Define Constants

In the section on Digital Output (...under Arduino Basics), you will see this code for blinking an external LED that is connected to pin 11.

void setup() {
  pinMode(11, OUTPUT);
}

void loop() {
  digitalWrite(11, HIGH);
  delay(1000);
  digitalWrite(11, LOW);
  delay(1000);
}

This code works just fine, but what if you need to change the wiring so that the LED is connected to pin 12 instead? You would need to modify the highlighted lines, changing all the 11 to 12, like this...

void setup() {
  pinMode(12, OUTPUT);
}

void loop() {
  digitalWrite(12, HIGH);
  delay(1000);
  digitalWrite(12, LOW);
  delay(1000);
}

As our program is rather short, and the 11 only appears 3 times, this modification isn't that hard to do. But what if the program is hundreds of lines long and the number 11 appears dozens of times? It's going to be much more tedious to manually change all the 11, and it's also likely that you may miss out changing one or two of them.

To avoid this problem, programmers often use #define to create a constant, like this...

#define LED_PIN 11

void setup() {
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  digitalWrite(LED_PIN, HIGH);
  delay(1000);
  digitalWrite(LED_PIN, LOW);
  delay(1000);
}

#define LED_PIN 11 : This defines a constant named LED_PIN and give it a value of 11. When the program is compiled, the compiler will search through the entire file and replace every LED_PIN with 11.

Now if you need to change the pin to 12, you just need to modify the first line...

#define LED_PIN 12

A few things to note about constants...

• A constant is not a variable. You cannot change the value of LED_PIN, so code like this... LED_PIN = LED_PIN + 1 ...is not valid.
• Unlike a normal statement, #define do not end with a semi-colon.
• There are no = sign between LED_PIN and 11.
• The names used for constants are usually written in all uppercase. This is not a requirement, but it is a common practice.