Map

// Declaring and initializing a map
var m map[KeyType]ValueType          // nil map (read-only)
m := make(map[KeyType]ValueType)     // writable map
m := map[KeyType]ValueType{          // initialized with a literal
	key1: value1,
	key2: value2,
}

// Operations
m[key] = value                       // add or update
value := m[key]                      // retrieve (zero value if key not found)
value, ok := m[key]                  // check if key exists
delete(m, key)                       // delete
length := len(m)                     // number of elements

package main

import "fmt"

func main() {
	// Create a map with make
	scores := make(map[string]int)
	scores["Tanaka"] = 85
	scores["Suzuki"] = 92
	scores["Sato"] = 78

	fmt.Println(scores["Tanaka"]) // 85

	// Initialize with a literal
	colors := map[string]string{
		"red":   "#FF0000",
		"green": "#00FF00",
		"blue":  "#0000FF",
	}

	// Check if a key exists
	if val, ok := colors["red"]; ok {
		fmt.Println("red:", val)
	}

	// Non-existent key (returns zero value)
	fmt.Println(scores["Yamada"]) // 0

	// Delete an entry
	delete(scores, "Sato")
	fmt.Println(len(scores)) // 2

	// Loop with range (order is not guaranteed)
	for name, score := range scores {
		fmt.Printf("%s: %d\n", name, score)
	}
}

Maps are reference types. Passing a map to a function or assigning it to another variable makes both point to the same underlying map. Declaring a map with just 'var m map[string]int' creates a nil map, and attempting to write to it will cause a panic. Always initialize a map with 'make()' or a map literal before use.

Only types that support the equality operator ('==') can be used as map keys. Slices, other maps, and function types cannot be used as keys.

The iteration order of a map is intentionally randomized by Go's specification and may differ on each run. If you need a consistent order, extract the keys into a slice and sort them first.