04. Sketch Structure, Variables & Types

What a sketch is

An Arduino program is called a sketch. After reset or power-up, the board runs your code from flash memory. Almost every sketch has the same backbone: optional global declarations, then setup(), then loop().

This lesson is about reading and writing that backbone clearly - before sensors and libraries pile on.

setup once, loop forever

setup() runs exactly once after reset. Use it for configuration: pinMode, Serial.begin, and one-time starts.

loop() then runs repeatedly for as long as the board is powered. Each pass typically reads inputs, makes decisions, writes outputs, then returns to the top of loop.

Both functions must exist. An empty body { } is legal if you truly have nothing to do there yet.

void setup() {
  // runs once
}

void loop() {
  // runs again and again
}

Variables store changing values

A variable is a named place in memory that holds a value your program can read and update. Declare it with a type, a name, and usually an initial value.

Example: unsigned long flashCount = 0; creates a counter starting at zero. Later, flashCount++; increases it.

unsigned long flashCount = 0;  // globals: above setup()
bool ledOn = false;

void loop() {
  flashCount++;   // updates go inside a function
  ledOn = !ledOn;
}

const for pins and fixed values

Use const when a name must not change after it is set - especially pin numbers. const byte ledPin = LED_BUILTIN; documents intent and lets the compiler reject accidental reassignment.

Course style: name pins clearly (ledPin, buttonPin). Some teams prefer LED_PIN in UPPERCASE for constants - pick one style and stay consistent in a sketch.

Choosing a type on the Uno

On a classic Uno (ATmega328P), common types have different ranges. Too small a type overflows - values wrap and look like random bugs.

Rules of thumb for this course: byte for pin numbers and small counts, int for everyday integers, unsigned long for millis() and long-running counters, float when you need decimals, bool for true/false flags.

Global vs local scope

Scope is where a name is visible. A global declared above setup can be used by setup, loop and other functions. A local declared inside a function exists only in that block.

If you declare bool ledOn = false; inside loop, it is recreated every pass - fine for a temporary, bad if you expected it to remember the previous toggle. Prefer the narrowest useful scope so names do not collide.

Names people can read

Good names beat long comments. Prefer ledPin and flashCount over x and temp2. Use camelCase for most variables and functions (buttonPressed, readSensor).

C++ is case-sensitive: setup and Setup are different. Arduino APIs use exact forms such as pinMode, digitalWrite, HIGH and LOW.

Syntax that trips beginners

Most early red errors are punctuation or spelling, not electronics:

- Missing semicolon - the compiler often points at the next line - Mismatched { } braces - count them or use the IDE brace highlight - Wrong case - PinMode will not compile; pinMode will - Comments: // for one line, /* ... */ for a block - explain why, units and pin roles

Course style: put the opening brace on the same line as the function or if header, and indent nested blocks by 2 spaces.

void loop() {
  digitalWrite(ledPin, HIGH);  // statement ends with ;
  // this whole line is a comment
}

What the worked sketch practises

The onboard LED toggles every 500 ms. A bool remembers on/off. An unsigned long counts flashes and prints to Serial so you can watch the value grow.

delay(500) is fine for this first demo. Later lessons replace long delays with millis() when several tasks must stay responsive.

Wiring and safe build sequence

1. No breadboard required - use the Uno onboard LED (LED_BUILTIN, usually pin 13)
2. Connect USB data cable, select board and port, open Serial Monitor at 9600 baud