Arduino C lesson 2: types, variables and constants

In robotics you handle a lot of numbers (speeds, distances, times) and states (on/off). In Arduino C, choosing the right type makes the program clearer and avoids bugs. This is lesson 2 of our Arduino course.

1. The most useful types

• int: whole number (counter, speed, threshold). On an UNO it is 16-bit, from −32,768 to 32,767 (Arduino reference).
• unsigned long: time from millis(), which returns this type.
• float: decimal number (measurement, average, calculation).
• bool: true/false (state, flag).
• char: one character ('A').
• const char*: constant text, handy for messages.

Rule of thumb: if you are counting events, it’s a whole number, so int, not float.

2. Global vs local variables (scope)

• Global: declared outside any function, visible everywhere.
• Local: declared inside a function, visible only in that block.

int global;        ← visible in setup() and loop()

void loop() {
  int local;       ← visible only in loop()
}

3. Constants: no magic numbers

const int LED_PIN = 13;
const int BAUD = 9600;
const int DELAY_MS = 100;

Give names that say why the number exists.

4. Common conversions

• int n = 3; float f = n; works (3 becomes 3.0).
• float f = 3.7; int n = (int)f; truncates (3.7 becomes 3).

For text output, keep numbers as int and print them with Serial.print rather than building global String objects.

Exercises (12)

Solutions are hidden: click Show solution.

Easy

Exercise 1: Declare int counter = 0; and increment it in loop(). Easy

int counter = 0;
void setup() {}
void loop() { counter++; }

Exercise 2: Declare bool alarm = false; and set it to true in setup(). Easy

bool alarm = false;
void setup() { alarm = true; }
void loop() {}

Exercise 3: Declare unsigned long t0 = 0; and store millis() in it in setup(). Easy

unsigned long t0 = 0;
void setup() { t0 = millis(); }
void loop() {}

Exercise 4: Print an int and a float on one line with Serial.print. Easy

int n = 7;
float f = 3.5;
void setup() {
  Serial.begin(9600);
  Serial.print("n=");
  Serial.print(n);
  Serial.print(" f=");
  Serial.println(f);
}
void loop() {}

Intermediate

Exercise 5: Scope: declare a local int x in loop() and explain in a comment why setup() cannot use it. Intermediate

void setup() {
  // x does not exist here if it is declared in loop()
  // Scope: a local variable is only visible inside its block.
}
void loop() {
  int x = 0;
  x++;
}

Exercise 6: Declare a text const char* msg = "OK"; and print it at start-up. Intermediate

const char* msg = "OK";
void setup() {
  Serial.begin(9600);
  Serial.println(msg);
}
void loop() {}

Exercise 7: Conversion: with float d = 12.9; compute int n = (int)d; and print n (it should print 12). Intermediate

void setup() {
  Serial.begin(9600);
  float d = 12.9;
  int n = (int)d;
  Serial.println(n); // 12
}
void loop() {}

Exercise 8: Write int clampInt(int v, int minV, int maxV) that keeps v between min and max. Intermediate

int clampInt(int v, int minV, int maxV) {
  if (v < minV) return minV;
  if (v > maxV) return maxV;
  return v;
}

Advanced

Exercise 9: Explain in a comment why millis() must be stored in an unsigned long. Advanced

void setup() {
  // millis() returns a large positive number (ms since start-up)
  // unsigned long matches its return type and keeps the full range.
  unsigned long t = millis();
  Serial.begin(9600);
  Serial.println(t);
}
void loop() {}

Exercise 10: Write bool isNear(int distanceCm, int threshold) that returns true if distanceCm <= threshold. Advanced

bool isNear(int distanceCm, int threshold) {
  return distanceCm <= threshold;
}

Exercise 11: static: write a function that counts how many times it has been called. Advanced

int countCalls() {
  static int c = 0;
  c++;
  return c;
}

Exercise 12: Mini-project: with int distance_cm = 42; print Distance: 42 cm. Advanced

void setup() {
  Serial.begin(9600);
  int distance_cm = 42;
  Serial.print("Distance: ");
  Serial.print(distance_cm);
  Serial.println(" cm");
}
void loop() {}

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