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Maps

Maps are similar to JavaScript objects, Python dictionaries, and Ruby hashes. Maps are a data structure that provides key->value mapping.

The zero value of a map is nil.

We can create a map by using the make() function:

ages := make(map[string]int)
ages["John"] = 37
ages["Mary"] = 24
ages["Mary"] = 21 // overwrites 24

Or by using a literal:

ages := map[string]int{
  "John": 37,
  "Mary": 21,
}

Map values can be structs too:

type car struct {
  registration string
  model        string
}

cars := map[string]car{
  "ABC-123": {registration: "ABC-123", model: "Civic"},
}

The len() function works on a map, it returns the total number of key/value pairs.

ages := map[string]int{
  "John": 37,
  "Mary": 21,
}
fmt.Println(len(ages)) // 2

Assignment

We can speed up our contact-info lookups by using a map!

Key-based map lookup: O(1)
Slice brute-force search: O(n)

Complete the getUserMap function. It takes a slice of names and a slice of phone numbers, and returns a map of name -> user structs and an error. A user struct just contains a user's name and phone number. The first element in the names slice pairs with the first phone number, and so on.

If the length of names and phoneNumbers is not equal, return an error with the string "invalid sizes".