GopherForgeA WORKBENCH FOR GO

Slices, maps and strings · 3 MIN READ

Reading a missing key is fine, writing to a nil map is not

Use comma-ok, and know which map operation panics.

Reading a key that is absent returns the value type's zero value, with no error. That is why comma-ok exists: v, ok := m[k] distinguishes "absent" from "present and zero".

The asymmetry to remember: reading from a nil map is legal and returns zero values, but writing to one panics. A map must be made before it can be written.

SEE THE IDEA

Reading and writing have different requirements

  1. 01

    Nil map

    go
    var m map[string]int

    Reads return the value type zero

  2. 02

    Absent key

    go
    m["missing"]

    Also returns zero

  3. 03

    Allocated map

    go
    m = make(map[string]int)

    Writes are now permitted

A safe read does not imply that a map is allocated for writes.

In practice

Initialize a map at the point where a function takes responsibility for storing entries. Use the two-result lookup when an absent key must be distinguished from a stored zero. Deleting a key from a nil map is safe, but assignment to a nil map panics.

Watch out

Maps are not safe for unsynchronized concurrent reads and writes. Protect access or transfer ownership.

Official reference ↗

Work through the exercise

go
package main

// countWords returns how many times each word appears.
func countWords(words []string) map[string]int {
	var counts map[string]int
	for _, word := range words {
		counts[word]++
	}
	return counts
}

func main() {}
Try it yourself

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