Map creation in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a map in Go?

A map in Go is a collection that stores key-value pairs. It allows you to look up a value by its key quickly, like a dictionary or a phone book.

beginner

How do you declare an empty map in Go?

You declare an empty map using the syntax: var m map[string]int or by using make: m := make(map[string]int). The second way creates an initialized map ready to use.

beginner

How do you add or update a key-value pair in a Go map?

You add or update a value by assigning it to a key: m["key"] = value. If the key exists, the value updates; if not, it adds a new pair.

intermediate

What happens if you try to read a key that does not exist in a Go map?

Reading a non-existent key returns the zero value for the map's value type. For example, if the value type is int, it returns 0. You can also check if the key exists using the two-value assignment.

intermediate

How do you check if a key exists in a Go map?

Use the two-value assignment: value, ok := m["key"]. The variable ok is true if the key exists, false otherwise.

How do you create an empty map of string keys and int values in Go?

A All of the above

B m := map[string]int{}

C var m map[string]int

D m := make(map[string]int)

All these ways create a map variable. However, only 'make' and '{}' initialize the map for use. 'var m map[string]int' declares but does not initialize the map.

What is the zero value returned when accessing a missing key in a map of type map[string]int?

A "" (empty string)

B nil

C 0

D false

For int values, the zero value is 0. So accessing a missing key returns 0.

Which syntax checks if a key exists in a Go map?

A value := m["key"]

B exists := m.contains("key")

C if m.hasKey("key")

D value, ok := m["key"]

The two-value assignment returns the value and a boolean indicating if the key exists.

What happens if you assign a value to a key in a nil map?

A It causes a runtime panic

B The map automatically initializes

C The assignment is ignored

D The key is added but value is zero

Assigning to a nil map causes a runtime panic because the map is not initialized.

How do you declare and initialize a map with some key-value pairs in one line?

A m := make(map[string]int){"a": 1, "b": 2}

B m := map[string]int{"a": 1, "b": 2}

C var m = map[string]int{}

D m := new(map[string]int)

You can declare and initialize a map with literals like map[string]int{"a": 1, "b": 2}.

Explain how to create, add, and check keys in a Go map.

Think about how you start a map, put things in it, and see if something is there.

Describe what happens when you read a key that does not exist in a Go map and how to handle it.

Remember the difference between missing keys and keys with zero values.