Golang笔记

1. :=

:=只能用在函数体里面, 不能作为全局变量

2. 声明多个变量 | 常量

var x, y int = 100, 200

3. iota

iota用于常量递增声明, 每一行加一

4. 小写字母开头的函数为私有函数

5. import

import 导入

import (
    "Project1/lib"
    "Project1/lib2"
)

import (
    p1 "Project1/lib"
    p2 "Project1/lib2"
)

// 点不要轻易使用; . 是引入包里的所有东西, 不需要再去用lib2去点
import (
    p1 "Project1/lib"
    . "Project1/lib2"
)

6. 指针

package main

import "fmt"

func main()  {
    var a ,b int = 10, 20
    fmt.Println("a =", a, "b =", b)
    exchangeAB(&a, &b)
    fmt.Println("a =", a, "b =", b)

    var x ,y int = 10, 20
    fmt.Println("x =", x, "y =", y)
    x, y = exchangeXY(x, y)
    fmt.Println("x =", x, "y =", y)
}

//使用指针； 这种方式不需要另外开辟内存
func exchangeAB(a, b *int) {

    fmt.Println(&*a)
    fmt.Println(&*b)

    var tmp = 0
    tmp = *a
    *a = *b
    *b = tmp
}

//不使用指针， 直接值传递， 最后再赋值
func exchangeXY(a, b int) (int, int){
    var tmp = 0
    tmp = a
    a = b
    b = tmp
    return a, b
}

7. defer: 函数最后运行

package main

import "fmt"

func main(){

    // 最后执行，比return还后
    // 使用堆栈模式，先进后出，所以hahaha2先执行
    defer fmt.Println("hahaha1")
    defer fmt.Println("hahaha2")

    fmt.Println("100000")
}

# 特殊测试

func main(){

  a := 1
  defer fmt.Println("a =", a)
  a += 1
  return a
}

# 这里打印 "a = 1"

8. 数组定义

package main

import "fmt"

func main()  {
    fmt.Println(".....")

    // 固定长度
    var a [10]int
    for i:=0; i<len(a); i++{
        fmt.Println("a", a[i])
        fmt.Println("i", a[i])
    }

    // 使用range来遍历
    for _, value := range a{
        fmt.Println(value)
    }

    fmt.Println("........................")

  // 动态数组
    myArray := []int {1, 2, 3, 4}
    modifyArr(myArray)
    fmt.Println("myArray=", myArray)

}

func modifyArr(arr []int){
    arr[0] = 11111
}

9. 数组长度,容量

当数组长度等于容量的时候

继续追加元素

数组会新开辟一个等于当前长度的容量 即 cap(Array) = len(Array) * 2

package main

import "fmt"

func main()  {
    //fmt.Println()
    // cap 容量, 容量要满的话, 继续开辟一倍的容量

    numbers := make([]int, 5)

    fmt.Printf("numbers %v len is %d, cap " +
        "is %d \n", numbers, len(numbers), cap(numbers))
    numbers = append(numbers, 1)
    fmt.Printf("numbers %v len is %d, cap " +
        "is %d \n", numbers, len(numbers), cap(numbers))

    // test defer
    defer fmt.Println("defer: numbers is ", numbers)

    numbers = append(numbers, 1)
    numbers = append(numbers, 1)
    numbers = append(numbers, 1)
    numbers = append(numbers, 1)
    numbers = append(numbers, 1)
    fmt.Printf("numbers %v len is %d, cap " +
        "is %d \n", numbers, len(numbers), cap(numbers))

    /*
        切片同python
    */
}

10. map

1. 关键点: map[string]string

2. 如果使用函数操作, 是地址传递而不是值传递

3. cap(map) 有如下报错; “cap: 要获取map的容量，可以用len函数”

   invalid argument m (type map[string]int) for cap

package main

import "fmt"

func main() {
    var myMap1 map[string]string
    if myMap1 == nil {
        fmt.Println("myMap is null")
    }

    myMap1 = make(map[string]string, 5)

    myMap1["one"] = "java"
    myMap1["two"] = "c++"
    myMap1["three"] = "python"

    fmt.Println(myMap1)
    printMap(myMap1)

    // 删除一个key
    delete(myMap1, "two")
    printMap(myMap1)

    myMap1["for"] = "golang"
    myMap1["one"] = "c#"

    printMap(myMap1)
}

func printMap(mymap map[string]string)  {
    fmt.Println("........................")
    fmt.Println("len is", len(mymap))
    //fmt.Println("cap is", cap(mymap))
    for k, v := range mymap{
        fmt.Println(k, v)
    }
}