Chapter 08: Maps

"Give me the ticket, I'll give you the coat."

Arrays and Slices are great if you want to find things by number (Index 0, Index 1). But what if you want to find a User by their Name? You don't want to loop through a list of 1,000,000 users. You need the Map (Hash Table).

8.1 The Coat Check Metaphor

Imagine a coat check at a theater.

1. You give your coat.
2. You get a specific ticket #123 (The Key).
3. Later, you give back ticket #123.
4. The attendant instantly finds your specific coat (The Value).

They don't search all coats. They go straight to hook #123. That is $O(1)$ complexity (Instant speed).

8.2 Syntax

// Key: String (Name) -> Value: Integer (Year)
phones := make(map[string]int)

// Add
phones["Alice"] = 12345
phones["Bob"] = 67890

// Get
fmt.Println(phones["Alice"]) // Prints 12345

8.3 The Zero Value Trap

If you ask for a key that doesn't exist, Go does not crash. It returns the "Zero Value" (0 for int, "" for string).

fmt.Println(phones["Ghost"]) // Prints 0. Wait, does a ghost have phone number 0?

The "Comma Ok" Idiom

To check if a key actually exists, we use the second return value.

price, ok := phones["Ghost"]
if !ok {
    fmt.Println("Ghost not found!")
} else {
    fmt.Println("Ghost's number is", price)
}

Visual Anchor:

The Library Card Catalog:

• You don't walk through every shelf looking for "Harry Potter" (Looping).
• You go to the Catalog (Map), look up "Harry Potter" (Key), and it tells you exactly "Aisle 4, Shelf B" (Value).
• You walk straight there.

🎓 Knowledge Check

: What two values does a map lookup return? Answer:

1. Value: The data you asked for (e.g., the price).
2. Exists: A boolean (true/false) telling you if the key was actually found. Always check this!