Arduino code basics — setup(), loop(), variables, if and for explained

#The short answer

An Arduino program is called a sketch. Every sketch has two parts:

• setup() runs once, when the board powers up or you press reset. You prepare things here.
• loop() runs again and again, forever, right after setup. Your project's behaviour goes here.

void setup() {
  // runs once
}

void loop() {
  // runs forever
}

Everything else — variables, if, for — goes inside (or above) those two.

#Your first real sketch, line by line

This is the code of our Blink project. It switches pin 13 on and off every second:

const int LED = 13;

void setup() {
  pinMode(LED, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  digitalWrite(LED, HIGH);   // 5 V on the pin: current flows, the LED lights
  Serial.println("on");
  delay(1000);
  digitalWrite(LED, LOW);    // 0 V: no current, the LED goes dark
  Serial.println("off");
  delay(1000);
}

Play in 3DRun the code · press the parts · turn it around

#Variables: boxes with names

A variable is a named box that holds a value. You say what kind of value it holds, give it a name, and (usually) a starting value:

int blinks = 0;          // a whole number
float celsius = 22.5;    // a number with decimals
bool fanOn = false;      // true or false
const int BUTTON = 2;    // const: a value that never changes

A variable made outside setup() and loop() (at the top) keeps its value between loops. That is how the Meet-the-board project counts: blinks = blinks + 1; adds one every time round.

#The three commands you will use most

• pinMode(pin, mode) — sets a pin up as OUTPUT, INPUT or INPUT_PULLUP. Call it in setup().
• digitalWrite(pin, value) — sets an output pin HIGH (5 V) or LOW (0 V).
• delay(ms) — pauses for that many milliseconds. 1000 ms = 1 s. While it waits, the board does nothing else.

Reading works the same way: digitalRead(pin) gives HIGH or LOW, and analogRead(A0) gives a number from 0 to 1023 (see digital vs analog).

#Comments and semicolons

• Comments are notes for humans. The board ignores them. // starts a one-line comment; /* … */ wraps several lines.
• Every statement ends with a semicolon ; — it is like a full stop in a sentence. Lines that open a block with { (like void loop() { or if (…) {) do not get one.

Upper and lower case matter too: digitalwrite is not digitalWrite, and high is not HIGH.

#if: making decisions

if runs a block only when a condition is true; else runs when it is not. This is from our Night light project:

if (light < DARK) {
  // the darker it is, the brighter the night light
  analogWrite(LED, map(light, 0, DARK, 255, 30));
} else {
  analogWrite(LED, 0);
}

Comparisons you can use: < > <= >= == (equal) and != (not equal).

#for: doing something many times

A for loop repeats a block while counting. It has three parts: where to start, when to keep going, and how to step. From our Motor project:

for (int speed = 0; speed <= 255; speed += 15) {
  analogWrite(MOTOR, speed);
  Serial.print("speed ");
  Serial.println(speed);
  delay(150);
}

Read it as: "start speed at 0; while speed is 255 or less, run the block; then add 15". The motor speeds up in 18 steps.

#Functions: your own commands

You can bundle lines into your own command. The RGB project does this so the main code stays short:

void setColor(int r, int g, int b) {
  analogWrite(RED, r);
  analogWrite(GREEN, g);
  analogWrite(BLUE, b);
}

Now setColor(255, 0, 0); makes red. void means the function gives nothing back.

#Check yourself

#Questions people ask

#What is the difference between setup and loop in Arduino?

setup() runs once at the start — use it for pinMode() and Serial.begin(). loop() runs over and over for as long as the board has power — put the things your project keeps doing there.

#Can an Arduino sketch work without loop()?

No. Both setup() and loop() must exist, or the code will not compile. If you have nothing to repeat, leave loop() empty: void loop() {}.

#Why does my Arduino code say "was not declared in this scope"?

You used a name the compiler does not know: usually a typo (led vs LED), a variable made inside another block, or a missing #include for a library. Check the spelling and where the variable is created.

#Is delay() bad?

Not for learning. But during a delay() the board cannot notice a button press. Bigger projects use millis() (the time since start) to do several things at once.

Last updated 2026-10-29