Arduino Sketch Structure and Libraries

A tidy program layout.

A sketch has setup(), which runs once, and loop(), which repeats forever. Put pin modes and library start-up in setup, and the repeating logic in loop.

A library bundles ready-made code for a sensor or display. Include it at the top, create an object, and call its functions. Read each library's documentation for its pin and address assumptions.

Avoid long delay() calls; use a millis() timestamp so the loop stays responsive.

unsigned long last = 0;
void setup() { pinMode(13, OUTPUT); }
void loop() {
  if (millis() - last >= 1000) {
    last = millis();
    digitalWrite(13, !digitalRead(13));
  }
}