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
http://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 prog/tour/pointers.go

* Structs

A `struct` is a collection of fields.

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

.play prog/tour/structs.go

* Struct Fields

Struct fields are accessed using a dot.

.play prog/tour/struct-fields.go

* Pointers to structs

Struct fields can be accessed through a struct pointer.

The indirection through the pointer is transparent.

.play prog/tour/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 prog/tour/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 prog/tour/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`.

.play prog/tour/slices.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 prog/tour/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 prog/tour/making-slices.go

* Nil slices

The zero value of a slice is `nil`.

A nil slice has a length and capacity of 0.

(To learn more about slices, read the [[http://golang.org/doc/articles/slices_usage_and_internals.html][Slices: usage and internals]] article.)

.play prog/tour/nil-slices.go

* Range

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

.play prog/tour/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 prog/tour/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` (to compute the latter function, use [[http://golang.org/pkg/math/#Pow][`math.Pow`]]).

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

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

.play prog/tour/exercise-slices.go

* Maps

A map maps keys to values.

Maps must be created with `make` (not `new`) before use; the `nil` map is empty and cannot be assigned to.

.play prog/tour/maps.go

* Map literals

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

.play prog/tour/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 prog/tour/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` and `elem` is the zero value for the map's element type.

Similarly, when reading from a map if the key is not present the result is the zero value for the map's element type.

.play prog/tour/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 [[http://golang.org/pkg/strings/#Fields][strings.Fields]] helpful.

.play prog/tour/exercise-maps.go

* Function values

Functions are values too.

.play prog/tour/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 prog/tour/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 fibonacci numbers.

.play prog/tour/exercise-fibonacci-closure.go


* Advanced Exercise: Complex cube roots

Let's explore Go's built-in support for complex numbers via the `complex64` and `complex128` types. For cube roots, Newton's method amounts to repeating:

.image /content/img/newton3.png

Find the cube root of 8, just to make sure the algorithm works. (There is a [[http://golang.org/pkg/math/cmplx/#Pow][Pow]] function in the `math/cmplx` package to check your results.)

.play prog/tour/advanced-exercise-complex-cube-roots.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]].