Maps

You can find all the code for this chapter here

In arrays & slices, you saw how to store values in order. Now, we will look at a way to store items by a key and look them up quickly.

Maps allow you to store items in a manner similar to a dictionary. You can think of the key as the word and the value as the definition. And what better way is there to learn about Maps than to build our own dictionary?

First, assuming we already have some words with their definitions in the dictionary, if we search for a word, it should return the definition of it.

Write the test first

In dictionary_test.go

package main

import "testing"

func TestSearch(t *testing.T) {
	dictionary := map[string]string{"test": "this is just a test"}

	got := Search(dictionary, "test")
	want := "this is just a test"

	if got != want {
		t.Errorf("got %q want %q given, %q", got, want, "test")
	}
}

Declaring a Map is somewhat similar to an array. Except, it starts with the map keyword and requires two types. The first is the key type, which is written inside the []. The second is the value type, which goes right after the [].

The key type is special. It can only be a comparable type because without the ability to tell if 2 keys are equal, we have no way to ensure that we are getting the correct value. Comparable types are explained in depth in the language spec.

The value type, on the other hand, can be any type you want. It can even be another map.

Everything else in this test should be familiar.

Try to run the test

By running go test the compiler will fail with ./dictionary_test.go:8:9: undefined: Search.

Write the minimal amount of code for the test to run and check the output

In dictionary.go

package main

func Search(dictionary map[string]string, word string) string {
	return ""
}

Your test should now fail with a clear error message

dictionary_test.go:12: got '' want 'this is just a test' given, 'test'.

Write enough code to make it pass

func Search(dictionary map[string]string, word string) string {
	return dictionary[word]
}

Getting a value out of a Map is the same as getting a value out of Array map[key].

Refactor

func TestSearch(t *testing.T) {
	dictionary := map[string]string{"test": "this is just a test"}

	got := Search(dictionary, "test")
	want := "this is just a test"

	assertStrings(t, got, want)
}

func assertStrings(t testing.TB, got, want string) {
	t.Helper()

	if got != want {
		t.Errorf("got %q want %q", got, want)
	}
}

I decided to create an assertStrings helper to make the implementation more general.

Using a custom type

We can improve our dictionary's usage by creating a new type around map and making Search a method.

In dictionary_test.go:

func TestSearch(t *testing.T) {
	dictionary := Dictionary{"test": "this is just a test"}

	got := dictionary.Search("test")
	want := "this is just a test"

	assertStrings(t, got, want)
}

We started using the Dictionary type, which we have not defined yet. Then called Search on the Dictionary instance.

We did not need to change assertStrings.

In dictionary.go:

type Dictionary map[string]string

func (d Dictionary) Search(word string) string {
	return d[word]
}

Here we created a Dictionary type which acts as a thin wrapper around map. With the custom type defined, we can create the Search method.

Write the test first

The basic search was very easy to implement, but what will happen if we supply a word that's not in our dictionary?

We actually get nothing back. This is good because the program can continue to run, but there is a better approach. The function can report that the word is not in the dictionary. This way, the user isn't left wondering if the word doesn't exist or if there is just no definition (this might not seem very useful for a dictionary. However, it's a scenario that could be key in other usecases).

func TestSearch(t *testing.T) {
	dictionary := Dictionary{"test": "this is just a test"}

	t.Run("known word", func(t *testing.T) {
		got, _ := dictionary.Search("test")
		want := "this is just a test"

		assertStrings(t, got, want)
	})

	t.Run("unknown word", func(t *testing.T) {
		_, err := dictionary.Search("unknown")
		want := "could not find the word you were looking for"

		if err == nil {
			t.Fatal("expected to get an error.")
		}

		assertStrings(t, err.Error(), want)
	})
}

The way to handle this scenario in Go is to return a second argument which is an Error type.

Notice that as we've seen in the pointers and error section here in order to assert the error message we first check that the error is not nil and then use .Error() method to get the string which we can then pass to the assertion.

Try and run the test

This does not compile

./dictionary_test.go:18:10: assignment mismatch: 2 variables but 1 values

Write the minimal amount of code for the test to run and check the output

func (d Dictionary) Search(word string) (string, error) {
	return d[word], nil
}

Your test should now fail with a much clearer error message.

dictionary_test.go:22: expected to get an error.

Write enough code to make it pass

func (d Dictionary) Search(word string) (string, error) {
	definition, ok := d[word]
	if !ok {
		return "", errors.New("could not find the word you were looking for")
	}

	return definition, nil
}

In order to make this pass, we are using an interesting property of the map lookup. It can return 2 values. The second value is a boolean which indicates if the key was found successfully.

This property allows us to differentiate between a word that doesn't exist and a word that just doesn't have a definition.

Refactor

var ErrNotFound = errors.New("could not find the word you were looking for")

func (d Dictionary) Search(word string) (string, error) {
	definition, ok := d[word]
	if !ok {
		return "", ErrNotFound
	}

	return definition, nil
}

We can get rid of the magic error in our Search function by extracting it into a variable. This will also allow us to have a better test.

t.Run("unknown word", func(t *testing.T) {
	_, got := dictionary.Search("unknown")
	if got == nil {
		t.Fatal("expected to get an error.")
	}
	assertError(t, got, ErrNotFound)
})

func assertError(t testing.TB, got, want error) {
	t.Helper()

	if got != want {
		t.Errorf("got error %q want %q", got, want)
	}
}

By creating a new helper we were able to simplify our test, and start using our ErrNotFound variable so our test doesn't fail if we change the error text in the future.

Write the test first

We have a great way to search the dictionary. However, we have no way to add new words to our dictionary.

func TestAdd(t *testing.T) {
	dictionary := Dictionary{}
	dictionary.Add("test", "this is just a test")

	want := "this is just a test"
	got, err := dictionary.Search("test")
	if err != nil {
		t.Fatal("should find added word:", err)
	}

	assertStrings(t, got, want)
}

In this test, we are utilizing our Search function to make the validation of the dictionary a little easier.

Write the minimal amount of code for the test to run and check output

In dictionary.go

func (d Dictionary) Add(word, definition string) {
}

Your test should now fail

dictionary_test.go:31: should find added word: could not find the word you were looking for

Write enough code to make it pass

func (d Dictionary) Add(word, definition string) {
	d[word] = definition
}

Adding to a map is also similar to an array. You just need to specify a key and set it equal to a value.

Pointers, copies, et al

An interesting property of maps is that you can modify them without passing as an address to it (e.g &myMap)

This may make them feel like a "reference type", but as Dave Cheney describes they are not.

A map value is a pointer to a runtime.hmap structure.

So when you pass a map to a function/method, you are indeed copying it, but just the pointer part, not the underlying data structure that contains the data.

A gotcha with maps is that they can be a nil value. A nil map behaves like an empty map when reading, but attempts to write to a nil map will cause a runtime panic. You can read more about maps here.

Therefore, you should never initialize a nil map variable:

var m map[string]string

Instead, you can initialize an empty map or use the make keyword to create a map for you:

var dictionary = map[string]string{}

// OR

var dictionary = make(map[string]string)

Both approaches create an empty hash map and point dictionary at it. Which ensures that you will never get a runtime panic.

Refactor

There isn't much to refactor in our implementation but the test could use a little simplification.

func TestAdd(t *testing.T) {
	dictionary := Dictionary{}
	word := "test"
	definition := "this is just a test"

	dictionary.Add(word, definition)

	assertDefinition(t, dictionary, word, definition)
}

func assertDefinition(t testing.TB, dictionary Dictionary, word, definition string) {
	t.Helper()

	got, err := dictionary.Search(word)
	if err != nil {
		t.Fatal("should find added word:", err)
	}
	assertStrings(t, got, definition)
}

We made variables for word and definition, and moved the definition assertion into its own helper function.

Our Add is looking good. Except, we didn't consider what happens when the value we are trying to add already exists!

Map will not throw an error if the value already exists. Instead, they will go ahead and overwrite the value with the newly provided value. This can be convenient in practice, but makes our function name less than accurate. Add should not modify existing values. It should only add new words to our dictionary.

Write the test first

func TestAdd(t *testing.T) {
	t.Run("new word", func(t *testing.T) {
		dictionary := Dictionary{}
		word := "test"
		definition := "this is just a test"

		err := dictionary.Add(word, definition)

		assertError(t, err, nil)
		assertDefinition(t, dictionary, word, definition)
	})

	t.Run("existing word", func(t *testing.T) {
		word := "test"
		definition := "this is just a test"
		dictionary := Dictionary{word: definition}
		err := dictionary.Add(word, "new test")

		assertError(t, err, ErrWordExists)
		assertDefinition(t, dictionary, word, definition)
	})
}

For this test, we modified Add to return an error, which we are validating against a new error variable, ErrWordExists. We also modified the previous test to check for a nil error.

Try to run test

The compiler will fail because we are not returning a value for Add.

./dictionary_test.go:30:13: dictionary.Add(word, definition) used as value
./dictionary_test.go:41:13: dictionary.Add(word, "new test") used as value

Write the minimal amount of code for the test to run and check the output

In dictionary.go

var (
	ErrNotFound   = errors.New("could not find the word you were looking for")
	ErrWordExists = errors.New("cannot add word because it already exists")
)

func (d Dictionary) Add(word, definition string) error {
	d[word] = definition
	return nil
}

Now we get two more errors. We are still modifying the value, and returning a nil error.

dictionary_test.go:43: got error '%!q(<nil>)' want 'cannot add word because it already exists'
dictionary_test.go:44: got 'new test' want 'this is just a test'

Write enough code to make it pass

func (d Dictionary) Add(word, definition string) error {
	_, err := d.Search(word)

	switch err {
	case ErrNotFound:
		d[word] = definition
	case nil:
		return ErrWordExists
	default:
		return err
	}

	return nil
}

Here we are using a switch statement to match on the error. Having a switch like this provides an extra safety net, in case Search returns an error other than ErrNotFound.

Refactor

We don't have too much to refactor, but as our error usage grows we can make a few modifications.

const (
	ErrNotFound   = DictionaryErr("could not find the word you were looking for")
	ErrWordExists = DictionaryErr("cannot add word because it already exists")
)

type DictionaryErr string

func (e DictionaryErr) Error() string {
	return string(e)
}

We made the errors constant; this required us to create our own DictionaryErr type which implements the error interface. You can read more about the details in this excellent article by Dave Cheney. Simply put, it makes the errors more reusable and immutable.

Next, let's create a function to Update the definition of a word.

Write the test first

func TestUpdate(t *testing.T) {
	word := "test"
	definition := "this is just a test"
	dictionary := Dictionary{word: definition}
	newDefinition := "new definition"

	dictionary.Update(word, newDefinition)

	assertDefinition(t, dictionary, word, newDefinition)
}

Update is very closely related to Add and will be our next implementation.

Try and run the test

./dictionary_test.go:53:2: dictionary.Update undefined (type Dictionary has no field or method Update)

Write minimal amount of code for the test to run and check the failing test output

We already know how to deal with an error like this. We need to define our function.

func (d Dictionary) Update(word, definition string) {}

With that in place, we are able to see that we need to change the definition of the word.

dictionary_test.go:55: got 'this is just a test' want 'new definition'

Write enough code to make it pass

We already saw how to do this when we fixed the issue with Add. So let's implement something really similar to Add.

func (d Dictionary) Update(word, definition string) {
	d[word] = definition
}

There is no refactoring we need to do on this since it was a simple change. However, we now have the same issue as with Add. If we pass in a new word, Update will add it to the dictionary.

Write the test first

t.Run("existing word", func(t *testing.T) {
	word := "test"
	definition := "this is just a test"
	dictionary := Dictionary{word: definition}
	newDefinition := "new definition"

	err := dictionary.Update(word, newDefinition)

	assertError(t, err, nil)
	assertDefinition(t, dictionary, word, newDefinition)
})

t.Run("new word", func(t *testing.T) {
	word := "test"
	definition := "this is just a test"
	dictionary := Dictionary{}

	err := dictionary.Update(word, definition)

	assertError(t, err, ErrWordDoesNotExist)
})

We added yet another error type for when the word does not exist. We also modified Update to return an error value.

Try and run the test

./dictionary_test.go:53:16: dictionary.Update(word, newDefinition) used as value
./dictionary_test.go:64:16: dictionary.Update(word, definition) used as value
./dictionary_test.go:66:23: undefined: ErrWordDoesNotExist

We get 3 errors this time, but we know how to deal with these.

Write the minimal amount of code for the test to run and check the failing test output

const (
	ErrNotFound         = DictionaryErr("could not find the word you were looking for")
	ErrWordExists       = DictionaryErr("cannot add word because it already exists")
	ErrWordDoesNotExist = DictionaryErr("cannot update word because it does not exist")
)

func (d Dictionary) Update(word, definition string) error {
	d[word] = definition
	return nil
}

We added our own error type and are returning a nil error.

With these changes, we now get a very clear error:

dictionary_test.go:66: got error '%!q(<nil>)' want 'cannot update word because it does not exist'

Write enough code to make it pass

func (d Dictionary) Update(word, definition string) error {
	_, err := d.Search(word)

	switch err {
	case ErrNotFound:
		return ErrWordDoesNotExist
	case nil:
		d[word] = definition
	default:
		return err
	}

	return nil
}

This function looks almost identical to Add except we switched when we update the dictionary and when we return an error.

Note on declaring a new error for Update

We could reuse ErrNotFound and not add a new error. However, it is often better to have a precise error for when an update fails.

Having specific errors gives you more information about what went wrong. Here is an example in a web app:

You can redirect the user when ErrNotFound is encountered, but display an error message when ErrWordDoesNotExist is encountered.

Next, let's create a function to Delete a word in the dictionary.

Write the test first

func TestDelete(t *testing.T) {
	word := "test"
	dictionary := Dictionary{word: "test definition"}

	dictionary.Delete(word)

	_, err := dictionary.Search(word)
	assertError(t, err, ErrNotFound)
}

Our test creates a Dictionary with a word and then checks if the word has been removed.

Try to run the test

By running go test we get:

./dictionary_test.go:74:6: dictionary.Delete undefined (type Dictionary has no field or method Delete)

Write the minimal amount of code for the test to run and check the failing test output

func (d Dictionary) Delete(word string) {

}

After we add this, the test tells us we are not deleting the word.

dictionary_test.go:78: got error '%!q(<nil>)' want 'could not find the word you were looking for'

Write enough code to make it pass

func (d Dictionary) Delete(word string) {
	delete(d, word)
}

Go has a built-in function delete that works on maps. It takes two arguments. The first is the map and the second is the key to be removed.

The delete function returns nothing, and we based our Delete method on the same notion. Since deleting a value that's not there has no effect, unlike our Update and Add methods, we don't need to complicate the API with errors.

Wrapping up

In this section, we covered a lot. We made a full CRUD (Create, Read, Update and Delete) API for our dictionary. Throughout the process we learned how to:

• Create maps

• Search for items in maps

• Add new items to maps

• Update items in maps

• Delete items from a map

• Learned more about errors

  • How to create errors that are constants

  • Writing error wrappers