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 the make() function. Note that maps must be initialized before attempting to write 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.