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My Journey of Go ⑤ (Maps, Ranges)

#go
k_penguin_sato profile image K-Sato ・4 min read

Maps

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

Declaring a map

You can define a map using the following syntax.

 make(map[key_type]value_type)
 map[key_type]value_type { key1: value1, key2: value2, ......., keyX: valueX}

① make(map[key_type]value_type)

You can initialize a map using the built-in make() function. The make() function returns a map of the given type, initialized and ready for use. Keys and corresponding values can be added to a map like the code below.

package main

import "fmt"

func main() {
  var map1 = make(map[int]string)

  fmt.Println(map1) //=> map[]

  //Add keys and values to 'map1'
  map1[1] = "Go"
  map1[2] = "Ruby"
  fmt.Println(map1) //=> map[1:Go 2:Ruby]
}

② map[key_type]value_type { key1: value1, key2: value2, ......., keyX: valueX}

By using the map literal, you can initialize a map with some initial data.

package main

import "fmt"

func main() {
  var map2 = map[int]string { 1: "Go", 2:"Ruby"}

  fmt.Println(map2) //=> map[1:Go 2:Ruby]
}

Nil maps

If you declare a map with the map literal without initial data, it would generate a nil-map. Needless to say, a nil-map does not contain any data. Moreover, any attempt to add any data to a nil-map causes a runtime error.

package main

import "fmt"

func main() {
  var nil_map map[int]string

  if nil_map == nil {
    fmt.Println("nil") //=> nil
  }

  nil_map[1] = "GO" //=>  assignment to entry in nil map
}

Modyfing a map

You can add data to a map and modify the data of a map like the folloing code.

package main

import "fmt"

func main(){
  var map_ex = make(map[int]string)


  //add keys and  values to a map
  map_ex[1] = "GO"
  map_ex[2] = "Ruby"

  fmt.Println(map_ex) //=> map[1:GO 2:Ruby]

  //modify the data of a map
  map_ex[1] = "Python"
  map_ex[2] = "Java"

  fmt.Println(map_ex) //=> map[1:Python 2:Java]
}

Retrieving values from a map

You can retrieve the value assigned to a key in a map using the syntax map[key].

package main

import "fmt"

func main(){
  var map_ex = make(map[int]string)

  map_ex[1] = "GO"
  map_ex[2] = "Ruby"

  fmt.Println(map_ex[1]) //=> GO

Checking the existence of a key in a map

When you retrieve the value assigned to a given key using the syntax map[key], it returns an additional boolean value as well. It returns true if the key exists in the map and returns false if it does not exist in the map.

package main

import "fmt"

func main(){
  var map_ex = make(map[int]string)

  map_ex[1] = "GO"
  map_ex[2] = "Ruby"

  //Trying to retrieve a key that exists in map_ex.
  lang, ok := map_ex[1]

 //Trying to retrieve a key that does not exist in map_ex.
  lang2, ok2 := map_ex[4]

  fmt.Println(lang, ok) //=> GO true
  fmt.Println(lang2, ok2) //=>  false
}

If you just want to check for the existence of a key without retrieving the value associated with that key, then you can use an _ (underscore) in place of the first value.

package main

import "fmt"

func main(){
  var map_ex = make(map[int]string)

  map_ex[1] = "GO"
  map_ex[2] = "Ruby"

  _, ok := map_ex[1]
  _, ok2 := map_ex[4]

  fmt.Println(ok) //=> true
  fmt.Println(ok2) //=> false
}

Deleting a key from a map

You can delete a key from a map using the built-in delete(map, key) function. The delete() function does not do anything if the key does not exist in the map.

package main

import "fmt"

func main(){
  var map_ex = make(map[int]string)

  map_ex[1] = "GO"
  map_ex[2] = "Ruby"

  delete(map_ex, 1)
  fmt.Println(map_ex) //=> map[2:Ruby]
}

Range

The range keyword is used in for loop to iterate over items of an array, slice, channel or map. When you are ranging over an array or a slice, it returns the index and the element at that index. When you are ranging over a map, it returns the key and the key-value.

Array

package main

import "fmt"

var arry_ex  = [3]string { "Go", "Ruby", "Python" }

func main(){
  for index, value := range arry_ex {
    fmt.Println(index, value)
    //=> 0 Go
    //=> 1 Ruby
    //=> 2 Python
  }
}

Slice

package main

import "fmt"

var slice_ex = []string { "Go", "Ruby", "Python" }

  for index, ele := range slice_ex {
    fmt.Println(index, ele)
    //=> 0 Go
    //=> 1 Ruby
    //=> 2 Python
  }
}

Map

package main

import "fmt"

var map_ex = map[string]string{ "Name":"Sam", "Gender":"Male" }

  for key, value := range map_ex {
    fmt.Println(key, value)
    //=> Name Sam
    //=> Gender Male
  }
}

Skipping the index, key or value

You can skip the index or value by assigning to _. If you only want the index, drop the value entirely.

Index Only

package main

import "fmt"

var arry_ex  = [3]string { "Go", "Ruby", "Python" }

func main(){
  for index, _ := range arry_ex {
    fmt.Println(index)
    //=> 0
    //=> 1
    //=> 2
  }
}

Value Only

package main

import "fmt"

var arry_ex  = [3]string { "Go", "Ruby", "Python" }

func main(){
  for _, value := range arry_ex {
    fmt.Println(value)
    //=> Go
    //=> Ruby
    //=> Python
  }
}

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