Go: Maps explained

A map is an unordered collection of key-value pairs, where each key is unique.

Basics

m := make(map[string]float64)

m["pi"] = 3.1416

fmt.Println(m) // "map[pi:3.1416]"

v1 := m["pi"]  // v1 == 3.1416
v2 := m["foo"] // v2 == zero value

_, exists := m["pi"] // exists == true

if x, ok := m["pi"]; ok {
        fmt.Println(x) // "3.1416"
}

delete(m, "pi")
fmt.Println(m) // "map[]"

• The default zero value of a map is nil. A nil map is equivalent to an empty map except that no elements may be added.
• Use make(mapType, n) to preallocate room for n entries.

Literals

m := map[string]float64{
        "e":  2.71828,
        "pi": 3.1416,
}

Size

n := len(m) // n == number of elements in m

Iteration

for key, value := range m {
        fmt.Println(key, ": ", value)
}

• Iteration order is not specified and may vary from iteration to iteration.
• If an entry that has not yet been reached is removed during iteration, the corresponding iteration value will not be produced.
• If an entry is created during iteration, that entry may or may not be produced during the iteration.

Implementation

• Maps are backed by hash tables.
• They provide lookup, insert, and delete operations in constant amortized time.
• The comparison operators == and != must be defined for the key type.