Software development

Working with Maps in Go

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In Go, a map is a data structure allowing you to store pairs of keys and values, while using the key to look up more complex data.

A few examples of maps, and a comparison to arrays/slices.

In other languages, you’ll find similar data structures called dictionary, hashtable, hashmap or even object. the ability to associate keys and values and the ability to perform this lookup very quickly makes maps extremely useful for many different applications, such as those in the image above and the list below.

  • Storing/retrieving customer data based on a government-issued ID number.
  • Translating English text to Morse code
  • Actual dictionaries, mapping words to a description, or words in one language to the equivalent in another
  • A telephone directory, associating names with telephone numbers
  • Storing product prices based on their barcodes
  • Grouping properties for a specific object (this can also be done with structs in Go, but only when all the properties are well-defined in advance)

The careful reader will note that arrays and slices have a similar capability of associating an index (key) with a value. However, there are two important differences:

  • Arrays and slices can only take zero or positive integer keys. Maps can use a wider variety of data types as keys, with strings being a popular choice.
  • Map keys have no particular order. Although they can be integers (similar to arrays and slices), they can be negative or have gaps (such as the “square roots” example in the image above).

Initialising Maps

A map is a generic data type so you need to decide the data type of the keys and values. (Interestingly, although general-purpose support for generics was added to the language as recently as 2022, built-in data structures such as arrays, slices and maps have been generic all along.) For instance, you can declare a map of string to string this way:

domainToCountry := map[string]string{}

Alternatively, you can use the built-in make() function. There’s no real difference between the two approaches if you’re declaring an empty map.

domainToCountry := make(map[string]string)

The map data type takes the form map[key]value. So if you want to declare a map of string to float32 instead, you do:

nameToPrice := map[string]float32{}

(Note that the use of float32 to represent money values isn’t a great idea due to floating-point error. This is just an example.)

If you want to initialise a map with data from the get-go, you initialise it with the curly brackets and add literal data between them:

	domainToCountry := map[string]string{
		"es": "Spain",
		"it": "Italy",
	}

Note that the comma is required even after the last item.

Outputting a Map

If you want to display the contents of a map for debugging or other purposes, simply dropping it into a fmt.Println() does the trick. If you want to display it as part of a format string, use the %v placeholder for the map.

package main

import "fmt"

func main() {
	domainToCountry := map[string]string{
		"es": "Spain",
		"it": "Italy",
	}
	fmt.Println(domainToCountry)
	fmt.Printf("My map: %v", domainToCountry)
}
The output displaying the map using fmt.Println() and fmt.Printf() can be seen in the Debug Console near the bottom of the window.

Getting Data from a Map

Use indexing syntax to get a value from a map by its key:

	country1, exists1 := domainToCountry["es"]
	country2, exists2 := domainToCountry["cat"]

	fmt.Printf("es: %s, %t\n", country1, exists1) // outputs: "es: Spain, true"
	fmt.Printf("es: %s, %t\n", country2, exists2) // outputs: "es: , false"

Doing this returns 2 values: the corresponding value of the key, and whether the key exists in the map. It’s a safe operation, so if the key doesn’t exist, the value returned will be the default value of the type (e.g. 0 for ints, "" for strings, etc), and the second return value will come back as false.

The second return value is in fact optional; you can omit it entirely if you just want the value back. But, it’s useful to check whether the key exists in the map, as a default value can otherwise be confused with a legit value (e.g. 0 could mean that the key isn’t in the map, or it could really be a value in the map).

country1 := domainToCountry["es"]

The first return value is also optional, so if you only care to check whether the key exists in the map, you can replace it with an underscore:

_, exists1 := domainToCountry["es"]

The existence check can also be done inline within an if statement. This has the advantage of limiting the scope of the key/value variables to the scope of the if statement, limiting the potential for accidental and erroneous usage in longer functions:

	if country1, exists1 := domainToCountry["es"]; exists1 {
		fmt.Printf("The entry for %s exists. Let's do something with it!\n", country1)
	}

Inserting/Updating Data in a Map

After a map has been initialised, you can add key-value pairs to it using indexing syntax:

domainToCountry["be"] = "Belgium"

If the key wasn’t present in the map, it gets added. If it was, then the value gets overwritten.

	domainToCountry["be"] = "Belgium"
	domainToCountry["be"] = "Belgium2"
	fmt.Println(domainToCountry) // outputs "map[be:Belgium2 es:Spain it:Italy]"

Removing Data from a Map

Use the built-in delete() function to remove a key and its corresponding value from the map. This function is safe and will do nothing if the key is not in the map.

	delete(domainToCountry, "be") // removed
	delete(domainToCountry, "aaa") // wasn't there, so no-op

Length of a Map

Use the built-in len() function to check how many keys are present in the map.

	length := len(domainToCountry)
	fmt.Println(length) // outputs 2

Iterating over a Map

Use a for ... range loop to iterate over the keys and/or values of a map:

	for domain, country := range domainToCountry {
		fmt.Printf("Extension %s belongs to %s\n", domain, country)
	}

The output of the above snippet would be:

Extension es belongs to Spain
Extension it belongs to Italy

Both the key and value are optional. If you want just the key, simply omit the value:

	for domain := range domainToCountry {
		fmt.Println(domain)
	}

Whereas if you just want the value, replace the key with an underscore:

	for _, country := range domainToCountry {
		fmt.Println(country)
	}

You could also omit both, but that’s not usually very useful:

	for range domainToCountry {
		fmt.Println("I don't know why I'm iterating over a map if I don't use its data")
	}

It’s important to note that when iterating over a map, there’s no clearly-defined order as there is in arrays and slices. If you iterate over the same map multiple times, don’t expect to see the data come out in the same order each time.

Iterating over the same map multiple times produces differently ordered results.

Clearing a Map

To delete all items from a map, all you need to do is re-initialise it. The memory used by the old keys and values will be freed when the garbage collector kicks in.

package main

import "fmt"

func main() {
	domainToCountry := map[string]string{
		"es": "Spain",
		"it": "Italy",
	}

	domainToCountry = map[string]string{} // clear map

	fmt.Println(domainToCountry) // outputs "map[]"
}

A Map of Slices

Now that we’ve covered basic usage of maps, let’s consider a few more elaborate scenarios. For starters, how do we store multiple values for each key? For instance, we want to create a telephone directory (name to telephone number) and each person can have multiple numbers. For that, we can use a map of string to slice of string ([]string):

telephoneDirectory := map[string][]string{}

Note that, as I wrote in “From .NET to GoLang: Where Did Everything Go?“, the map syntax starts to be very confusing when you go beyond maps of simple types, due to overuse of square brackets. Note also that I’m opting to use strings to represent telephone numbers because the latter sometimes have length or characters that integer data types can’t handle.

When we add entries to our directory, we have to be careful to check whether a list of numbers already exists for that particular name. If it does, we add to it; otherwise we initialise a new one.

package main

import "fmt"

func addToDirectory(telephoneDirectory map[string][]string, name, telephoneNumber string) {
	if _, exists := telephoneDirectory[name]; !exists {
		telephoneDirectory[name] = []string{}
	}

	telephoneDirectory[name] = append(telephoneDirectory[name], telephoneNumber)
}

func main() {
	telephoneDirectory := map[string][]string{}

	addToDirectory(telephoneDirectory, "Bob", "12345678")
	addToDirectory(telephoneDirectory, "Bob", "87654321")
	addToDirectory(telephoneDirectory, "Charlie", "20202020")

	fmt.Println(telephoneDirectory) // outputs "map[Bob:[12345678 87654321] Charlie:[20202020]]"
}

This could have been written in a few different ways, but the one I chose in this example is to use the inline existence check to initialise an empty slice of strings for the name if it isn’t found in the directory. The subsequent addition of the number to the corresponding slice thus works the same way whether the name was previously in the directory or not.

A Map of Maps

Sometimes you need multiple dimensions in a map. I don’t have a really good example for this as it’s not a very common use case unless you’re grouping a lot of data for batch processing. So I’ll just show how it’s done:

package main

import "fmt"

func main() {
	myMap := map[string]map[string]int{} // map of string -> (map of string -> int)

	myMap["John"] = map[string]int{} // initialise inner map for key "John"
	myMap["John"]["age"] = 12
	myMap["John"]["height"] = 76

	fmt.Println(myMap) // outputs "map[John:map[age:12 height:76]]"
}

This is quite similar to what we saw in the previous section: the map syntax is rather confusing, and you have to make sure to initialise the inner map properly before using it. Otherwise it starts off as nil and if you try to use it, your program will panic.

Maps of Structs

Instead of maps of maps, it’s more common to have maps of structs. That allows us to look up data records based on some kind of identifier. For instance:

package main

import "fmt"

type Product struct {
	Name  string
	Price float32
}

func main() {

	products := map[string]Product{}

	products["pen"] = Product{
		Name:  "A fine blue pen",
		Price: 12.0,
	}

	fmt.Println(products) // outputs: "map[pen:{A fine blue pen 12}]"
}

However, as I showed in “From .NET to GoLang: Here We Go Again“, there’s a nasty surprise to be seen if you try to update a struct’s field when it’s in a map:

products["pen"].Price = 15.0
Attempting to update a property in a struct in a map causes a compiler error.

This is an unfortunate peculiarity in Go resulting from the concept of addressable values. In short, because values in a map are stored by value (rather than by reference), they can’t be manipulated directly. So there are 2 ways we can carry out this update.

The first is to replace the entire struct. So:

package main

import "fmt"

type Product struct {
	Name  string
	Price float32
}

func main() {

	products := map[string]Product{}

	products["pen"] = Product{
		Name:  "A fine blue pen",
		Price: 12.0,
	}

	products["pen"] = Product{
		Name:  products["pen"].Name,
		Price: 15.0,
	}

	fmt.Println(products) // outputs "map[pen:{A fine blue pen 15}]"
}

The second is to store pointers to products, instead of products by value.

package main

import "fmt"

type Product struct {
	Name  string
	Price float32
}

func main() {

	products := map[string]*Product{}

	products["pen"] = &Product{
		Name:  "A fine blue pen",
		Price: 12.0,
	}

	products["pen"].Price = 15.0

	fmt.Println(products) // outputs "map[pen:0xc000010030]"
}

Note however that this messes up the output when we print the map, because the map is no longer storing products directly. So the value that gets printed out is the address of the Product that the pointer is pointing to.

A Set Data Structure

Go doesn’t have a set data structure (you know, the mathematical kind in which values are unique and unordered). For simple use cases like eliminating duplicates, we can emulate the behaviour of a set using a map:

package main

import "fmt"

func main() {

	numbers := []int{1, 5, 8, 1, 3, 2, 4, 5}

	deduplicated := map[int]struct{}{}

	for _, number := range numbers {
		deduplicated[number] = struct{}{}
	}

	fmt.Println(deduplicated) // outputs "map[1:{} 2:{} 3:{} 4:{} 5:{} 8:{}]"
}

What we’re doing here is creating a map where we only care about the key (and not the value). The use of an empty struct{} is a tip I picked up on Stack Overflow because it doesn’t allocate any memory (as opposed to, for example, a bool). The syntax may appear a little confusing, but when you see two pairs of curly brackets next to each other, think of struct{} as the data type and the second {} as the initialisation syntax.

So then, all we do is feed each number from the slice into the map. As we’ve seen before, assigning another value to a key that already exists will simply overwrite it, leaving a single value for the key. That’s pretty much the same functionality we need for a set.

However, a set can do much more than just deduplicate items. If you need typical set operations such as intersection, union or difference, then check out my article “GoLang Set Data Structure” which shows how to use the third-party golang-set library which should have all the features you need.

Maps and Concurrency

The 2013 official blog post about maps states clearly that maps are not thread-safe, and suggests the use of locks to prevent data races arising from concurrent access to maps.

However, a concurrent version of the map data structure was released in 2017 with Go 1.9, i.e. sync.Map. While I haven’t had the chance to explore it in detail and it’s outside the scope of this article anyway, those looking for such a thing will be pleased to note that it exists and can do the necessary research to learn how to use it.

Summary and Further Reading

The map data structure will be familiar to anyone who has used something similar in other languages. It is easy enough to work with, but does have some quirks of its own that are unique to Go.

Read more about Go maps at the following locations:

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