Unit A - Foundations

05. Operators & Decisions

Calculate, compare and select actions.

Estimated time 3 hours

Learning outcomes

Use arithmetic, comparison and Boolean operators
Distinguish assignment from equality
Write if, else if and else chains
Combine conditions with AND, OR and NOT

Parts and preparation

Arduino Uno, two push buttons and onboard LED.

Before power: inspect wiring, confirm supply voltage and ensure all connected circuits share GND.

Operators & Decisions instructional connection diagram — Open the diagram for a larger view. Trace every connection before building.

Arithmetic

+, -, , / and % calculate values. Integer division discards the remainder. Use a decimal operand such as 5.0 when a floating-point result is required.

Assignment and comparison

= stores a value. == compares equality. Other comparisons are !=, <, <=, > and >=.

Boolean logic

A && B requires both conditions. A || B requires either condition. !A reverses a condition. Parentheses make complex conditions easier to read.

Decision chains

Only the first true branch of an if / else if / else chain runs. Separate if statements are each tested independently.

Wiring and safe build sequence

Button 1: D2 to GND
Button 2: D3 to GND
Use INPUT_PULLUP for both
Use onboard LED as output

Power rule: switch off before moving wires. Arduino I/O pins are control signals; high-current loads require a driver and suitable external supply.

Worked sketch

Download .ino sketch

const byte buttonA = 2;
const byte buttonB = 3;

void setup() {
  pinMode(buttonA, INPUT_PULLUP);
  pinMode(buttonB, INPUT_PULLUP);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  bool aPressed = digitalRead(buttonA) == LOW;
  bool bPressed = digitalRead(buttonB) == LOW;

  if (aPressed && bPressed) {
    digitalWrite(LED_BUILTIN, HIGH);
  } else if (aPressed || bPressed) {
    digitalWrite(LED_BUILTIN, millis() / 250 % 2);
  } else {
    digitalWrite(LED_BUILTIN, LOW);
  }
}

How the code works

Pressed is LOW because INPUT_PULLUP holds an open input HIGH.
Both buttons give steady light; either button gives flashing light.

Test and record evidence

Expected result: No button: off. One button: flashing. Both buttons: steady on.

Practical evidence checklist

Labelled wiring or block diagram Uploaded sketch and observed result At least one measured/tested value Fault or improvement recorded in logbook

Common faults and checks

If behavior is inverted, remember that pressed reads LOW.
Check && versus || and == versus =.

Extension challenge: Add a condition that gives button A priority over button B.

Check your understanding

Q1. What is the difference between = and ==?

Show answer

= assigns; == compares.

Q2. When does A || B evaluate true?

Show answer

When A, B or both are true.