Go Maps

External

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

Internal

• Built-in Types
• Arrays
• Slices

Overview

A map is an unordered collection of key-value pairs. The map implementation uses the key as an index, quickly retrieving the associated value.

The map key can be a value from any built-in or struct type as long as the value can be used in an expression with an == operator. Slices, functions and struct types that contain slices can't be used as map keys, an attempt will generate a compiler error.

Lexically, a map type is a reference type. The map instances must be initialized before attempting to use into them, either by using the make() function or a map literal, otherwise you will get a runtime error, because a zero value for a map is nil:

panic: assignment to entry in nil map

Implementation Details

In order to store a key/value pair in the map, the map hash function is applied to the key, which produces the index of a bucket. The purpose of the hash function is to generate the index that is evenly distributed across the bucket population. Once a bucket is selected, the map stores the key/value pair in the bucket.

In order to retrieve the value corresponding to a key, the same hash function is applied to select the bucket, and then the map iterates over the keys stored in that bucket.

Declaration

Long Declaration

var map_identifier map[key_type]value_type

Example of a map of string to ints:

var m map[string]int

A map declaration and initialization with a map literal:

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

A map declaration and initialization with make():

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

Short Declaration

Short Declaration with Literals

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

Using a literal is the idiomatic way to create a map.

Short Declaration with make()

m := make(map[string]string)

More about make() is available here.

nil Maps and Empty Maps

Declaring a map without initializing it creates a nil map - the map variable is initialized with nil. This is common behavior for reference types:

var m map[string]string

For a nil slice, fmt.Printf("%p", s) prints 0x0. The underlying data structures are initialized as follows: the array pointer is initialized to nil, the length is initialized to 0 and the capacity is initialized to 0.

A nil slice is different from an empty slice:

s := []int{}

s := make([]int, 0)

For an empty slice, fmt.Printf("%p", s) prints a pointer: 0x19d730. An empty slice points to a zero length array. The length is initialized to 0 and the capacity is initialized to 0.

Regardless of whether a nil or empty slice is used, the built-in functions append(), len() and cap() work the same.

Map Operators and Functions

Indexing Operator

Assigning a key/value pair to the map is performed using the indexing [] and the assignment operators =:

m["something"] = "something else"

Indexing operator [] returns the copy of 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 ...
}

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.