More types: structs, slices, and maps.
Learn how to define types based on existing ones: this lesson covers structs, arrays, slices, and maps.

The Go Authors
https://golang.org

* Pointers

Go has pointers.
A pointer holds the memory address of a variable.

The type `*T` is a pointer to a `T` value. Its zero value is `nil`.

	var p *int

The `&` operator generates a pointer to its operand.

	i := 42
	p = &i

The `*` operator denotes the pointer's underlying value.

	fmt.Println(*p) // read i through the pointer p
	*p = 21         // set i through the pointer p

This is known as "dereferencing" or "indirecting".

Unlike C, Go has no pointer arithmetic.

.play moretypes/pointers.go

* Structs

A `struct` is a collection of fields.

(And a `type` declaration does what you'd expect.)

.play moretypes/structs.go

* Struct Fields

Struct fields are accessed using a dot.

.play moretypes/struct-fields.go

* Pointers to structs

Struct fields can be accessed through a struct pointer.

To access the field `X` of a struct when we have the struct pointer `p` we could
write `(*p).X`.
However, that notation is cumbersome, so the language permits us instead to
write just `p.X`, without the explicit dereference.

.play moretypes/struct-pointers.go

* Struct Literals

A struct literal denotes a newly allocated struct value by listing the values of its fields.

You can list just a subset of fields by using the `Name:` syntax. (And the order of named fields is irrelevant.)

The special prefix `&` returns a pointer to the struct value.

.play moretypes/struct-literals.go

* Arrays

The type `[n]T` is an array of `n` values of type `T`.

The expression

	var a [10]int

declares a variable `a` as an array of ten integers.

An array's length is part of its type, so arrays cannot be resized.
This seems limiting, but don't worry;
Go provides a convenient way of working with arrays.

.play moretypes/array.go

* Slices

A slice points to an array of values and also includes a length.

`[]T` is a slice with elements of type `T`.

`len(primes)` returns the length of slice `primes`.

.play moretypes/slices.go

* Slices of Slices

Slices can contain any type, including other slices.

.play moretypes/slices-of-slice.go

* Slicing slices

Slices can be re-sliced, creating a new slice value that points to the same array.

The expression

	s[lo:hi]

evaluates to a slice of the elements from `lo` through `hi-1`, inclusive. Thus

	s[lo:lo]

is empty and

	s[lo:lo+1]

has one element.

.play moretypes/slicing-slices.go

* Making slices

Slices are created with the `make` function. It works by allocating a zeroed array and returning a slice that refers to that array:

	a := make([]int, 5)  // len(a)=5

To specify a capacity, pass a third argument to `make`:

	b := make([]int, 0, 5) // len(b)=0, cap(b)=5

	b = b[:cap(b)] // len(b)=5, cap(b)=5
	b = b[1:]      // len(b)=4, cap(b)=4

.play moretypes/making-slices.go

* Nil slices

The zero value of a slice is `nil`.

A nil slice has a length and capacity of 0.

.play moretypes/nil-slices.go

* Adding elements to a slice

It is common to append new elements to a slice, and so Go provides a built-in
`append` function. The [[https://golang.org/pkg/builtin/#append][documentation]]
of the built-in package describes `append`.

	func append(s []T, vs ...T) []T

The first parameter `s` of `append` is a slice of type `T`, and the rest are
`T` values to append to the slice.

The resulting value of `append` is a slice containing all the elements of the
original slice plus the provided values.

If the backing array of `s` is too small to fit all the given values a bigger
array will be allocated. The returned slice will point to the newly allocated
array.

(To learn more about slices, read the [[https://blog.golang.org/go-slices-usage-and-internals][Slices: usage and internals]] article.)

.play moretypes/append.go

* Range

The `range` form of the `for` loop iterates over a slice or map.

When ranging over a slice, two values are returned for each iteration.
The first is the index, and the second is a copy of the element at that index.

.play moretypes/range.go

* Range continued

You can skip the index or value by assigning to `_`.

If you only want the index, drop the ", value" entirely.

.play moretypes/range-continued.go

* Exercise: Slices

Implement `Pic`. It should return a slice of length `dy`, each element of which is a slice of `dx` 8-bit unsigned integers. When you run the program, it will display your picture, interpreting the integers as grayscale (well, bluescale) values.

The choice of image is up to you. Interesting functions include `(x+y)/2`, `x*y`, and `x^y`.

(You need to use a loop to allocate each `[]uint8` inside the `[][]uint8`.)

(Use `uint8(intValue)` to convert between types.)

.play moretypes/exercise-slices.go

* Maps

A map maps keys to values.

Maps must be initialized before use; the `nil` map is empty and cannot be written to.

The `make` function initializes an empty map of the given type.

.play moretypes/maps.go

* Map literals

Map literals are like struct literals, but the keys are required.

.play moretypes/map-literals.go

* Map literals continued

If the top-level type is just a type name, you can omit it from the elements of the literal.

.play moretypes/map-literals-continued.go

* Mutating Maps

Insert or update an element in map `m`:

	m[key] = elem

Retrieve an element:

	elem = m[key]

Delete an element:

	delete(m, key)

Test that a key is present with a two-value assignment:

	elem, ok = m[key]

If `key` is in `m`, `ok` is `true`. If not, `ok` is `false`.

If `key` is not in the map, then `elem` is the zero value for the map's element type.

_Note_: if `elem` or `ok` have not yet been declared you could use a short declaration form:

	elem, ok := m[key]

.play moretypes/mutating-maps.go

* Exercise: Maps

Implement `WordCount`.  It should return a map of the counts of each “word” in the string `s`. The `wc.Test` function runs a test suite against the provided function and prints success or failure.

You might find [[https://golang.org/pkg/strings/#Fields][strings.Fields]] helpful.

.play moretypes/exercise-maps.go

* Function values

Functions are values too. They can be passed around just like other values.

Function values may be used as function arguments and return values.

.play moretypes/function-values.go

* Function closures

Go functions may be closures. A closure is a function value that references variables from outside its body. The function may access and assign to the referenced variables; in this sense the function is "bound" to the variables.

For example, the `adder` function returns a closure. Each closure is bound to its own `sum` variable.

.play moretypes/function-closures.go

* Exercise: Fibonacci closure

Let's have some fun with functions.

Implement a `fibonacci` function that returns a function (a closure) that
returns successive [[https://en.wikipedia.org/wiki/Fibonacci_number][fibonacci numbers]]
(0, 1, 1, 2, 3, 5, ...).

.play moretypes/exercise-fibonacci-closure.go

* Congratulations!

You finished this lesson!

You can go back to the list of [[/list][modules]] to find what to learn next, or continue with the [[javascript:click('.next-page')][next lesson]].