Arduino C++ basics

Learn

What you’ll learn

• The shape of every Arduino program (a sketch)
• How to switch pins on and off and read them
• Variables, if statements and printing messages with Serial

Every sketch has two parts

void setup() {
  // Runs ONCE when the board powers on or resets
}

void loop() {
  // Runs AGAIN and AGAIN, forever
}

This is the sense, think, act loop in code: loop() is where your robot senses, decides and acts, thousands of times per second.

Blink, line by line

const int LED = 13;          // a name for pin 13 (the built-in LED)

void setup() {
  pinMode(LED, OUTPUT);      // pin 13 will SEND signals out
}

void loop() {
  digitalWrite(LED, HIGH);   // turn the pin ON (5 V)
  delay(1000);               // wait 1000 milliseconds = 1 second
  digitalWrite(LED, LOW);    // turn the pin OFF (0 V)
  delay(1000);
}

Code	Meaning
const int LED = 13;	Make a named number. const means it never changes
pinMode(pin, OUTPUT)	Set a pin to send signals (LEDs, buzzers)
pinMode(pin, INPUT_PULLUP)	Set a pin to read a button (more below)
digitalWrite(pin, HIGH/LOW)	Turn an output pin on or off
digitalRead(pin)	Read an input pin: gives HIGH or LOW
delay(ms)	Pause for that many milliseconds

Grammar rules (syntax)

C++ is fussy. Most beginner errors are one of these:

• Every statement ends with a semicolon ;
• Code blocks live inside curly braces { }, and every { needs a matching }
• Names are case-sensitive: digitalWrite works, DigitalWrite doesn’t
• // starts a comment, a note for humans that the board ignores

Tip

When the IDE shows an error, look at the line it points to and the line above. A missing ; is usually reported on the next line.

Variables

A variable is a named box that holds a value that can change.

int count = 0;          // whole numbers
float distance = 12.5;  // numbers with decimals
bool pressed = false;   // true or false

Making decisions with if

if (pressed) {
  digitalWrite(LED, HIGH);
} else {
  digitalWrite(LED, LOW);
}

Compare with == (equal), != (not equal), <, >, <=, >=. Remember: = stores a value, == compares two values.

Talking to your computer: Serial

Serial lets the board send text to the IDE’s Serial Monitor (magnifying-glass icon, top right). It’s your number-one debugging tool.

int count = 0;

void setup() {
  Serial.begin(9600);          // start talking at 9600 bits per second
}

void loop() {
  Serial.print("Count: ");
  Serial.println(count);       // println adds a new line
  count = count + 1;
  delay(500);
}

Set the Serial Monitor’s speed to 9600 baud to match, or you’ll see gibberish.

Examples

Example: a countdown on the Serial Monitor

void setup() {
  Serial.begin(9600);
  for (int i = 5; i > 0; i--) {
    Serial.println(i);
    delay(1000);
  }
  Serial.println("Lift off!");
}

void loop() {}

Example: counting button presses

const int BUTTON = 2;
int presses = 0;
bool wasPressed = false;

void setup() {
  pinMode(BUTTON, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  bool pressed = digitalRead(BUTTON) == LOW;
  if (pressed && !wasPressed) {
    presses = presses + 1;
    Serial.print("Presses: ");
    Serial.println(presses);
    delay(50); // ignore button bounce
  }
  wasPressed = pressed;
}

Practice

Try it

Use the virtual Arduino below (or your real board). Paste the Serial example into sketch.ino and press ▶. The Serial Monitor opens under the board automatically.

Virtual Arduino UNOOpen full screen ↗

Start simulator

Runs on Wokwi, free, no account needed.

1. Run the Serial example. Watch the count go up.
2. Change delay(500) to delay(100). What happens?
3. Add an if so it prints "Big number!" whenever count > 20.

Faster countingEasy

Make the Serial example count twice as fast. Change delay(500) to delay(250).

Count to ten and stopMedium

Print 1 to 10, then stop printing. Put the counting in setup(), or check count with an if in loop().

Show an answer

void setup() {
  Serial.begin(9600);
  for (int i = 1; i <= 10; i++) Serial.println(i);
}
void loop() {}

Even or oddHard

Print each number from 1 to 20 followed by “even” or “odd”. n % 2 gives the remainder after dividing by 2.

Show an answer

void setup() {
  Serial.begin(9600);
  for (int n = 1; n <= 20; n++) {
    Serial.print(n);
    if (n % 2 == 0) Serial.println(" even");
    else Serial.println(" odd");
  }
}
void loop() {}

Quick check

1. Which function runs only once, at start-up?

loop()setup()delay()main()

2. What does delay(250) do?

Waits 250 secondsWaits a quarter of a secondRepeats 250 timesTurns pin 250 on

3. Which line correctly checks whether count equals 10?

if (count = 10)if (count == 10)if count is 10if (count === 10);

4. Your code fails with an error on line 8, but line 8 looks fine. What should you check first?

Line 7 for a missing semicolonThe USB cableThe LED colourYour internet

Check my answers