Maps

Sample code link: (https://repl.it/@jjoco/go-maps)

Go implements map, a key-value data structure similar to Dictionaries in Python and Hashtables/HashMaps in Java.

Creating a Map

A dev can use literal-based syntax or the make function to allocate memory for a new map.

exampleMap := make(map[KeyType]ValueType)

//Functionally identical to using the make function
otherExampleMap := map[KeyType]ValueType{}

//Use interface{} if the ValueType can be anything
anyValueMap := make(map[string]interface{})

• KeyType must be a type that is comparable => "boolean, numeric, string, pointer, channel, and interface types, and structs or arrays that contain only those types"
• ValueType can be of any type, including custom structs or other maps
  • use interface{} as your ValueType to have the map's valueType be of anything

Reading and Writing Map Elements

Like other languages, one can directly read, write, and delete values from Go maps.

//Setting Map Elements
cardMap["ace"] = 1
cardMap["jack"] = 11
cardMap["queen"] = 12
cardMap["king"] = 13

// cardMap == map[ace:1 jack:11 king:13 queen:12]

Reading Map Element

j := cardMap["queen"]
// j == 12

Deleting Map Element

delete(cardMap, "king")
// cardMap == map[ace:1 jack:11 queen:12]

Map Literals

Use map literals to define the initial elements in a map.

trafficLightMap := map[string]string{
  "red": "stop",
  "green": "go",
  "yellow": "slow",
}

Iterating Through Maps

Similar in syntax to slice iteration, a dev can use range to iterate through the key-value pairs in a map.

for key, value := range exampleMap{
  //Process key and/or value
}

• Can use _ in place of key or value if they aren't needed

Example:

for key, value := range cardMap{
  fmt.Println("Key:", key, "; Value:", value)
}
/*
Console output
Key: ace ; Value: 1
Key: jack ; Value: 11
Key: queen ; Value: 12
*/

Getting a Key's Value if the pair exists

There is an optional second return value when reading from a map that tells the dev if a key-value pair exists.

Syntax

value, ok := someMap[key]

if !ok{
  //Do something if there is no key-value pair
}

Checking for key-value existence

If you don't care for retrieving the value, one can just check the existence of the key-value pair.

_, ok := someMap[key]

if !ok{
  //Do something if there is no key-value pair  
}

Example:

//Given: "king" and its value don't exist in cardMap
k, ok := cardMap["king"]

if !ok{
  fmt.Println("king isn't there")
}
//Output = "king isn't there"