primenumbers.cpp

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// Exercise 20.12: PrimeNumbers.cpp
// Determines all prime numbers between a user-specified range.
#include <iostream> // required to perform C++ stream I/O
#include <cmath>    // required to use the sqrt function
#include <string>   // required to access string functions
#include <exception> // required to use exceptions

using namespace std; // for accessing C++ Standard Library members

// function prototype
bool isPrime( int );

// function main begins program execution
int main()
{
   // define variables
   int lowerBound; // stores lower bound
   int upperBound; // stores upper bound
   int counter;    // stores counter value

   // prompt user for and input lower bound
   cout << "\nEnter lower bound: ";
   cin >> lowerBound;

   // prompt user for and input upper bound
   cout << "Enter upper bound: ";
   cin >> upperBound;

   // validate the input values
   if ( ( lowerBound <= 0 ) || ( upperBound <= 0 ) )
   {
      cout << "\nBounds must be greater than 0\n" << endl;

      return 1; // indicate that program ended erroneously
   } // end if
   else if ( upperBound < lowerBound )
   {
      cout << "\nUpper bound cannot be less than lower bound\n" << endl;

      return 1; // indicate that program ended erroneously
   } // end else if
   else
   {
	  cout << endl;
	  
	  // loop from lower bound to upper bound
	  for ( counter = lowerBound; counter <= upperBound; counter++ )
      {
         // if prime number, display the value
         if ( isPrime( counter ) )
         {
            cout << counter << endl;
         } // end if

      } // end for

   } // end else
   
   cout << endl; // insert newline for readability
   return 0; // indicate that program ended successfully

} // end function main

// determine whether the number is prime
bool isPrime( int testValue )
{
   // convert int to double
   double number = testValue;

   // 1 is not a valid prime number
   if ( number == 1 )
   {
      return false;
   } // end if

   // set the square root of number as limit
   double limit = sqrt( number );

   // loop until count reaches the square root of number
   for ( int count = 2; count <= limit; count++ )
   {
      // determine if count is a factor of testValue
      if ( ( testValue % count ) == 0 )
      {
         return false; // number is not prime
      } // end if

   } // end for

   return true; // number is prime

} // end function isPrime


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