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