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course/9-maps/exercises/1-maps/readme.md

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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 a literal or by using the `make()` function:
+
+```go
+ages := make(map[string]int)
+ages["John"] = 37
+ages["Mary"] = 24
+ages["Mary"] = 21 // overwrites 24
+```
+
+```go
+ages = map[string]int{
+  "John": 37,
+  "Mary": 21,
+}
+```
+
+The `len()` function works on a map, it returns the total number of key/value pairs.
+
+```go
+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! Looking up a value in a map by its key is much faster than searching through a slice.
+
+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 potentially an `error`. A `user` struct just contains a user's name and phone number.
+
+If the length of `names` and `phoneNumbers` is not equal, return an error with the string "invalid sizes".
+
+The first name in the `names` slice matches the first phone number, and so on.