Creating Maps

You can create maps with literals or with make. A nil map can be read safely but cannot receive a new entry.

Use make(map[string]int) for an empty, writable map. A map literal is convenient when starting data is known. Declaring var counts map[string]int without initialization produces a nil map.

Reading a missing key from either a nil or non-nil map returns the value type’s zero value. Writing into a nil map causes a panic. make optionally accepts a capacity hint, but this is not a fixed size or indexable capacity property.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func main() {
	var missing map[string]int
	fmt.Println("Nil map:", missing == nil)
	fmt.Println("Nil lookup:", missing["x"])
	counts := make(map[string]int)
	counts["apple"] = 2
	counts["banana"] = 3
	fmt.Println("Apple:", counts["apple"])
	fmt.Println("Count:", len(counts))
}

Output

Nil map: true
Nil lookup: 0
Apple: 2
Count: 2

The uninitialized map is nil, and reading "x" returns the zero int value 0. The map created with make is writable, so the two assignments succeed and its length becomes two.

• Do not assign m[key] = value while m is nil.
• A make capacity hint does not establish a fixed maximum number of map entries.

Declare a nil map, initialize it with make, and store three item counts. Check len after every insert.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Creating Maps in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.