Go from zero: make the map before you write

A map is a set of keys and values. A menu is a map from the drink's name to its price in cents. The zero value of a map is nil. Reading a missing key from a nil map returns the zero of the value type. Writing to a nil map panics. The tip is short: make the menu before anyone writes a price into it.

A nil map can be read and must not be written#

The zero of a map is nil, and nil is not an empty menu. A read of a missing key returns the zero of the value type, so a nil map answers 0 cents for a drink and does not panic. A write needs a header that points at buckets. A nil map has no header, so the write stops the program. map[string]int{} allocates that header and stores no pairs. After that, a missing drink is still 0, which is why a two-result read exists when 0 is also a legal stored price.

1. menu["latte"] = 450nil mappanic

package main

import "fmt"

func main() {
    menu := map[string]int{}
    menu["latte"] = 450
    fmt.Println(menu["latte"])
}

450

The braces make the menu#

map[string]int{} is a map from string keys to int values, and the {} makes it. It starts empty, which is different from nil. menu["latte"] = 450 is a write. menu["latte"] is a read, and it prints 450, the cents you would charge.

var menu map[string]int with no {} is nil. The line menu["latte"] = 450 then stops the program with a panic: assignment to entry in nil map. The panic names the line of the write. The fix is the {} you see here, or make(map[string]int). Both give you a menu that can take a price.

A missing drink is still a zero#

After the map exists, menu["missing"] is 0. That 0 is the zero of int, the same zero as the previous lesson. It does not tell you whether the drink was stored at 0 cents or never stored. When that difference matters, use the two-result read: cents, ok := menu["latte"]. ok is true when the drink is on the menu. The next lesson puts a price in a struct and changes it through a method.