Chapter 04: Variables & Data

Computers are not magic. They are machines that manipulate data in Memory. To be a professional engineer, you must understand what happens inside the machine.

4.1 The Memory Cell (The Box)

Imagine your computer's RAM (Random Access Memory) as a giant warehouse filled with millions of small numbered boxes.

Each box has:

1. An Address: A unique number (e.g., 0x001).
2. A Value: What is inside (e.g., 10).

When you create a Variable, you are claiming one of these boxes and giving it a human name.

var age int = 25

4.2 Declaration Styles (Why two ways?)

In Go, you will see two ways to create variables.

A. The Formal Way (Explicit)

var username string = "Gopher"

Use this when:

• You want to be very clear about the type.
• You want to declare it now but fill it later.

B. The Short Way (Inference)

username := "Gopher"

• :=: Short variable declaration.
• It means: "Create the box, look at the value on the right ("Gopher"), guess the type (String), and put it in."
• Constraint: You can ONLY use this inside functions.

TIP

🛡️ Interview Defense: "The Legend"

Interviewer: "When do you use := vs var?"

You: "I prioritize readability and scope control:

• I use := (short declaration) for local variables where the type is obvious or inferred, to keep code concise.
• I use var when I need to declare a variable without initializing it immediately (zero value), or when specifically initializing package-level variables where := is not allowed."

4.3 The Zero Value

Go initializes declared variables to a zero value. Some types have nil as their zero value, so initialization does not make every operation safe: dereferencing a nil pointer or writing to a nil map fails. Java local variables require definite assignment; Python names must be bound before use. Neither is accurately described as automatically becoming null or None.

In Go, declared variables have a value. If you don't provide one, Go gives it a "Zero Value".

Type	Zero Value	Meaning
int	0	Empty Count
float64	0.0	Approximate zero
bool	false	Default Off
string	""	Blank Paper
pointer	nil	No Address

var count int
fmt.Println(count) // Output: 0

The box is not missing. The box is just empty (contains 0).

4.4 Basic Types (The Shop Objects)

Let's map Go types to real-world objects in our Gopher Shop.

Go Type	Shop Concept	Example
int	Inventory Count	var stock int = 50
int64	Price in minor units	var priceMinor int64 = 1999 (USD cents)
float64	Approximate measurement	var weight float64 = 1.25
string	Book Title	var title string = "Go Guide"
bool	Is On Sale?	var onSale bool = true

4.5 Constants (The Stone Tablets)

Variables can change (age = 25, then age = 26). Constants cannot.

const Pi = 3.14159

Metaphor: Carved in stone. You cannot overwrite it.

4.6 Basic Math (The Calculator)

Computer = Compute.

• + (Add): 10 + 5 -> 15
• - (Sub): 10 - 5 -> 5
• * (Mul): 10 * 5 -> 50
• / (Div): 10 / 2 -> 5
  • Note: 10 / 3 (Integers) -> 3 (Cuts off decimal).
• % (Mod): 10 % 3 -> 1 (The Remainder).

---

Visual Anchor:

A Variable is a Labeled Box at a specific Address in the memory warehouse.

🎓 Knowledge Check

1. What is the difference between var and :=?var can be used anywhere. := (short declaration) can ONLY be used inside functions.
2. What is the zero value of an int?0.
3. Can you change the type of a variable? No. Go is statically typed. An int is always an int.