Golang Maps

In Go language, a map is a powerful, ingenious, and versatile data structure. Golang Maps is a collection of unordered pairs of key-value. It is widely used because it provides fast lookups and values that can retrieve, update or delete with the help of keys.

• It is a reference to a hash table.
• Due to its reference type it is inexpensive to pass, for example, for a 64-bit machine it takes 8 bytes and for a 32-bit machine, it takes 4 bytes.
• In the maps, a key must be unique and always in the type which is comparable using == operator or the type which support != operator. So, most of the built-in type can be used as a key like an int, float64, rune, string, comparable array and structure, pointer, etc. The data types like slice and noncomparable arrays and structs or the custom data types which are not comparable don’t use as a map key.
• In maps, the values are not unique like keys and can be of any type like int, float64, rune, string, pointer, reference type, map type, etc.
• The type of keys and type of values must be of the same type, different types of keys and values in the same maps are not allowed. But the type of key and the type values can differ.
• The map is also known as a hash map, hash table, unordered map, dictionary, or associative array.
• In maps, you can only add value when the map is initialized if you try to add value in the uninitialized map, then the compiler will throw an error.

How to creating and initializing Maps?

In Go language, maps can create and initialize using two different ways: 

1. Simple: In this method, you can create and initialize a map without the use of make() function:

Creating Map: You can simply create a map using the given syntax:

// An Empty map
map[Key_Type]Value_Type{}

// Map with key-value pair
map[Key_Type]Value_Type{key1: value1, ..., keyN: valueN}

Example:

var mymap map[int]string

In maps, the zero value of the map is nil and a nil map doesn’t contain any key. If you try to add a key-value pair in the nil map, then the compiler will throw runtime error. 
Initializing map using map literals: Map literal is the easiest way to initialize a map with data just simply separate the key-value pair with a colon and the last trailing colon is necessary if you do not use, then the compiler will give an error. 

Example:

Go

// Go program to illustrate how to
// create and initialize maps
package main

import "fmt"

func main() {

    // Creating and initializing empty map
    // Using var keyword
    var map_1 map[int]int

    // Checking if the map is nil or not
    if map_1 == nil {

        fmt.Println("True")
    } else {

        fmt.Println("False")
    }

    // Creating and initializing a map
    // Using shorthand declaration and
    // using map literals
    map_2 := map[int]string{

            90: "Dog",
            91: "Cat",
            92: "Cow",
            93: "Bird",
            94: "Rabbit",
    }

    fmt.Println("Map-2: ", map_2)
}

Output:

True
Map-2:  map[90:Dog 91:Cat 92:Cow 93:Bird 94:Rabbit]

2. Using make function: You can also create a map using make() function. This function is an inbuilt function and in this method, you just need to pass the type of the map and it will return an initialized map.]

Syntax:

make(map[Key_Type]Value_Type, initial_Capacity)
make(map[Key_Type]Value_Type)

Example:

Go

// Go program to illustrate how to
// create and initialize a map
// Using make() function
package main

import "fmt"

func main() {

    // Creating a map
    // Using make() function
    var My_map = make(map[float64]string)
    fmt.Println(My_map)

    // As we already know that make() function
    // always returns a map which is initialized
    // So, we can add values in it
    My_map[1.3] = "Rohit"
    My_map[1.5] = "Sumit"
    fmt.Println(My_map)
}

Output:

map[]
map[1.3:Rohit 1.5:Sumit]

Important Points 

1. How to iterate over a map?: You can iterate a map using the range for loop. The value of this loop may vary because the map is an unordered collection.

Example:

Go

// Go program to illustrate how
// to iterate the map using for
// range loop

package main

import "fmt"

// Main function
func main() {

    // Creating and initializing a map
    m_a_p := map[int]string{

        90: "Dog",
        91: "Cat",
        92: "Cow",
        93: "Bird",
        94: "Rabbit",
    }

    // Iterating map using for range loop
    for id, pet := range m_a_p {

        fmt.Println(id, pet)
    }
}

Output:

90 Dog
91 Cat
92 Cow
93 Bird
94 Rabbit

2. How to add key-value pairs in the map?: In maps, you are allowed to add key-value pairs in the initialized map using the given syntax:

map_name[key]=value

In maps, if you try to add an already existing key, then it will simply override or update the value of that key with the new value.

Example:

Go

// Go program to illustrate how to add
// a key-value pair in the map using
// make() function
package main

import "fmt"

// Main function
func main() {

    // Creating and initializing a map
    m_a_p := map[int]string{
        90: "Dog",
        91: "Cat",
        92: "Cow",
        93: "Bird",
        94: "Rabbit",
    }

    fmt.Println("Original map: ", m_a_p)

    // Adding new key-value pairs in the map
    m_a_p[95] = "Parrot"
    m_a_p[96] = "Crow"
    fmt.Println("Map after adding new key-value pair:\n", m_a_p)

    // Updating values of the map
    m_a_p[91] = "PIG"
    m_a_p[93] = "DONKEY"
    fmt.Println("\nMap after updating values of the map:\n", m_a_p)
}

Output:

Original map:  map[90:Dog 91:Cat 92:Cow 93:Bird 94:Rabbit]
Map after adding new key-value pair:
 map[90:Dog 91:Cat 92:Cow 93:Bird 94:Rabbit 95:Parrot 96:Crow]

Map after updating values of the map:
 map[90:Dog 91:PIG 92:Cow 93:DONKEY 94:Rabbit 95:Parrot 96:Crow]

3. How to retrieve a value related to a key in the maps?: In maps, you can retrieve a value with the help of key using the following syntax:

map_name[key]

If the key doesn’t exist in the given map, then it will return zero value of the map, i.e, nil. And if the key exists in the given map, then it will return the value related to that key.

Example:

Go

// Go program to illustrate how to
// retrieve the value of the key

package main

import "fmt"

// Main function
func main() {

    // Creating and initializing a map
    m_a_p := map[int]string{
        90: "Dog",
        91: "Cat",
        92: "Cow",
        93: "Bird",
        94: "Rabbit",
    }
    fmt.Println("Original map: ", m_a_p)

    // Retrieving values with the help of keys
    value_1 := m_a_p[90]
    value_2 := m_a_p[93]
    fmt.Println("Value of key[90]: ", value_1)
    fmt.Println("Value of key[93]: ", value_2)
}

Output:

Original map:  map[90:Dog 91:Cat 92:Cow 93:Bird 94:Rabbit]
Value of key[90]:  Dog
Value of key[93]:  Bird

4. How to check the existence of the key in the map?: In maps, you can check whether the given key exists or not using the following syntax:

// With value
// It will gives the value and check result
value, check_variable_name:= map_name[key]

or

// Without value using the blank identifier
// It will only give check result
_, check_variable_name:= map_name[key]

Here, if the value of the check_variable_name is true which means the key exists in the given map and if the value of check_variable_name is false which means the key does not exist in the given map. 

Example:

Go

// Go program to illustrate how to
// check the key is available or not

package main

import "fmt"

// Main function
func main() {

    // Creating and initializing a map
    m_a_p := map[int]string{
        90: "Dog",
        91: "Cat",
        92: "Cow",
        93: "Bird",
        94: "Rabbit",
    }

    fmt.Println("Original map: ", m_a_p)

    // Checking the key is available
    // or not in the m_a_p map
    pet_name, ok := m_a_p[90]
    fmt.Println("\nKey present or not:", ok)
    fmt.Println("Value:", pet_name)

    // Using blank identifier
    _, ok1 := m_a_p[92]
    fmt.Println("\nKey present or not:", ok1)
}

Output:

Original map:  map[90:Dog 91:Cat 92:Cow 93:Bird 94:Rabbit]

Key present or not: true
Value: Dog

Key present or not: true

5. How to delete key from the map?: In maps, you are allowed to delete the key present in the map using the delete() function. It is inbuilt function and does not return any value and does not do anything if the key does not present in the given map. In this function, you just simply pass the map and key which you want to delete from the map.

Syntax:

delete(map_name, key)

Example:

Go

// Go program to illustrate how to delete a key

package main

import "fmt"

// Main function
func main() {

    // Creating and initializing a map
    m_a_p := map[int]string{
        90: "Dog",
        91: "Cat",
        92: "Cow",
        93: "Bird",
        94: "Rabbit",
    }

    fmt.Println("Original map: ", m_a_p)

    // Deleting keys
    // Using delete function
    delete(m_a_p, 90)
    delete(m_a_p, 93)

    fmt.Println("Map after deletion: ", m_a_p)
}

Output:

Original map:  map[90:Dog 91:Cat 92:Cow 93:Bird 94:Rabbit]
Map after deletion:  map[91:Cat 92:Cow 94:Rabbit]

6. Modifying map: As we know that maps are of reference type. So, when we assign an existing map to a new variable, both the maps still refer to the same underlying data structure. So, when we update one map it will reflect in another map.

Example:

Go

// Go program to illustrate the
// modification concept in map

package main

import "fmt"

// Main function
func main() {

    // Creating and initializing a map
    m_a_p := map[int]string{
        90: "Dog",
        91: "Cat",
        92: "Cow",
        93: "Bird",
        94: "Rabbit",
    }
    fmt.Println("Original map: ", m_a_p)

    // Assigned the map into a new variable
    new_map := m_a_p

    // Perform modification in new_map
    new_map[96] = "Parrot"
    new_map[98] = "Pig"

    // Display after modification
    fmt.Println("New map: ", new_map)
    fmt.Println("\nModification done in old map:\n", m_a_p)
}

Output:

Original map:  map[90:Dog 91:Cat 92:Cow 93:Bird 94:Rabbit]
New map:  map[90:Dog 91:Cat 92:Cow 93:Bird 94:Rabbit 96:Parrot 98:Pig]

Modification done in old map:
 map[90:Dog 91:Cat 92:Cow 93:Bird 94:Rabbit 96:Parrot 98:Pig]