
17
19

Heres the code for the program, what Im doing here is testing all values that are divided by 2 (a prime number) to see if they leave any remainder. Im not sure Im understanding this correctly but I though this would work. Thanks guys.

 
// Bradley Latreille 
// Chapter 4 Exercise 11
// 
/* 
    Fine all prime numbers between 1 and 100. 
*/ 

#include "std_lib_facilities.h"	

vector<double> prime_numbers; // create a vector to store our prime_numbers

bool is_prime(int val) {
    if(val%2 == 0) { 
        return false;
    }else{
        return true; 
    }
}
int main()
try
{
    // Loop through numbers 1 to 100 
    for(int i = 1; i < 100; i++) {
        // check if the number is prime 
        if(is_prime(i)) {
            //cout << i << " is a prime number! Adding to primes list!\n"; 
            prime_numbers.push_back(i); // add the number to our prime_numbers vector 
        }else{
            //cout << i << " is not a prime number! Skipping number!\n"; 
        }
    }
    
    for(int i = 1; i < 100; i++) {
        cout << prime_numbers[i] << endl; 
    }
    
}
catch (runtime_error e) {	// this code is to produce error messages; it will be described in Chapter 5
	cout << e.what() << '\n';
	keep_window_open("~");	// For some Windows(tm) setups
}
catch (...) {	// this code is to produce error messages; it will be described in Chapter 5
	cout << "exiting\n";
	keep_window_open("~");	// For some Windows(tm) setups
}

[Repeater]

what Im doing here is testing all values that are divided by 2 (a prime number)

That's not what a prime number is. A prime number is a number exactly divisible by itself and one. You seem to be testing for odd numbers.

[bradltr95]

Okay sounds good :) It actually turned out the exercise was much simpler. He wanted me to test a loop of numbers 1 - 100 against a vector of already prime numbers, and store the found primes in a new vector. In later exercises he asks to do it without a vector of primes offers the "Sieve of Eratosthenes" approach to solving the problem :) Thanks for your help though!