Chapter 20: In-Memory Store

20.1 The Map (Hash Table)

We need a way to find a Book by its ID instantly. In Computer Science, this is a Hash Map (or Dictionary).

var storage = make(map[string]Book)

Anatomy of the Declaration

1. var: Create a variable.
2. make: Crucial Keyword. Maps must be initialized before use.
   • If you just did var m map[string]int, it is nil. Writing to it causes a Panic (Crash).
   • make allocates the memory bucket for the map.
3. map: The type.
4. [string]: The Key Type (The ID is a string).
5. Book: The Value Type (The full Book struct).

20.2 Pointers vs Values (*Book vs Book)

This is the hardest concept for beginners coming from Python/JS.

func GetBook() *Book  // Returns a Pointer (Address)
func GetBook() Book   // Returns a Value (Photocopy)

The Metaphor

• Value (Book): You ask for the Mona Lisa. I take a Photocopy and hand it to you. If you draw a mustache on the copy, the real Mona Lisa is unchanged.
• Pointer (*Book): You ask for the Mona Lisa. I write "Room 303, Wall 2" on a card and hand it to you. If you go there and draw a mustache, the real painting is changed forever.

Why use Pointers?

1. Performance: Copying a huge book is slow. Passing a small address card is fast.
2. Mutability: We want to modify the original (e.g., updating the price).

Comparison: Memory

Language	Default Behavior
Python	Everything is a Reference (Pointer-like).
Java	Objects are References. Primitives (int) are Values.
Go	Everything is a Value (Copy) by default. You must explicitly ask for a Pointer (*).

🎓 Knowledge Check: If I pass a Book (not *Book) to a function and change its title, what happens to the original book?

Answer: Nothing! You only changed the photocopy. The original book remains untouched. To change the original, you must pass a Pointer (*Book).