Imagine your BOE Shield-Bot will enter a contest where you have to make it travel in a circle, but the radius of the circle will only be announced a few minutes before the contest.  In this situation, you’ll have an advantage if your code can calculate the circumference of the circle.  The circumference is 2 × π × r, where r is the circle’s radius and π ≈ 3.14159.  This calculation would be a lot easier to do with floating point math.

Here is a snippet of code that gets the job done.  Notice that it uses PI instead of 3.14159.  PI is a built-in C language constant (a named value that does not change throughout the sketch).  Also notice that all the values have decimal points.  That makes them all floating-point values.

  float r = 0.75;
  float c = 2.0 * PI * r;

Example Sketch - Circumference

• Enter the Circumference sketch into the Arduino editor and save it.
• Make sure to use the values 0.75 and 2.0.  Do not try to use 2 instead of 2.0.
• Upload your sketch to the Arduino and check the results with the Serial Monitor.

// Robotics with the BOE Shield - Circumference

void setup()
{
  Serial.begin(9600);

  float r = 0.75;
  float c = 2.0 * PI * r;
 
  Serial.print("circumference = ");
  Serial.println(c);
}

void loop()
{
  // Empty, no repeating code.
}

Your Turn – Circle Area

The area of a circle is a = π × r2.  Hint: r2 can be expressed as simply r × r.

• Save your sketch as CircumferenceArea.
• Add another float variable to store the result of an area calculation, and code to display it. Run the sketch, and test the answer against a calculator.