Chapter 06: Loops

Managing one variable is easy. Managing a thousand is hard. We need containers.

6.1 The Egg Carton (Arrays)

An Array is a fixed-size container.

• Rule: When you buy an egg carton for 12 eggs, it always has 12 slots. You cannot magically stretch it to hold 13.
• Syntax: [3]int (An array of exactly 3 integers).

var carton [3]int
carton[0] = 10
carton[1] = 20
// carton[3] = 30 // ERROR! Index out of bounds.

Arrays are rarely used directly because they are too rigid.

6.2 The Magic Shelf (Slices)

A Slice is a window into an underlying array. Ideally, it feels like a dynamic list that can grow.

• Syntax: []int (No number).

shelf := []string{"Book A", "Book B"}
shelf = append(shelf, "Book C") // Magic! It grows.

6.3 The Conveyor Belt (For Loops)

How do we process 1,000 items? We don't write 1,000 lines of code. We use a Loop.

The Standard Loop

"Init; Condition; Post"

// Start at 0; Keep going while i < 5; Add 1 to i after each step
for i := 0; i < 5; i++ {
    fmt.Println(i)
}

The Range Loop (Best for Libraries)

Ideal for Slices. It gives you the Index (Position) and the Value (Copy of the item).

library := []string{"Go", "Rust", "Python"}

for index, title := range library {
    fmt.Printf("%d: %s\n", index, title)
}

6.4 Maps (The Keychains)

Arrays are ordered lists (1st, 2nd, 3rd). Maps are labelled storage (Key -> Value).

// [Key] -> Value
menu := make(map[string]float64)
menu["Coffee"] = 2.50

Metaphor: A Coat Check. You give a ticket (Key), you get the coat (Value). You don't say "Give me the 5th coat". You say "Give me the coat for ticket 'Coffee'".

Visual Anchor:

• Array: Solid concrete block with slots.
• Slice: A flexible viewing window.
• Loop: A factory conveyor belt processing items one by one.

🎓 Knowledge Check

1. How do you make an infinite loop in Go?for { }.
2. What does range do? It iterates over elements of an array, slice, string, map, or channel.
3. How do you stop a loop? Use the break keyword.