Go Maps

Declaring and Creating Maps

Maps are written as map[KeyType]ValueType. A nil map (the zero value) can be read from but panics if you try to write to it, so in practice you almost always create maps with the built-in make function or a map literal before storing anything in them.

package main

import "fmt"

func main() {
	// Using make - starts empty
	ages := make(map[string]int)
	ages["Nia"] = 24
	ages["Tom"] = 31

	// Using a map literal - pre-filled
	capitals := map[string]string{
		"France": "Paris",
		"Japan":  "Tokyo",
	}

	fmt.Println(ages)
	fmt.Println(capitals)
}

Reading Keys Safely with the Comma-ok Idiom

Looking up a missing key does not panic; it simply returns the value type's zero value, which can hide bugs if zero is also a valid value in your data. Go solves this with a special two-value form of the lookup that also tells you whether the key was actually present.

package main

import "fmt"

func main() {
	scores := map[string]int{"Ada": 95, "Lin": 88}

	value, ok := scores["Ada"]
	fmt.Println(value, ok) // 95 true

	value, ok = scores["Zed"]
	fmt.Println(value, ok) // 0 false

	if v, found := scores["Lin"]; found {
		fmt.Println("Lin scored", v)
	}
}

Deleting Keys and Iterating

The built-in delete function removes a key-value pair from a map; calling it on a key that doesn't exist is a harmless no-op. Iterating over a map with range visits every key-value pair, but Go deliberately randomizes the order on each run so you never accidentally depend on a fixed sequence.

package main

import "fmt"

func main() {
	inventory := map[string]int{"apples": 10, "pears": 4, "limes": 0}

	delete(inventory, "limes")

	total := 0
	for item, qty := range inventory {
		fmt.Println(item, "->", qty)
		total += qty
	}
	fmt.Println("total stock:", total)
}

• Always create maps with make(...) or a literal before writing to them.
• Use the comma-ok form when a missing key and a zero value need to mean different things.
• delete(m, key) is safe to call even if the key isn't present.
• Map iteration order is intentionally random - sort keys yourself if you need a stable order.
• len(m) returns the number of key-value pairs currently stored in the map.

Maps vs Slices at a Glance