Go Maps

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External

Internal

Overview

Maps and Pass-by-Value

Pointers to Maps

nil and Empty Map

nil Map

Empty Map

Declaration and Initialization

Initialization with make()

Initialization with a Composite Literal

Composite literals can be used for map initialization.

Long variable declaration and initialization with type inference:

var mm = map[string]int{"A": 1, "B": 2}

Short variable declaration:

mm := map[string]int{"A": 1, "B": 2}

Operators

Indexing Operator []

Values stored in a map can be referenced with the indexing operator [], using the [key] syntax.

The indexing operator returns two values: the value associated with the key, or zero-value for the type if the key does not exist in map, and a boolean that says whether the key exists or not in the map. The second returned value can be omitted, but given the fact that the indexing operator returns the zero value for the type and not nil on missing keys, it is impossible to say without it whether the key is missing, or a zero-value was associated with the key.

mm := map[string]int{"A": 1, "B": 2}
v, ok := mm["A"]
fmt.Printf("%d, %t\n", v, ok) // will display 1, true

If the key does not exist in map, the retuned value is not nil, but the zero value for the type.

The indexing operator can be used to change the value associated with a key, or to add a new key/value pair:

mm["A"] = 7
mm["C"] = 9

Map Functions

delete()

len()

len() returns the number of keys. Note that the built-in cap() function does not work on maps. Unlike slices, maps do not have the concept of capacity.

make()

new()

Iterating over a Map

TODO

Go_Maps_TODEPLETE