Maps Cheatsheet

Creation

• Make: m := make(map[K]V)
• Literal: m := map[string]int{"A": 1}
• Nil: var m map[string]int (Cannot be written to!)

Operations

• Insert/Update: m[key] = val
• Get: v := m[key] (Returns zero value if missing)
• Delete: delete(m, key)
• Length: len(m)

Comma OK Idiom

if val, ok := m[key]; ok {
    // Exists
}

Constraints

• Keys: Must be comparable (no slices, maps, or functions).
• Values: Can be anything.
• Reference: Maps are passed by reference; functions can mutate them.

Common Patterns

• Sets: map[T]struct{}
• Nested Maps: map[K]map[K]V (Must initialize inner map!)
• Grouping: Using maps to count frequencies or group items by a key.