How Sketches Work

A sketch is C++ with training wheels. You don't need to learn C++ first — you need about ten ideas, and you'll pick up the rest from examples.

The non-negotiables

void setup() {
  // runs once
}

void loop() {
  // runs forever
}

Every sketch has these two functions. Miss one and it won't compile. // starts a comment — ignored by the compiler, read by humans. Comment generously; future-you won't remember why.

Statements end with ; — every single one. Forgetting it is the #1 compile error, and the error message points at the next line, which confuses everyone exactly once:

digitalWrite(LED_BUILTIN, HIGH)   // missing ; — error reported on the NEXT line
delay(1000);

Variables

int blinkCount = 0;          // whole numbers
float voltage = 3.3;         // decimals
bool ledOn = true;           // true/false
char letter = 'A';           // single character (single quotes)
String message = "hello";    // text (double quotes, capital S)

On the Uno, an int holds -32,768 to 32,767. Count past that and it wraps around — a classic bug source. Need bigger? long (-2 billion to 2 billion). With 2 KB of RAM, you think about this more than on a PC — but for beginner projects, int is almost always right.

Careful: String (capital S) is convenient but fragments the Uno's tiny memory over time, eventually crashing long-running programs. Fine for learning; for devices that run months, C-style strings (char msg[] = "hello";) are the grown-up choice.

if, for, while — the usual suspects

if (buttonPressed) {
  digitalWrite(LED_BUILTIN, HIGH);
} else {
  digitalWrite(LED_BUILTIN, LOW);
}

for (int i = 0; i < 10; i++) {
  // do something 10 times
}

If you've seen any programming language, this is familiar. If not: if chooses, for repeats a counted number of times, while repeats until something changes. Curly braces {} group statements together.

delay() and its problem

delay() freezes everything — no button reads, no sensor checks — until the time passes. Fine for blinking; terrible for responsive projects. The grown-up pattern is millis() (milliseconds since power-on) — "is it time yet?" instead of "wait here":

unsigned long previousMillis = 0;
const long interval = 1000;

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

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));  // toggle
  }
  // other code runs freely here — nothing is blocked
}

This is the famous "Blink Without Delay" example (File → Examples → 02.Digital → BlinkWithoutDelay). Learn it when delay() starts annoying you — roughly your third project.

Next: Digital I/O