Go map

In Go, the map data structure stores elements in key/value pairs. Here, keys are unique identifiers that are associated with each value on a map.

Create a map in Golang

The syntax to create a Go map is:

subjectMarks := map[string]float32{"Golang": 85, "Java": 80, "Python": 81}

This code creates a map named subjectMarks. Here,

• [string] - indicates that keys of the map are of string type
• float32 - indicates that values of the map are of float type
• {Golang", "Java", "Python"} - keys of the map
• {85, 80, 81} - values of the map

Example: Map in Golang

// Program to create a map and print its keys and values

package main
import "fmt"

func main() {

  // create a map
  subjectMarks := map[string]float32{"Golang": 85, "Java": 80, "Python": 81}

  fmt.Println(subjectMarks)
}

Output

map[Golang:85 Java:80 Python:81]

Here, we have created a map and printed the key-values of a map.

var subjectMarks = map[string]float32{"Golang": 85, "Java": 80, "Python": 81}

Access Values of a Map in Golang

We can access the value of a map by using the corresponding key. For example,

package main
import "fmt"

func main() {

  // create a map
  flowerColor := map[string]string{"Sunflower": "Yellow", "Jasmine": "White", "Hibiscus": "Red"}

  // access value for key Sunflower
  fmt.Println(flowerColor["Sunflower"])  // Yellow

  // access value for key Hibiscus
  fmt.Println(flowerColor["Hibiscus"])  // Red

}

Here, we have used expressions

• flowerColor["Sunflower"] to access the value of the key Sunflower
• flowerColor["Hibiscus"] to access the value of the key Hibiscus

Change value of a map in Golang

To change the value of a map, we can directly assign a new value to the corresponding key. For example,

package main
import "fmt"

func main() {

  // create a map
  capital := map[string]string{ "Nepal": "Kathmandu", "US": "New York"}
  fmt.Println("Initial Map: ", capital)

  // change value of US to Washington DC
  capital["US"] = "Washington DC"

  fmt.Println("Updated Map: ", capital)
}

Output

Initial Map:  map[Nepal: Kathmandu US: New York]
Updated Map:  map[Nepal: Kathmandu US: Washington DC]

In the above example, we have changed the value of a map by re-assigning the key "US" with a new value "Washington DC"

Add Element of Go Map Element

So far, we have created a map with a predefined set of elements. However, we can also add elements to a map.

To add an element, we can assign a new value along with a new key. For example,

package main
import "fmt"

func main() {

  // create a map
  students := map[int]string{1: "John", 2: "Lily"}
  fmt.Println("Initial Map: ", students)

  // add element with key 3
  students[3] = "Robin"

  // add element with key 5
  students[5] = "Julie"

  fmt.Println("Updated Map: ", students)
}

Output

Initial Map:  map[1:John 2:Lily]
Updated Map:  map[1:John 2:Lily 3:Robin 5:Julie]

Here, the code

• students[3] = "Robin" - adds a new element with key 3 and value Robin
• students[5] = "Julie" - adds a new element with key 5 and value Julie

Delete Element of Go Map Element

To delete an element of the map, we can use the delete() function. For example,

package main
import "fmt"

func main() {

  // create a map
  personAge := map[string]int{"Hermione": 21, "Harry": 20, "John": 25}
  fmt.Println("Initial Map: ", personAge)

  // remove element of map with key John
  delete(personAge, "John")

  fmt.Println("Updated Map: ", personAge)
}

Output

Initial Map:  map[Harry:20 Hermione:21 John:25]
Updated Map:  map[Harry:20 Hermione:21]

Here, we have used the delete() function to remove the element denoted by the key "John".

delete(personAge, "John")

The function takes two arguments:

• personAge - name of the map
• John - key of the element which is to be deleted

Note: If the key passed to the delete() function is not present inside the map, the function does nothing.

Looping through the map in Golang

We can use a Go for range loop to iterate through each element of the map. For example,

package main
import "fmt"

func main() {

  // create a map
  squaredNumber := map[int]int{2: 4, 3: 9, 4: 16, 5: 25}

  // for-range loop to iterate through each key-value of map
  for number, squared := range squaredNumber {
    fmt.Printf("Square of %d is %d\n", number, squared)
  }

}

Output

Square of 2 is 4
Square of 3 is 9
Square of 4 is 16
Square of 5 is 25

In the above code, the for range loop accesses each key/value pair of the map.

Working of the for range loop

Note: We have used the Printf() function to make our output look much cleaner and understandable.

package main
import "fmt"

func main() {

  // create a map using make()
  student := make(map[int]string)

  // add elements to the map
  student[1] = "Harry"
  student[2] = "Lilly"
  student[5] = "Harmonie"

  fmt.Println(student)
}

map[1:Harry 2:Lilly 5:Harmonie]

package main
import "fmt"

func main() {

  // create a map
  places := map[string]string{"Nepal": "Kathmandu", "US": "Washington DC", "Norway": "Oslo"}

  // access only the keys of the map
  for country := range places {
    fmt.Println(country)
  }

}

Nepal
US
Norway

for country := range places {

package main
import "fmt"

func main() {
  places := map[string]string{"Nepal": "Kathmandu", "US": "Washington DC", "Norway": "Oslo"}

  for _, capital := range places {
    fmt.Println(capital)
  }

}

Kathmandu
Washington DC
Oslo

places := map[string]string{"Nepal": "Kathmandu", "US": "Washington DC", "Norway": "Oslo"}

// country variable is declared but not used
for country, capital := range places {
  // throws an error
  fmt.Println(capital)
}