Writing Your Own void Function

By now some of your sketches have a loop() forty lines long, with the same five or six lines appearing twice. A function is how you give a piece of code a name and stop repeating it.

void beep() {
  tone(8, 880);
  delay(100);
  noTone(8);
  delay(100);
}

Written once. Now beep(); anywhere in the sketch runs those four lines. Three beeps is three words.

void means gives nothing back — it does a job, it does not answer a question. That is the next lesson's subject.

A function can take parameters — values handed to it when it is called:

void beep(int pitch, int length) { ... }

beep(880, 100);
beep(440, 300);

One function, any number of different beeps.

The real gain is not fewer lines. It is that beep() says what it does. A reader who trusts the name never has to look inside, and a loop() made of well-named calls can be read like a sentence.

No wiring. Read everything in the Serial Monitor at 9600 baud.

→ Functions — the same idea with an LED.

Writing one

void sayHello() {
  Serial.println("Hello");
}

void setup() {
  Serial.begin(9600);
  sayHello();
  sayHello();
}

void loop() {
}

• void — hands nothing back.
• sayHello — the name. Same rules as a variable name.
• () — no parameters this time.
• { } — the body.

Calling it needs the brackets: sayHello();. Without them, sayHello; compiles and does nothing at all.

You have been writing functions all along. setup() and loop() are both void functions — the Arduino calls them for you.

Parameters

void countTo(int limit) {
  for (int i = 1; i <= limit; i++) {
    Serial.println(i);
  }
}

countTo(5);
countTo(10);

limit is a variable that exists only inside countTo, and it takes whatever value was passed in.

More than one, separated by commas — each with its own type:

void printLabelled(char tag, int value) {
  Serial.print(tag);
  .();
  .(value);
}

printLabelled(, );

Refactor the dice game.

Go back to the ARD-PG-09 dice game — roll, categorise, score, print.

Break it into functions:

• int rollDice() — you cannot write this one yet, it returns a value. Use a global for now.
• void describeRoll(int roll) — prints the message for a roll
• void updateScore(int roll) — changes the running total
• void printScore() — prints the total

Your loop() should end up about four lines long and read almost like English.

Think about it: the sketch is now longer in total lines than it was. What did you gain? And which of the four functions could you drop into a completely different sketch tomorrow without changing a character?