Go Maps

External

• Go Specification - Deletion of Map elements https://golang.org/ref/spec#Deletion_of_map_elements

Internal

• Go Built-in Types

Overview

A map is an unordered collection of key-value pairs.

Lexically, a map type is a reference type and in order to get an instance of it, you need to use either the make() function or a map literal. The map instances must be initialized before attempting to use into them, otherwise you will get a runtime error on write. This is because a zero value for a map is nil Also see Assinging a Value to an Unitialized Map.

Declaration

Long Declaration

var map_identifier map[key_type]value_type

Example of a map of string to ints:

var m map[string]int

Declaration and initialization:

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

Short Declaration

m := make(map[string]string)

m := map[string]string {
  "A": "B",
  "C": "D",
}

Map Operators and Functions

Indexing Operator

Indexing operator [] returns the value corresponding to the specified key and a boolean value that says whether the key exists or not. If the key does not exist, the zero value for the value type is returned.

value := m["key"] // only the first return value can be used, as long as we are prepared to deal with the zero value
value, exists := m["key"]

Idiom:

if value, exists := m["key"]; exists {
   // it exists ...
}

Assigning a Value to an Uninitialized Map

If the indexing operator is also used to assign a value to a key for an uninitialized map, we get a runtime error:

panic: assignment to entry in nil map

Map Length

len() returns the number of keys.

delete()

Removes the element corresponding to the given key from the map:

delete(m, "something")

It is a no-op if the key does not exist.

make()

The make() function creates the map:

m := make(map[key_type]value_type)

Note that make() returns the map instance, not a pointer.