map-basics

Go Map

This example highlights common operations and some relevant tips on using maps in Go.

Tips and Gotchas

Getting non-existing value from map

In Go, a map returns a zero-value if key isn't found through the [key] operator. And zero value is contextual depending on the data type of the map values. If the map value is declared as a pointer, the map returns a nil, but if the map value is declared as a value, the map returns a zero value corresponding to the type.

Because of this, when checking for the existence of a key, always use the second parameter that's returned from the [key] operator to check if a boolean value if the key exists.

	type State struct {
		name string
		population int
	}

	states := map[string]State{
		"CA": State{name: "California", population: 39512223},
		"MA": State{name: "Massachusetts", population: 6892503},
	}

	state := states["WA"]
	fmt.Println(state)  // Prints {"", "", 0} - zero value

	// Better
	state, ok := states["WA"]
	fmt.Println("Does it exist? ", ok)  // Prints false

Here we declare the values of our map as pointers:

	states := map[string]*State{
		"CA": &State{name: "California", population: 39512223},
		"MA": &State{name: "Massachusetts", population: 6892503},
	}

	state := states["WA"]
	fmt.Println(state)  // Prints <nil>, the zero value is now a nil pointer

You can only add to an empty (or initialized) map

The following will cause panic: assignment to entry in nil map at runtime because the map variable is nil.

var states map[string]State
states["CA"] = State{name: "California", population: 39512223}  // Panics

We need to initialize the map before we can add data to it.

states := make(map[string]State)
// This works too: var states = map[string]State{}
states["CA"] = State{name: "California", population: 39512223}  // Works now

Setup

1. Run the program

   $ make run

Reference

• The Go Blog: Arrays, slices (and strings): The mechanics of 'append'
• The Go Blog: Go maps in action