Module 2 - Programming Fundamentals

07. Loops & Arrays

Remove repetition with for and while loops, and keep related pins and readings together in arrays.

Estimated time 2-3 hours

Learning outcomes

Explain the init, condition and update parts of a for loop
Choose for or while for a counting or waiting task
Store related values in an array and index from zero safely
Calculate an array's length with sizeof so loops and arrays stay matched
Avoid off-by-one bounds errors and infinite loops

Parts and preparation

Arduino Uno, four LEDs, four 330 ohm resistors, breadboard and jumper wires.

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

Loops & Arrays instructional connection diagram

Why loops and arrays matter

Copy-pasting the same digitalWrite four times works once - then a pin change means four edits and four chances to mistype. An array holds a list of related values. A loop walks that list.

This lesson builds a four-LED chase and an all-on flash from one list of pins. Lesson 08 then packages those patterns into reusable functions.

Flow from LED array through for loop and setAll function to outputs — Array holds pins. for walks them. The next lesson names the repeated actions as functions.

for loop anatomy

A for header has three parts separated by semicolons: initialisation, condition and update.

Initialisation runs once. The condition is tested before each body pass - if false, the loop ends. After the body, the update runs (often i++), then the condition is tested again.

Use for when the count is known: walk every LED, repeat N times, scan an array.

for header broken into init, condition and update cards — Init once, test the condition, run the body, update, repeat until the test fails.

for (byte i = 0; i < ledCount; i++) {
  digitalWrite(leds[i], HIGH);
  delay(150);
  digitalWrite(leds[i], LOW);
}

while when you wait for a change

while (condition) { ... } repeats as long as the condition stays true. It suits waiting until a button releases, draining Serial bytes, or looping until a flag clears.

Danger: if nothing inside the body can make the condition false, the rest of loop() never runs. That is an infinite loop - fine for a deliberate hang demo, fatal in a multi-task sketch.

Side by side cards comparing for for known counts and while for waiting until something changes — for = known count. while = until something changes. Prefer the clearer shape.

// Example idea - not in the worked sketch
while (digitalRead(buttonPin) == LOW) {
  // wait until released (blocks other work)
}

An array holds several values of the same type under one name. Indexing starts at zero: the first item is leds[0], not leds[1].

For four pins, valid indices are 0, 1, 2 and 3. Declaring const byte leds[] = {2, 3, 4, 5}; keeps pin numbers in one place.

Four cells leds 0 to 3 holding pin numbers 2, 3, 4 and 5 — Four items means indices 0-3. leds[4] does not exist.

Expression	Meaning
leds[0]	First pin (2)
leds[3]	Last pin (5)
ledCount	How many items (4)
leds[4]	Out of bounds - bug

sizeof for length

Writing const byte ledCount = 4; works, but then you must update two places when you add a pin. sizeof(leds) / sizeof(leds[0]) computes the element count from the array itself.

Use that length in every for condition so the loop and the array stay matched.

const byte leds[] = {2, 3, 4, 5};
const byte ledCount = sizeof(leds) / sizeof(leds[0]);

The bounds trap

The classic off-by-one error is writing i <= ledCount instead of i < ledCount. With four LEDs that tries index 4 - memory that is not your array.

Symptoms look random: a wrong pin toggles, the board resets, or nothing obvious happens until later. Always ask: what is the last valid index?

Safe i less than ledCount versus buggy i less than or equal to ledCount — i < ledCount stops at the last valid index. i <= ledCount walks off the end.

What the worked sketch practises

setup configures every pin in the array with a for loop. loop chases the LEDs one by one, then uses a second loop to switch all four on and a third to switch them off.

Notice that the all-on and all-off loops have the same shape - that repetition is exactly what Lesson 08 removes. delay is used so the pattern is easy to see; Lesson 11 replaces it when several tasks must stay responsive.

Wiring and safe build sequence

D2 -> 330 ohm -> LED anode; cathode -> GND
D3 -> 330 ohm -> LED anode; cathode -> GND
D4 -> 330 ohm -> LED anode; cathode -> GND
D5 -> 330 ohm -> LED anode; cathode -> GND
Confirm LED polarity: long leg is usually the anode

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 leds[] = {2, 3, 4, 5};
const byte ledCount = sizeof(leds) / sizeof(leds[0]);

void setup() {
  for (byte i = 0; i < ledCount; i++) {
    pinMode(leds[i], OUTPUT);
  }
}

void loop() {
  // Chase: one LED at a time
  for (byte i = 0; i < ledCount; i++) {
    digitalWrite(leds[i], HIGH);
    delay(150);
    digitalWrite(leds[i], LOW);
  }

  // All on together
  for (byte i = 0; i < ledCount; i++) {
    digitalWrite(leds[i], HIGH);
  }
  delay(300);

  // All off together
  for (byte i = 0; i < ledCount; i++) {
    digitalWrite(leds[i], LOW);
  }
  delay(300);
}

How the code works

sizeof expression calculates the array length instead of hard-coding 4.
Every loop uses i < ledCount, so adding a pin to leds[] needs no other change.
Valid indices are 0 to ledCount - 1. Never use i <= ledCount.
The all-on and all-off loops repeat the same shape - Lesson 08 turns them into one setAll() call.

Test and record evidence

Expected result: Four LEDs light in order (chase), then all turn on together, then off, and the pattern repeats.

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 only some LEDs work, check that LED's resistor and polarity first.
An i <= ledCount condition accesses one item past the array - change it to i < ledCount.
If the chase skips a pin, confirm the leds[] values match your wiring.
Dim or dead LED: try swapping that LED with a known-good one.

Extension challenge: Add a reverse chase from the last LED back to the first. Use int i = ledCount - 1 with the condition i >= 0, then explain why the same loop with byte i would never end.

Check your understanding

Q1. What are the three parts inside a for (...) header?

Show answer

Initialisation, condition and update.

Q2. What is the first array index?

Show answer

Q3. Why is i <= ledCount dangerous for a 4-element array?

Show answer

It also uses index 4, which is outside the array.

Q4. How do you calculate how many items are in leds[]?

Show answer

sizeof(leds) / sizeof(leds[0]).

Q5. When is while a better fit than for?

Show answer

When you repeat until a condition changes, rather than a known count.

Learning outcomes

Parts and preparation

Why loops and arrays matter

for loop anatomy

while when you wait for a change

Arrays store related values

sizeof for length

The bounds trap

What the worked sketch practises

Wiring and safe build sequence

Worked sketch

How the code works

Test and record evidence

Practical evidence checklist

Check your understanding