Variables and data

Learn

What you’ll learn

• How to create and change variables
• Common Arduino data types and their limits
• Scope: where a variable can be used
• Why const is useful

Variables

A variable is a named box for a value:

int speed = 150;   // create it, starting at 150
speed = 200;       // change it
speed = speed + 10;

Data types

Type	Holds	Range on an UNO
bool	true / false	—
int	Whole numbers	−32,768 to 32,767
long	Bigger whole numbers	about ±2.1 billion
unsigned long	Big non-negative numbers	0 to about 4.3 billion; used for millis()
float	Decimals	about 6–7 significant digits
char	One character	‘A’, ‘7’
String	Text	“Hello”

Overflow

An int that goes past 32,767 wraps round to −32,768. millis() passes that after about 33 seconds, so always store it in an unsigned long.

Scope

• Global variables (outside any function) keep their value and can be used everywhere.
• Local variables (inside a function) exist only while that function runs and start fresh each time.

Constants

const int MOTOR_PIN = 9; can’t be changed by accident. Use constants for pin numbers and settings, so you only change them in one place.

Examples

Example: global vs local

int laps = 0;                 // global: remembered between loops

void loop() {
  int reading = analogRead(A0);   // local: fresh each loop
  if (reading < 200) laps++;
}

Example: integer division

int a = 7 / 2;       // 3 (whole numbers drop the .5)
float b = 7 / 2;     // still 3.0! both sides are ints
float c = 7.0 / 2;   // 3.5

Example: settings at the top

const int LEFT_MOTOR = 5;
const int RIGHT_MOTOR = 6;
const int CRUISE_SPEED = 180;
const int STOP_DISTANCE = 20;   // cm

Practice

Pick the typeEasy

Which type would you use for: a distance in cm with decimals; whether a button is pressed; the time from millis(); a lap counter? float, bool, unsigned long, int.

Spot the overflowMedium

int start = millis(); works for a while, then the robot behaves strangely. Why, and what’s the fix? After about 32.8 s millis() is bigger than an int can hold, so it overflows to a negative number. Use unsigned long start = millis();.

Average speedHard

A robot drove 237 cm in 4 s. Write code that prints the speed as a decimal in cm/s. float speed = 237.0 / 4; Serial.println(speed); prints 59.25. Without the .0, integer division would give 59.

Quick check

1. What type should store millis()?

intfloatunsigned longchar

2. What is 9 / 2 when both are int?

4.545Error

3. A local variable inside loop()…

Keeps its value foreverStarts fresh each time loop() runsCan be used in setup()Is always zero

Check my answers