Lesson 20 Source Code

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Lesson 20 Video

Lesson 20: The while loop part 3: break statement

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A prime number algorithm


Here is an infinite loop that we have encountered in the past. This is a bug that can be very difficult to find.
#include <iostream>
using namespace std;

int main()
{
 int count=0;
 while(true)
 {
  cout << count << endl;
  count++;
 }
 system("pause");
 return 0;
}


We can add a condition in the while loop parameter that changes each time by using a counting variable.
#include <iostream>
using namespace std;

int main()
{
 int count=0;
 while(count < 10)
 {
  cout << count << endl;
  count++;
 }
 system("pause");
 return 0;
}



Prime number algorithm


In this program, you will enter a positive Integer value for a, and it will tell you if that number is prime or not
#include <iostream>
using namespace std;

int main()
{
 int a, n=2;

 cout << "Enter a number for a: ";
 cin >> a;
 while(n<=a)
 {
  if(n==a)
  {
   cout << a << " is Prime!\n";
   break;
  }
  if(a%n==0)
  {
   cout << a << " is Divisible by" << n << endl;
   break;
  }
  n=n+1;
 }
 system("pause");
 return 0;
}


Here is the algorithm that prints off many prime numbers to the screen. Just enter an Integer value for x and it will print off that 

many random numbers.
#include <iostream>
using namespace std;

int main()
{
 int a=2, n=2, x;

 cout << "Enter a number for x: ";
 cin >> x;
 while(a<=x)
 {
  while(n<=a)
  {
   if(n==a)
   {
    cout << a << " is Prime!\n";
    break;
   }
   if(a%n==0)
   {
    cout << a << " is Divisible by" << n << endl;
    break;
   }
   n=n+1;
  }
  a++;
  n=2;
 }
 system("pause");
 return 0;
}

from 2 to x is the range of values that will be printed. a is the variable we are testing to see if its prime or not. n is the divisor 

that determines if it really is prime or not.

Notice that when you escape from the second while loop, n has to be set back equal to 2 because in the we need to start off dividing the 

next number by 2 just as we have been doing for the first number.