Go Academy · Lesson

Maps: Key-Value Stores

Creating, reading, writing and deleting map entries

Maps: Key-Value Stores is a free Go Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What Is a Map?

A map in Go is an unordered collection of key-value pairs. Keys must be a comparable type (strings, ints, etc.). Maps are reference types backed by a hash table.

Creating Maps

Use a literal or make:

package main
import "fmt"

func main() {
    ages  := map[string]int{"Alice": 30, "Bob": 25}
    cache := make(map[string]string)
    fmt.Println(ages, cache)
}

Reading and Writing

Access values with m[key] and assign with m[key] = value. Reading a missing key returns the zero value:

package main
import "fmt"

func main() {
    m := map[string]int{}
    m["hits"] = 10
    m["hits"]++
    fmt.Println(m["hits"])    // 11
    fmt.Println(m["missing"]) // 0
}

Checking Key Existence

The two-value form distinguishes a missing key from a zero value:

package main
import "fmt"

func main() {
    m := map[string]int{"a": 0}
    v, ok := m["a"]
    fmt.Println(v, ok) // 0 true
    v2, ok2 := m["z"]
    fmt.Println(v2, ok2) // 0 false
}

Deleting Entries

Use the built-in delete function. Deleting a non-existent key is a no-op:

package main
import "fmt"

func main() {
    m := map[string]int{"x": 1, "y": 2}
    delete(m, "x")
    delete(m, "z") // no-op
    fmt.Println(m) // map[y:2]
}

Iterating a Map

Use range to iterate. Order is not guaranteed:

package main
import "fmt"

func main() {
    scores := map[string]int{"Alice": 90, "Bob": 85}
    for name, score := range scores {
        fmt.Printf("%s: %d\n", name, score)
    }
}

Maps with Struct Values

Maps can hold structs as values. Update via a temporary variable (structs in maps are not addressable):

package main
import "fmt"

type Point struct{ X, Y int }

func main() {
    points := map[string]Point{"A": {1, 2}}
    p := points["A"]
    p.X = 99
    points["A"] = p
    fmt.Println(points["A"]) // {99 2}
}

nil Maps

A nil map reads like an empty map (zero values), but writing to a nil map panics:

package main

func main() {
    var m map[string]int
    _ = m["key"]   // ok — returns 0
    // m["key"] = 1 // PANIC: assignment to nil map
}

Maps Are Not Safe for Concurrent Use

Go's built-in map is not goroutine-safe. Concurrent reads are fine, but concurrent writes (or read+write) require a mutex or sync.Map:

package main
import "sync"

func main() {
    var mu sync.Mutex
    m := map[string]int{}
    mu.Lock()
    m["key"] = 1
    mu.Unlock()
    _ = m
}

Counting Frequencies

A classic map pattern — counting occurrences:

package main
import "fmt"

func main() {
    words := []string{"go", "is", "fast", "go", "is", "go"}
    freq := map[string]int{}
    for _, w := range words {
        freq[w]++
    }
    fmt.Println(freq) // map[fast:1 go:3 is:2]
}

Quick Check

What happens when you read a key that doesn't exist in a Go map?

Recap: Maps

Key takeaways:

Maps are unordered key-value stores
Use the two-value form to check key existence
Never write to a nil map
Iteration order is random — sort keys if needed
Protect maps with a mutex in concurrent code

Practice Prompt

Write a function that takes a string and returns a map[rune]int counting how many times each character appears. Test it with the string "hello".

Frequently asked questions

Is the “Maps: Key-Value Stores” lesson free?

Yes — the full text of “Maps: Key-Value Stores” is free to read here on the web, and the Go Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Go Academy course, upgrade to CoddyKit PRO.

What will I learn in “Maps: Key-Value Stores”?

Creating, reading, writing and deleting map entries You practise Go Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Go Academy?

No prior experience is required. Go Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Maps: Key-Value Stores” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Go Academy lesson?

Yes. Every Go Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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