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Go Tutorial · Data Structures · Lesson 14 of 30

Go Maps

Key-value maps: create, read, check existence, delete and iterate.

Creating and using maps

Example 1
package main

import (
	"fmt"
	"strings"
)

func main() {
	stock := map[string]int{"apples": 5, "pears": 2}
	stock["plums"] = 12
	stock["apples"] += 3
	delete(stock, "pears")
	fmt.Println(stock["apples"], len(stock))

	counts := make(map[string]int)
	for _, w := range strings.Fields("the cat and the hat") {
		counts[w]++
	}
	fmt.Println(counts["the"], counts["dog"])
}

Output

8 2
2 0

Run this on your computer — see “Get Started” for setup.

Reading a missing key returns the zero value. Use the "comma ok" form to tell "missing" from "zero":

Example 2
package main

import "fmt"

func main() {
	ages := map[string]int{"ada": 36, "baby": 0}
	if age, ok := ages["baby"]; ok {
		fmt.Println("baby is", age)
	}
	if _, ok := ages["lin"]; !ok {
		fmt.Println("lin not found")
	}
}

Output

baby is 0
lin not found

Run this on your computer — see “Get Started” for setup.

Iterating in a stable order

Map iteration order is randomised on purpose. Sort the keys when output order matters:

Example 3
package main

import (
	"fmt"
	"slices"
)

func main() {
	prices := map[string]float64{"tea": 2.5, "coffee": 3, "cake": 4.25}
	keys := make([]string, 0, len(prices))
	for k := range prices {
		keys = append(keys, k)
	}
	slices.Sort(keys) // Go 1.23+: keys := slices.Sorted(maps.Keys(prices))
	for _, k := range keys {
		fmt.Printf("%-7s %.2f\n", k, prices[k])
	}
}

Output

cake    4.25
coffee  3.00
tea     2.50

Run this on your computer — see “Get Started” for setup.

A nil map can be read but panics when written — always create maps with make or a literal.

Test yourself

How do you check whether a key exists in a map?

  1. m.has(key)
  2. v, ok := m[key]
  3. key in m
  4. m[key] != nil
Show answer

Answer: v, ok := m[key]

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