Introduction to Golang Maps With Examples

A map is a data structure that provides you with an unordered collection of key/value pairs (maps are also sometimes called associative arrays in Php, hash tables in Java, or dictionaries in Python). Maps are used to look up a value by its associated key. You store values into the map based on a key.

The strength of a map is its ability to retrieve data quickly based on the key. A key works like an index, pointing to the value you associate with that key.

A map is implemented using a hash table, which is providing faster lookups on the data element and you can easily retrieve a value by providing the key. Maps are unordered collections, and there's no way to predict the order in which the key/value pairs will be returned. Every iteration over a map could return a different order.

Go provides a built-in map type. In this article, we’ll learn how to use Golang’s built-in map type.

Declaring a map in Golang

A Go map type looks like the following.

syntax

var m map[KeyType]ValueType

where KeyType may be any comparable type (more on this later), and ValueType may be any type at all, including another map.

See the variable m is a map of string keys to int values.

var m map[string]int

Map types are reference types, like pointers or slices, and so the value of m above is nil.

A nil map has no keys. A nil map behaves like a blank map when reading. Any attempt to add keys to a nil map will result in a runtime error.

See the below code of nil variable in Go.

// hello.go

package main

import "fmt"

func main() {
    var app map[string]int
    fmt.Println(app)
    if app == nil {
        fmt.Println("app variable is nil")
    }
}

See the below output.

Now, let’s add the items to the nil map and see the output.

// hello.go

package main

import "fmt"

func main() {
    var app map[string]int
    app["dividend"] = 21
    fmt.Println(app)
}

Output

It is, therefore, necessary to initialize a map before adding items to it.

Initializing a map

1. Initializing a map using the built-in make() function

You can initialize a map using the built-in make() function. You just need to pass the type of the map to the make() function as in the example below. The function will return an initialized and ready to use map -

// Initializing a map using the built-in make() function
var m = make(map[string]int)

Let’s see a complete example -

package main
import "fmt"

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

	fmt.Println(m)

	if m == nil {
		fmt.Println("m is nil")
	} else {
		fmt.Println("m is not nil")
	}

	// make() function returns an initialized and ready to use map.
	// Since it is initialized, you can add new keys to it.
	m["one hundred"] = 100
	fmt.Println(m)
}

Output

map[]
m is not nil
map[one hundred:100]

2. Initializing a map using a map literal

A map literal is a very convenient way to initialize a map with some data. You just need to pass the key-value pairs separated by colon inside curly braces like this -

var m = map[string]int{
	"one": 1,
	"two": 2,
	"three": 3,
}

Note that the last trailing comma is necessary, otherwise, you’ll get a compiler error.

Let’s check out a complete example -


package main
import "fmt"

func main() {
	var m = map[string]int{
		"one":   1,
		"two":   2,
		"three": 3,
		"four":  4,
		"five":  5, // Comma is necessary
	}

	fmt.Println(m)
}

Output

map[one:1 two:2 three:3 four:4 five:5]

You can also create an empty map using a map literal by leaving the curly braces empty -

// Initialize an empty map
var m = map[string]int{}

The above statement is functionally identical to using the make() function.

Adding items (key-value pairs) to a map

You can add new items to an initialized map using the following syntax -

m[key] = value

The following example initializes a map using the make() function and adds some new items to it -

package main
import "fmt"

func main() {
	// Initializing a map
	var tinderMatch = make(map[string]string)

	// Adding keys to a map
	tinderMatch["Rajeev"] = "Nano" // Assigns the value "Nano" to the key "Rajeev"
	tinderMatch["Jack"] = "Sophia"
	tinderMatch["David"] = "Tina"

	fmt.Println(tinderMatch)

	/*
	  Adding a key that already exists will simply override
	  the existing key with the new value
	*/
	tinderMatch["Rajeev"] = "Jony"
	fmt.Println(tinderMatch)
}

Output


map[Rajeev:Nano Jack:Sophia David:Tina]
map[Rajeev:Jony Jack:Sophia David:Tina]

If you try to add a key that already exists in the map, then it will simply be overridden by the new value.

Retrieving the value associated with a given key in a map

You can retrieve the value assigned to a key in a map using the syntax m[key]. If the key exists in the map, you’ll get the assigned value. Otherwise, you’ll get the zero value of the map’s value type.

Let’s check out an example to understand this -

package main
import "fmt"

func main() {
	var personMobileNo = map[string]string{
		"John":  "+33-8273658526",
		"Steve": "+1-8579822345",
		"David": "+44-9462834443",
	}

	var mobileNo = personMobileNo["Steve"]
	fmt.Println("Steve's Mobile No : ", mobileNo)

	// If a key doesn't exist in the map, we get the zero value of the value type
	mobileNo = personMobileNo["Jack"]
	fmt.Println("Jack's Mobile No : ", mobileNo)
}

Output

Steve's Mobile No :  +1-8579822345
Jack's Mobile No :

In the above example, since the key "Jack" doesn’t exist in the map, we get the zero value of the map’s value type. Since the map’s value type is string, we get " ".

Unlike other languages, we do not get a runtime error in Golang if the key doesn’t exist in the map.

But what if you want to check for the existence of a key? In the above example, the map would return " " even if the key "Jack" existed with the value " ". So how do we distinguish between cases where a key exists with the value equal to the zero value of the value type, and the absence of a key?

Well, let’s find out.

Checking if a key exists 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 which is true if the key exists in the map, and false if it doesn’t exist.

So you can check for the existence of a key in a map by using the following two-value assignment

value, ok := m[key]

The boolean variable ok will be true if the key exists, and false otherwise.

Consider the following map for example. It maps employeeIds to names -

var employees = map[int]string{
	1001: "Rajeev",
	1002: "Sachin",
	1003: "James",
}

Accessing the key 1001 will return "Rajeev" and true, since the key 1001 exists in the map -

name, ok := employees[1001]  // "Rajeev", true

However, If you try to access a key that doesn’t exist, then the map will return an empty string "" (zero value of strings), and false -


name, ok := employees[1010]  // "", 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 -

_, ok := employees[1005]

Now let’s check out a complete example


package main
import "fmt"

func main() {
	var employees = map[int]string{
		1001: "John",
		1002: "Steve",
		1003: "Maria",
	}

	printEmployee(employees, 1001)
	printEmployee(employees, 1010)

	if isEmployeeExists(employees, 1002) {
		fmt.Println("EmployeeId 1002 found")
	}
}

func printEmployee(employees map[int]string, employeeId int) {
	if name, ok := employees[employeeId]; ok {
		fmt.Printf("name = %s, ok = %v\n", name, ok)
	} else {
		fmt.Printf("EmployeeId %d not found\n", employeeId)
	}
}

func isEmployeeExists(employees map[int]string, employeeId int) bool {
	_, ok := employees[employeeId]
	return ok
}

Output

name = Rajeev, ok = true
EmployeeId 1010 not found
EmployeeId 1002 found

In the above example, I’ve used a short declaration in the if statement to initialize the name and ok values, and then test the boolean value ok. It makes the code more concise.

Deleting a key from a map

You can delete a key from a map using the built-in delete() function. The syntax looks like this -

// Delete the `key` from the `map`
delete(map, key)

The delete() function doesn’t return any value. Also, it doesn’t do anything if the key doesn’t exist in the map.

Here is a complete example


package main

import "fmt"

func main() {
	var fileExtensions = map[string]string{
		"Python": ".py",
		"C++":    ".cpp",
		"Java":   ".java",
		"Golang": ".go",
		"Kotlin": ".kt",
	}

	fmt.Println(fileExtensions)

	delete(fileExtensions, "Kotlin")

	// delete function doesn't do anything if the key doesn't exist
	delete(fileExtensions, "Javascript")

	fmt.Println(fileExtensions)
}

Output

map[Python:.py C++:.cpp Java:.java Golang:.go Kotlin:.kt]
map[Python:.py C++:.cpp Java:.java Golang:.go]

Maps are reference types

Maps are reference types. When you assign a map to a new variable, they both refer to the same underlying data structure. Therefore changes done by one variable will be visible to the other -

package main
import "fmt"

func main() {
	var m1 = map[string]int{
		"one":   1,
		"two":   2,
		"three": 3,
		"four":  4,
		"five":  5,
	}

	var m2 = m1
	fmt.Println("m1 = ", m1)
	fmt.Println("m2 = ", m2)

	m2["ten"] = 10
	fmt.Println("\nm1 = ", m1)
	fmt.Println("m2 = ", m2)
}

Output

m1 =  map[one:1 two:2 three:3 four:4 five:5]
m2 =  map[one:1 two:2 three:3 four:4 five:5]

m1 =  map[one:1 two:2 three:3 four:4 five:5 ten:10]
m2 =  map[one:1 two:2 three:3 four:4 five:5 ten:10]

The same concept applies when you pass a map to a function. Any changes done to the map inside the function is also visible to the caller.

Iterating over a map

You can iterate over a map using range form of the for loop. It gives you the key, value pair in every iteration -

package main
import "fmt"

func main() {
	var personAge = map[string]int{
		"Rajeev": 25,
		"James":  32,
		"Sarah":  29,
	}

	for name, age := range personAge {
		fmt.Println(name, age)
	}

}

Output

James 32
Sarah 29
Rajeev 25

Note that, A map is an unordered collection and therefore the iteration order of a map is not guaranteed to be the same every time you iterate over it.

So if you run the above program multiple times, you’ll get the results in different orders.
The article ends here,Thanks for reading !

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