Maps

A map maps key to value.

Golang provides map data structure which implements hashtable.

1. Creating a Maps

The zero value of a map is nil. A nil map has no keys, nor can keys be added.

Method 1: like array

// nil map
var m map[keyTpye]valueType

• Note:

  如果使用此方法來create map，還需要使用make()來初始化此map，否則無法存放key-value.

Method 2: use make()

The make function returns a map od the given type, initialized and ready for use.

m := make(map[keyTpye]valueType)

// or

// Define a map
var m map[keyTpye]valueType
// Using make function to initiate map
m = make(map[keyTpye]valueType)

2. Maps initialize

Map literals are like struct literals, but the keys are required.

var m = map[keyType]valueType {
    key1: value1,
    key2: value2,
}

3. Mutating Maps

Insert or Update

Insert or update an element in map m

m[key] = value

Detele

Delete an element

delete(m, key)

Check if the key is exist

Test that a key is present with a two-value assignment:

v, ok := m[key]

• If key is in map, ok is true. If not, ok is false.
• If key is not in map, then v is zero value for the map’s element type.

Exercise: Maps

Implement WordCount. It should return a map of the counts of each “word” in the string s. The wc.Test function runs a test suite against the provided function and prints success or failure.

func WordCount(s string) map[string]int {
    m := make(map[string]int)
    //a := strings.Split(s, " ")
    a := string.Fields(s)

	for _, c := range a {
		m[c]++
	}

	return m
}