text: //source video: https://www.youtube.com/watch?v=F5feTW3CAZs&index=4&list=PLhQjrBD2T381wyZt81eGNZuZ4rzOos-AF
  
  #include <stdio.h>
  #include <cs50.h>
  
  // If we initialize pi out here, at the global scope, it will be accessible everywhere.
  float pi = 3.141592654;
  // BUT! this is generally a bad idea. What if some piece of code somewhere changes the value pi?
  
  float area(float radius);
  float circumference(float diameter);
  
  int main (void)
  {
    // Get the size of the pizza from the user
    printf("What's the diameter of the pizza? ");
    float pizza_diameter = GetFloat();
    
    // Calculate circumference and area of the pizza
    float pizza_circumference = circumference(pizza_diameter);
    float pizza_area = area( pizza_diameter / 2.0 );
    
    // Tell the user all about their pizza
    printf("The pizza is %f inches around.\n", pizza_circumference);
    printf("The pizza has %f square inches.\n", pizza_area);
  }
  
  /*
   * returns the area of a circle with a given radius
   */
  float area(float radius)
  {
    // now we can just use pi here, because we declared it globally (see top of file)
    return pi * (radius * radius); // pi r squared
  }
  
  /*
   * returns the circumference of a circle with a given diameter
   */
  float circumference(float diameter)
  {
    return pi * diameter;
  }
  