DAY 4 — Maps and Zero Values

Goal: Learn how Go represents key–value data and how zero values affect program correctness.

1. What a Map Is

• A map is a built-in key–value data structure.
• Keys must be comparable types (e.g. string, int).
• Values can be any type.
• Map syntax: map[K]V

Example: map[string]int

2. Declaring and Creating Maps

• A map must be created before use.
• A nil map cannot store values.

Valid ways to create maps:

• make(map[string]int)
• map[string]int{}

Invalid:

• var m map[string]int (nil until make)

Rule: A map must be initialized before writing to it.

3. Reading From a Map

• Reading a missing key does NOT panic.
• Missing keys return the zero value of the value type.

Example: m["x"] returns 0 if value type is int.

Important: You cannot tell if the key existed just from the value.

4. The “Comma ok” Pattern

• Used to distinguish “missing key” from “zero value”.
• Syntax: value, ok := m[key]

Rules:

• ok == true → key exists
• ok == false → key does not exist

This is mandatory when correctness depends on key presence.

5. Writing to a Map

• Assigning to a key inserts or overwrites.
• There is no append; assignment is the only write operation.

Example: m["a"] = 10

If the key already exists, the value is replaced.

6. Deleting From a Map

• delete(m, key) removes a key.
• Deleting a missing key is safe (no error).

delete(m, "x") is always valid.

7. Iterating Over a Map

• Use range to iterate.
• Order is NOT guaranteed.
• Never rely on iteration order.

Syntax: for k, v := range m { ... }

Rule: Map iteration order is intentionally randomized.

8. Maps Are Reference Types

• Passing a map to a function does NOT copy its contents.
• All references point to the same underlying data.

Rule: You do NOT need pointers to modify a map.

This is different from structs.

9. Zero Value of a Map

• The zero value of a map is nil.
• nil maps can be read from but not written to.

Rule: Always initialize maps before writing.

10. When to Use Maps vs Slices

Use a map when:

• You need fast lookup by key
• Order does not matter
• Keys are meaningful identifiers

Use a slice when:

• Order matters
• You need indexed iteration
• Duplicates are allowed

End of Day 4 Append key learnings to learnt.txt after completion.