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

An array has a fixed size.
A slice, on the other hand, is a dynamically-sized,
flexible view into the elements of an array.
In practice, slices are much more common than arrays.

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

This expression creates a slice of the first five elements of the array `a`:

	a[0:5]


.play moretypes/slices.go


* Slices are like references to arrays

A slice does not store any data,
it just describes a section of an underlying array.

Changing the elements of a slice modifies the
corresponding elements of its underlying array.

Other slices that share the same underlying array will see those changes.

.play moretypes/slices-pointers.go


* Slice literals

A slice literal is like an array literal without the length.

This is an array literal:

	[3]bool{true, true, false}

And this creates the same array as above,
then builds a slice that references it:

	[]bool{true, true, false}

.play moretypes/slice-literals.go


* Slice defaults

When slicing, you may omit the high or low bounds to use their defaults instead.

The default is zero for the low bound and the length of the slice for the high bound.

For the array

	var a [10]int

these slice expressions are equivalent:

	a[0:10]
	a[:10]
	a[0:]
	a[:]

.play moretypes/slice-bounds.go


* Slice length and capacity

A slice has both a _length_ and a _capacity_.

The length of a slice is the number of elements it contains.

The capacity of a slice is the number of elements in the underlying array,
counting from the first element in the slice.

The length and capacity of a slice `s` can be obtained using the expressions
`len(s)` and `cap(s)`.

You can extend a slice's length by re-slicing it,
provided it has sufficient capacity.
Try changing one of the slice operations in the example program to extend it
beyond its capacity and see what happens.

.play moretypes/slice-len-cap.go


* Nil slices

The zero value of a slice is `nil`.

A nil slice has a length and capacity of 0
and has no underlying array.

.play moretypes/nil-slices.go


* Creating a slice with make

Slices can be created with the built-in `make` function;
this is how you create dynamically-sized arrays.

The `make` function allocates a zeroed array
and returns 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


* Slices of slices

Slices can contain any type, including other slices.

.play moretypes/slices-of-slice.go


* Appending 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.

The zero value of a map is `nil`.
A `nil` map has no keys, nor can keys be added.

The `make` function returns a map of the given type,
initialized and ready for use.

.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]].