Need to find prime numbers between two numbers

[kevin guo95]

The program works, but it includes the two number I input. I have to find a way to stop the program from outputing the input numbers.

#include <iostream>
using namespace std;

int main(){
    int one;
    int two;
    
    cout << "Please enter two different numbers and I will tell you what numbers between are not prime numbers." << endl << endl;
    cout << "Enter your first number" << endl;
    cin >> one;
    cout << "Enter your second number" << endl;;
    cin >> two;
    
    while(one <= two){
        if(one%2 == 0){
            cout << one << " is not a prime number." << endl;
        }
        else if((one%3 == 0)&&(one!=3)){
            cout << one << " is not a prime number." << endl;
        }
        else if((one%5 == 0)&&(one!=5)){
            cout << one << " is not a prime number." << endl;
        }
        else if((one%7 == 0)&&(one!=7)){
            cout << one << " is not a prime number." << endl;
        }
        one++;
    }
    system("pause");
}

[newbiee999]

It's very simple.
replace lines 13 and line 14 with the following code:

one++;    // [line 13] (before while loop condition) You start by checking the number after the 'one'
while(one <= two-1) // [line 14] and you check until the number before 'two'  

This way, you don't include 'one' and 'two'

Hope this helps!

[kevin guo95]

thank you so much

[TheIdeasMan]

Hi Kevin,

Welcome to cplusplus :+)

I just wanted to mention about your algorithm, it is only going to work for numbers up to 49 (7²), which is rather limited, and your code is not scalable. If one has a list of all the numbers, then removes multiples of all the numbers up to N, then the numbers up to N² will be prime.

So rather than repeating your code for each number, try to think of a way of doing it using loops.

On the wiki page for prime numbers, at the end there is the Euler's method. Have a go at implementing that.

Good Luck! :+)

[Momothegreat]

First you need to figure out which of the 2 numbers entered is the smallest.
Then start a loop for the smallest integer till the highest integer.
Second you need to remember what the conditions for a prime is.
A prime number is a number that has only two factors: 1 and itself.
You need to setup a for loop that checks for factors for each number in the range entered.

Also you should check if the numbers are the same and only display prime for that number.

#include <iostream>
using namespace std;

int main(){
    int one;
    int two;
int small;
int large;
int factorcount;
    
    cout << "Please enter two different numbers and I will tell you what numbers between are not prime numbers." << endl << endl;
    cout << "Enter your first number" << endl;
    cin >> one;
    cout << "Enter your second number" << endl;;
    cin >> two;

while(cin)
{
if (one == two)
{
if (one == 1 || one == 2 || one <= 0) // Integers must be:postive,non-zero,not "1" or "2."
        cout << number << " is not a prime." << endl << endl;
      else 
        {
          for (int i=1; i<=one; i++) // Determines factors
            if (one% i == 0)
              factorcount++;
          if ( factorcount == 2) // If factor are 2 then display prime
            cout << number << " is a prime." << endl << endl;
          else
            cout << number << " is not a prime." << endl << endl;
          factorcount = 0;
        }
}

if( one < two)
{
one = small;
two = large;
}
else 
{
two = small;
one = large;
} // this if, else loop will set the smallest integer as "small" and the larger integer as "large"

for (int i=small; i < large; i++)
{
if (i== 1 || i== 2 || i<= 0) // Integers must be:postive,non-zero,not "1" or "2."
        cout << i<< " is not a prime." << endl << endl;
      else 
        {
          for (int j=1; j<=i; j++) // Determines factors
            if (i% j == 0)
              factorcount++;
          if ( factorcount == 2) // If factor are 2 then display prime
            cout << number << " is a prime." << endl << endl;
          else
            cout << number << " is not a prime." << endl << endl;
          factorcount = 0;
        }

    }
    system("pause");
}

I didn't check brackets or anything; not sure if this will even compile.