[Go] Ch1: Go Basics - 04 Go Data Structure: `Array` & `Slice`

    Go   Go  Comments

Array

1. Creating an Array

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

The expression

var a [10]int
  • An array’s length is part of its type, so arrays cannot be resized. But Go provides a convenient way of working with array.

2. Array initialize

a := [6]int{2, 3, 5, 7, 11, 13}

// or
a := []int{2, 3, 5, 7, 11, 13}

Slices

A slice is a dynamically-size, flexible view into the elements of an array.

Slice的長度是可變的，與Array相比，提供一種更高階的觀點看待一片資料。
  • Slice also has continuous segments of memory locations
  • The default value of uninitialized slice is nil
  • Slices does not store the data. It just provides reference to an array

1. Creating a Slice

Method 1: like array but no size

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

var s []int // length: 0, capacity: 0 -> nil

Method 2: use make()

Create slice by using make() which is available in builtin package of golang

s := make([]int, 5) // lenght == capacity == 5

// or
s := make([]int, 5, 10) // lenght: 5, capacity: 10

Method 3: [low:high]

A slice is formed by specifying two indices, a low and high bound, seperated by a colon:

var s []type = a[low:high]

//or
s := a[low:high]

Example:

// Elements 1 through 3
// a = [1, 2, 3, 4, 5]

//var s []int = a[1:4]
s := a[1:4] // [2, 3, 4]
  • More
// slice initialize
a := []int{2,3,5,7,11}

a1 = a[1:4] // [3 5 7]

a2 = a[:2] // [2 3]

a3 = a[1:] // [3 5 7 11]

a4 = a[:] // [2 3 5 7 11]

2. 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.
a := [6]int{1, 2, 3, 4, 5, 6}
fmt.Println(a) // [1 2 3 4 5 6]
b := a[1:4]
fmt.Println(b) // [2 3 4]
b[1] = 10
fmt.Println(b) // [2 10 4]
fmt.Println(a) // [1 2 10 4 5 6]

3. Slice length and capacity

A slice has both a length and a capacity.

  • length

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

    Number of elements

  • capacity

    The capacity of a slice is the number of elements in the underlying array

    Total size

// length
len(s)

// capacity
cap(s)

4. Slices of slices

Slices can contain any type, including other slices

Example: 2D Array

3x3 Array

matrix := [][]int{
    make([]int, 3),
    make([]int, 3),
    make([]int, 3),
}

tic-tac-toe boart(井字遊戲)

board := [][]string{
    []string{"-", "-", "-"},
    []string{"-", "-", "-"},
    []string{"-", "-", "-"},
}

5. append function

Appending new values to the slice using built-in append function.

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

  • 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.

    當append的元素個數超過原slice的capacity時，append利用現有的slice建立一個新的slice，並將後續的參數附加在這個slice之後。

var s []int
s = append(s, 0)

6. copy function

Copying one slice into another using built-in copy function.

func copy(dst []T, src []T) int
  • copy will return the number of elements copied which is the minimum of len(dst) and len(src).
  • 複製時，目的slice的容量必須足夠，否則會發生 cap out of range的錯誤，copy函式若執行成功，會傳回複製的元素個數。

s1 := []int{1,2,3}
s2 := make([]int, 2)
copy(s1, s2)

7. range function

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.

  1. index
  2. copy of the element at that index.
var a = []int{1,2,3,4,5}

for i, v := range a {
    fmt.Printf("a[%d]: %d\n", i, v)
}

Skip the index or value

Can skip the index or value by assigning to _

for i, _ := range array
for _, v := range array

If only want the index, can omit the second variable.

for i := range array

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.

func Pic(dx, dy int) [][]uint8 {
    image := make([][]uint8, dy)
    for i := range image {
        image[i] = make([]uint8, dx)
        for j := range image[i] {
            image[i][j] = uint8((i+j) / 2)
        }
    }

    return image
}

Kaka LinSoftware Engineer

AIoT