Maps

Maps map keys to values, similarly to dicts in Python, but with many more restrictions.

Quick Facts

• a map’s zero value is nil
• you cannot set/get values from nil
• len() will give number of key/value pairs
• cap() doesn’t work

Basic Usage

var sounds map[string]string
sounds = make(map[string]string)

weights := make(map[string]float64)

sounds["frog"] = "ribbit"
weight["frog"] = 2.4
fmt.Println(weights) // map[frog:2.4]

Map literals

counts := map[string]int {"frog":2, "submarine":1}
names := map[string][]string {
  "frog": []string {"jim", "fred"},
  "submarine": []string {"bob"},
}

names["frog"] // {jim, fred}
names["cat"] // nil

Working with Maps

Since exceptions do not exist in go, the , ok pattern is a common way to deal with things that would raise an exception in other languages.

, ok

counts := map[string]int{"dogs":3, "cats":0}
counts["dinosaurs"] = 10
fmt.Println(counts["dinosours"]) // 10
delete(counts, "dinosours") // remove key-value pair

counts["giraffe"] // zero value for missing keys
if val, ok := counts["giraffe"]; ok {
  // case that the key exists
} else {
  // case that the key doesn't exist
}

The , ok pattern allows you to distinguish from a zero value returned by the map, and a zero value returned because a key doesn’t exist.