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Updated 23 October 2026 By Emmanuel SAUVAGE

Arduino C lesson 3: if/else conditions for sensors and actuators

Comparisons, AND/OR/NOT and else if: write the rules that make a robot stop, slow down or beep from a distance reading. Lesson 3 of our Arduino C course, 12 exercises with solutions.

Conditions (if, else if, else) decide what to do based on the state of the world: measured distance, button pressed, low light. This is lesson 3 of our Arduino course.

In robotics you constantly write rules like:

IF distance <= 15 cm → STOP
ELSE → MOVE FORWARD

1. Comparisons

  • == equal (careful: two = signs in C)
  • != not equal
  • <, >, <=, >=

2. Combining conditions

  • &&: AND
  • ||: OR
  • !: NOT
if (distance_cm <= 15 && speed > 0) {
  // obstacle close and robot moving
}

3. Readable if statements

  • use constants: const int DIST_STOP = 15;
  • avoid deep “pyramids” of nested if: split into functions;
  • name boolean helpers like questions: isNear, isPressed.

On a real robot the distance usually comes from an ultrasonic sensor such as the HC-SR04 found in most Arduino car kits.

Exercises (12)

If you don’t have a sensor at hand, simulate a reading:

int distance_cm = 42; // test value

Easy

Exercise 1: With int distance_cm = 12; print STOP if distance_cm <= 15, otherwise GO. Easy

Show solution 
void setup() {
  Serial.begin(9600);
  int distance_cm = 12;
  if (distance_cm <= 15) Serial.println("STOP");
  else Serial.println("GO");
}
void loop() {}

Exercise 2: Create const int DIST_WARN = 35; and print WARN if distance_cm <= DIST_WARN. Easy

Show solution 
const int DIST_WARN = 35;
void setup() {
  Serial.begin(9600);
  int distance_cm = 30;
  if (distance_cm <= DIST_WARN) Serial.println("WARN");
}
void loop() {}

Exercise 3: Three zones: d >= 70 prints SILENT, d >= 35 prints SLOW BEEP, otherwise FAST BEEP. Easy

Show solution 
void setup() {
  Serial.begin(9600);
  int d = 50;
  if (d >= 70) Serial.println("SILENT");
  else if (d >= 35) Serial.println("SLOW BEEP");
  else Serial.println("FAST BEEP");
}
void loop() {}

Exercise 4: With bool pressed = true; print CLICK if pressed is true. Easy

Show solution 
void setup() {
  Serial.begin(9600);
  bool pressed = true;
  if (pressed) Serial.println("CLICK");
}
void loop() {}

Intermediate

Exercise 5: Noisy sensor: treat it as an intrusion only if abs(reading - reference) > 3 (write int absInt(int v)). Intermediate

Show solution 
int absInt(int v) { return v < 0 ? -v : v; }

void setup() {
Serial.begin(9600);
int reference = 40;
int reading = 44;
if (absInt(reading - reference) > 3) Serial.println(“INTRUSION”);
else Serial.println(“OK”);
}
void loop() {}

Exercise 6: AND: print DANGER if distance <= 15 AND speed > 0. Intermediate

Show solution 
void setup() {
  Serial.begin(9600);
  int distance = 12;
  int speed = 80;
  if (distance <= 15 && speed > 0) Serial.println("DANGER");
}
void loop() {}

Exercise 7: OR: print ALERT if distance <= 15 OR soundDetected is true. Intermediate

Show solution 
void setup() {
  Serial.begin(9600);
  int distance = 50;
  bool soundDetected = true;
  if (distance <= 15 || soundDetected) Serial.println("ALERT");
}
void loop() {}

Exercise 8: With int speed = 80;, set speed = 30 if distance <= 50, then print the speed. Intermediate

Show solution 
void setup() {
  Serial.begin(9600);
  int distance = 40;
  int speed = 80;
  if (distance <= 50) speed = 30;
  Serial.println(speed);
}
void loop() {}

Advanced

Exercise 9: Write int radarZone(int d) returning 0, 1 or 2 (silent, slow, fast), then print the matching text. Advanced

Show solution 
int radarZone(int d) {
  if (d >= 70) return 0;
  if (d >= 35) return 1;
  return 2;
}

void setup() {
Serial.begin(9600);
int z = radarZone(20);
if (z == 0) Serial.println(“SILENT”);
else if (z == 1) Serial.println(“SLOW BEEP”);
else Serial.println(“FAST BEEP”);
}
void loop() {}

Exercise 10: Range check: print OK if d is between 35 and 70 inclusive. Advanced

Show solution 
void setup() {
  Serial.begin(9600);
  int d = 50;
  if (d >= 35 && d <= 70) Serial.println("OK");
}
void loop() {}

Exercise 11: Explain = vs == in a comment, then write a correct test. Advanced

Show solution 
void setup() {
  Serial.begin(9600);
  int mode = 2;
  // = assigns a value; == compares two values
  if (mode == 2) Serial.println("MODE 2");
}
void loop() {}

Exercise 12: Mini-project, slow-down rule: if distance <= 50 then speed = 40, otherwise speed = 80. Print speed. Advanced

Show solution 
void setup() {
  Serial.begin(9600);
  int distance = 60;
  int speed = 0;
  if (distance <= 50) speed = 40;
  else speed = 80;
  Serial.println(speed);
}
void loop() {}

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