Go/Maps

Go maps are data structures that use a hash table under the hood to provide fast, O(1) lookup of key-value pairs.

Basics

Initialization

To declare a map type, use the notation:

map[KeyType]ValueType

Example: declare a variable that is a map of string keys to integer values:

var m map[string]int

Like pointers/slices, map types are reference types, so if they are not initialized with a value, they have a nil value. A nil map has not been initialized and so has no underlying space in memory, so you cannot write to a nil map.

To initialize space for a map:

m = make(map[string]int)

The make function allocates space in memory for the hash table. It returns a map value pointing to this location in memory. It is important to note that the actual implementation of a map is not specified by the language, so it depends on the implementation of the Go runtime being used.

To initialize with literal data, use brackets:

commits := map[string]int{
  "asdf": 10,
  "qwerty": 20,
}

This also gives a shorter way of creating maps than the make function:

commits := map[string]int{}

Working with maps

To set a key value pair:

m["route"] = 66

To look up a key and assign the value to a variable:

i := m["route"]

Unlike Python, if a key does not exist, Go will return the value type's zero value. For integers, the value is 0; for strings, empty string; for booleans, False; etc.

j := m["root"]
// j == 0

Two useful built-in functions are length (len) and delete (delete):

// number of items in a map
n := len(m)

// remove the specified key from the map
delete(m, "route")

Like Pyhton, Go supports the two-value assignment format:

i, ok := m["route"]

i is the value in the map if the key exists (zero value otherwise), ok is a boolean indicating whether the key is in the map or not.

Also like Python, you can use underscore to skip assignment of one of the multi-assignment values. To check for existence of a key:

_, ok := m["root"]

Iteration

To iterate over a map, use the range keyword, like with lists:

for key, value := range m {
    fmt.Println("Key:", key, "Value:", value)
}