Go in Action 不完全阅读笔记

// declare an integer array of five elements
var array [5]int

//采用字面量创建数组
array :=[5]int{10, 20, 30, 40, 50}

// 使用...时会根据初始化的元素数量觉得数组长度
array := [...]int{10, 20, 30, 40, 50}

// 指定第一个和第二个元素， 其它元素用0填充
array := [5]int{1: 10, 2: 20}

// 定义一个数组，元素是指向整形的指针，初始化前两个指针（后面的为nil指针）
//Declare an integer pointer array of five elements.
// Initialize index 0 and 1 of the array with integer pointers.
array := [5]*int{0: new(int), 1: new(int)}
// Assign values to index 0 and 1.
*array[0] = 10
*array[1] = 20


// 相同类型的数组直接赋值
// Declare a string array of five elements.
var array1 [5]string
// Declare a second string array of five elements.
// Initialize the array with colors.
array2 := [5]string{"Red", "Blue", "Green", "Yellow", "Pink"}
// Copy the values from array2 into array1.
array1 = array2


// 当赋值的数组时指针数组时，复制的是指针的值，不是指向的元素
// Declare a string pointer array of three elements.
var array1 [3]*string
// Declare a second string pointer array of three elements.
// Initialize the array with string pointers.
array2 := [3]*string{new(string), new(string), new(string)}
// Add colors to each element
*array2[0] = "Red"
*array2[1] = "Blue"
*array2[2] = "Green"
// Copy the values from array2 into array1.
array1 = array2

//参见下面图片:
[[file:static/images/go-goinaction-ch3.png]]


// 声明和初始化多维数组
// Declare a two dimensional integer array of four elements
// by two elements.
var array [4][2]int
// Use an array literal to declare and initialize a two
// dimensional integer array.
array := [4][2]int{{10, 11}, {20, 21}, {30, 31}, {40, 41}}
// Declare and initialize index 1 and 3 of the outer array.
array := [4][2]int{1: {20, 21}, 3: {40, 41}}
// Declare and initialize individual elements of the outer
// and inner array.
array := [4][2]int{1: {0: 20}, 3: {1: 41}}


// 多维数组间也可以赋值，只要他们长度和类型相同

// Allocate an array of 8 megabytes.
var array [1e6]int

// Pass the address of the array to the function foo.
foo(&array)

// Function foo accepts a pointer to an array of one million integers.
func foo(array *[1e6]int) {
...
}

/* 1. 使用make */
// Create a slice of strings.
// Contains a length and capacity of 5 elements.
slice := make([]string, 5)

// Create a slice of integers.
// Contains a length of 3 and has a capacity of 5 elements.
slice := make([]int, 3, 5)


/* 2. 使用字面量  */
// Create a slice of strings.
// Contains a length and capacity of 5 elements.
slice := []string{"Red", "Blue", "Green", "Yellow", "Pink"}

/ Create a slice of integers.
// Contains a length and capacity of 3 elements.
slice := []int{10, 20, 30}

// Create a slice of strings.
// Initialize the 100th element with an empty string.
slice := []string{99: ""}


/* 3. WTF  使用字面量时指定长度就是创建数组，不指定就是创建slice*/
// Create an array of three integers.
array := [3]int{10, 20, 30}
// Create a slice of integers with a length and capacity of three.
slice := []int{10, 20, 30}

/* 4 nil and empty  slice */
// Create a nil slice of integers.
var slice []int

// Use make to create an empty slice of integers.
slice := make([]int, 0)
// Use a slice literal to create an empty slice of integers.
slice := []int{}

/* 1. slice切片 */

// Create a slice of integers.
// Contains a length and capacity of 5 elements.
slice := []int{10, 20, 30, 40, 50}

/ Create a new slice.
newSlice := slice[1:3]

结果参见图片:
[[file:static/images/go-goincation-ch4-02.png]]

/* 2 */
slice的length以内是可被访问的，capacity长度是用来增长的

/* 3 使用append给slice添加元素*/

// Create a slice of integers.
// Contains a length and capacity of 5 elements.
slice := []int{10, 20, 30, 40, 50}
// Create a new slice.
// Contains a length of 2 and capacity of 4 elements.
newSlice := slice[1:3]
// Allocate a new element from capacity.
// Assign the value of 60 to the new element.
newSlice = append(newSlice, 60)

模型图如下:
[[file:static/images/go-goinaction-ch4-04.png]]

当底层数组长度不够时，会新建一个更大的数组替换它，把原来的元素复制过来
增加的算法（可能会改变）是：当长度1000以内时每次增长1倍，超过1000后，每次增长25%

/* 4. 当给一个切片添加元素时，会改变底层数组，如果当两个切片共享同一底层数组时，势必会相互影响
   可以采用一种模式来避免这种互相影响
*/

// Create a slice of strings.
// Contains a length and capacity of 5 elements.
source := []string{"Apple", "Orange", "Plum", "Banana", "Grape"}
// Slice the third element and restrict the capacity.
// Contains a length and capacity of 1 element.
slice := source[2:3:3]
// Append a new string to the slice.
slice = append(slice, "Kiwi")

如图:
[[file:static/images/go-goinaction-ch4-05.png]]
切片的时候，指定低3个元素和第二个一样，这样得到的切片是容量和长度一样的，这种情况下新切片上append新元素的话
因为底层容量不够，会新创建一个底层数组，把元素复制过去，和之前的切片底层数组就可以隔离开了。


/* 5. appendke接受多个元素， 使用...表示所有元素 */

// Create two slices each initialized with two integers.
s1 := []int{1, 2}
s2 := []int{3, 4}
// Append the two slices together and display the results.
fmt.Printf("%v\n", append(s1, s2...))
Output:
[1 2 3 4]

/* 6. 遍历slice */

// 可用range配合for
// Create a slice of integers.
// Contains a length and capacity of 4 elements.
slice := []int{10, 20, 30, 40}
// Iterate over each element and display each value.
for index, value := range slice {
  fmt.Printf("Index: %d  Value: %d\n", index, value)
}

Output:
Index: 0  Value: 10
Index: 1  Value: 20
Index: 2  Value: 30
Index: 3  Value: 40

要注意的是，循环里的index 是索引位置，value是slice元素值的copy.而不是指向元素的地址， 看下面的例子
// Create a slice of integers.
// Contains a length and capacity of 4 elements.
slice := []int{10, 20, 30, 40}
// Iterate over each element and display the value and addresses.
for index, value := range slice {
   fmt.Printf("Value: %d  Value-Addr: %X  ElemAddr: %X\n",
       value, &value, &slice[index])
}
Output:
Value: 10  Value-Addr: 10500168  ElemAddr: 1052E100
Value: 20  Value-Addr: 10500168  ElemAddr: 1052E104
Value: 30  Value-Addr: 10500168  ElemAddr: 1052E108
Value: 40  Value-Addr: 10500168  ElemAddr: 1052E10C

// 通常的for循环遍 遍历切片，可以控制起始位置等

// Create a slice of integers.
// Contains a length and capacity of 4 elements.
slice := []int{10, 20, 30, 40}
// Iterate over each element starting at element 3.
for index := 2; index < len(slice); index++ {
    fmt.Printf("Index: %d  Value: %d\n", index, slice[index])
}
Output:
Index: 2  Value: 30
Index: 3  Value: 40



/* 7. 多维切片 */

// Create a slice of a slice of integers.
slice := [][]int{{10}, {100, 200}}

底层构造:
[[file:static/images/go-goinaction-ch4-07.png]]

// 同样使用append的结果

// Create a slice of a slice of integers.
slice := [][]int{{10}, {100, 200}}
// Append the value of 20 to the first slice of integers.
slice[0] = append(slice[0], 20)

底层构造:
[[file:static/images/go-goinaction-ch4-08.png]]


/* 8. 切片作为函数参数值，直接传递就好
   因为切片本身是由3部分组成: 一个addr, 一个length, 一个 Capacity，
   在64-bit架构上，每个部分各需8bytes，总共24bytes, 传递给函数是没问题的
*/

/* 使用make 或者 字面量创建 */

// Create a map with a key of type string and a value of type int.
dict := make(map[string]int)

// Create a map with a key and value of type string.
// Initialize the map with 2 key/value pairs.
dict := map[string]string{"Red": "#da1337", "Orange": "#e95a22"}

/* map的key只用能用 == 操作符比较即可。
切片，函数，struct type that contains slice 不能用作map keys, 作value没问题
*/

// Create a map using a slice of strings as the key.
dict := map[[]string]int{}
Compiler Exception:
invalid map key type []string

// Create a map using a slice of strings as the value.
dict := map[int][]string{}

/* 1. 赋值方法和python类似 */

// Create an empty map to store colors and their color codes.
colors := map[string]string{}
// Add the Red color code to the map.
colors["Red"] = "#da1337"

/* 2. nil map 赋值的时候会报运行时错误 */

// Create a nil map by just declaring the map.
var colors map[string]string
// Add the Red color code to the map.
colors["Red"] = "#da1337"
Runtime Error:
panic: runtime error: assignment to entry in nil map

/* 3. 从map 取值时， 会返回两个值，第一个是值，第二个是表示存不存在*/

// Retrieve the value for the key "Blue".
value, exists := colors["Blue"]
// Did this key exist?
if exists {
    fmt.Println(value)
}


/* 4. 当然也可以直接根据但回到值是不是零值来判断key是否存在，
当然这局限于零值不是map里的有效值
零值的意思是，比如整形是0，字符串是""等
*/

// Retrieve the value for the key "Blue".
value := colors["Blue"]
// Did this key exist?
if value != "" {
    fmt.Println(value)
}


/* 5 遍历map 用for range */

// Create a map of colors and color hex codes.
colors := map[string]string{
    "AliceBlue":   "#f0f8ff",
    "Coral":       "#ff7F50",
    "DarkGray":    "#a9a9a9",
    "ForestGreen": "#228b22",
}
// Display all the colors in the map.
for key, value := range colors {
    fmt.Printf("Key: %s  Value: %s\n", key, value)
}

/* 6. 删除key/value pair 用 delete */
// Remove the key/value pair for the key "Coral".
delete(colors, "Coral")


/* 7. 函数间传递map 也只是传递的它的引用 */