Table of Contents

• Basics of map
• Mimic set type with a map
• Implement a nested map which works like a defaultdict in Python

Basics of map tutorial

map[KeyType]ValueType

• Maps are reference types, so you don't need to pass maps as pointers.
• The KeyType must be comparable using ==.
• So, types like slice cannot be a KeyType.
• The zero value for a map is nil.
• nil map behaves like an empty map. Most operations without assignments are safe.

var m map[string]int
// or
m := map[string]int{}
// or
m := make(map[string]int)
// or (with initial values)
ages := map[string]int{
    "yeonghoey": 32,
}

m := map[string]int
for key := range m {}
for key, value := range m {}
for _, value := range m {}

x, ok := m["key"]
// or concisely
if x, ok := m["key"]; !ok {
}

• The second value ok is a bool that is true if the key exists in the map.
• When ok is false, x is going to be the zero value of the ValueType.
• As a map element is not a variable, you cannot take its address.

delete(m, "route")

• delete() doesn't return anything.
• delete() will do nothing if the specified key doesn't exist.

Mimic set type with a map howto

m := make(map[string]bool)
if !m[x] {
    m[x] = true
}

Implement a nested map which works like a defaultdict in Python howto

graph := make(map[string]map[string]bool)

func addEdge(from, to string) {
    edges := graph[from]
    if edges == nil {
        edges = make(map[string]bool)
        graph[from] = edges
    }
    edges[to] = true
}