15.切片
切片是基于数组类型做的一层封装。它非常灵活，可以自动扩容。
切片的定义
1: var a []int
// 定义一个int类型的空切片
// 切片不初始化是无法操作的，这点和数组不一样。切片的初始化：
1: 1.arr[start:end]start到end-1(包括end-1)之间的所有元素
2: 2.arr[start:]
3: 3.arr[:end]
4: 4.arr[:]包括整个数组的所有元素
例：
1: a := [5]int{76,77,78,79,80} 2: var b []int = a[1:4]
3: c:= []int{6,7,8} 切片的修改：
1: func main() { 2: darr := [...]int{57,89,90,82,100,78,67,69,59} 3: dslice := darr[2:5]
4: fmt.Println("array before", darr) 5: for i,_ := range dslice { 6: dslice[i] ++
7: }
8: fmt.Println("array after", darr) 9: }
//这种类似于python里的浅拷贝
1: func main() { 2: numa := [3]int{78,79,80} 3: nums1 := numa[:]
4: nums2 := numa[:]
5: fmt.Println("array before change 1", numa) 6: nums1[0] = 100
7: fmt.Println(numa)
8: nums2[1] = 101
9: fmt.Println(numa)
10: }
11: >>> [100 79 80]
12: [100 101 80]
使用make创建切片
1: i := make([]int, 5, 5)
// (类型 当前长度 容量上线)
// 未指定的元素默认为01: func main() { 2: var a []int
3: a = make([]int, 1, 10)
4: a[0] = 10
5: // 这里a[1] = 20 会报错，操作越界了,这是个重要的细节,忽略很容易犯错
6: a = append(a, 11)
7: fmt.Printf(a)
8: }
1: >>> [10 11]
扩容(cap(a)是表示a的容量上限)
1: for i:=0;i<10;i++ { 2: a = append(a, i)
3: fmt.Printf("a=%v addr:%p len:%d cap:%d\n", a,a,len(a),cap(a)) 4: }
输出：
1: >>>a=[10 11 0] addr:0xc000014140 len:3 cap:10
2: a=[10 11 0 1] addr:0xc000014140 len:4 cap:10
3: a=[10 11 0 1 2] addr:0xc000014140 len:5 cap:10
4: a=[10 11 0 1 2 3] addr:0xc000014140 len:6 cap:10
5: a=[10 11 0 1 2 3 4] addr:0xc000014140 len:7 cap:10
6: a=[10 11 0 1 2 3 4 5] addr:0xc000014140 len:8 cap:10
7: a=[10 11 0 1 2 3 4 5 6] addr:0xc000014140 len:9 cap:10
8: a=[10 11 0 1 2 3 4 5 6 7] addr:0xc000014140 len:10 cap:10
9: a=[10 11 0 1 2 3 4 5 6 7 8] addr:0xc00008a000 len:11 cap:20
//观察扩容的结果，发现扩容的策略是翻倍扩容
1: func testCap() { 2: a := [...]string{"a","b","c","d","e","f","g","h"} 3: b := a[1:3]
4: fmt.Printf("b:%v len:%d cap:%d\n",b,len(b),cap(b)) 5: }
1: >>> b:[b c] len:2 cap:7
空切片
1: var a []int
2: fmt.Println(a, len(a), cap(a))
3: a = append(a, 100)
4: fmt.Println(a, len(a), cap(a))
1: >>> [] 0 0
2: [100] 1 1
切片传参
1: veggies := []string{"potatoes", "tomatoes", "brinjal"} 2: fruits := []string{"oranges", "apples"} 3: food := append(veggies, fruits...)
4: // friuts后面的3个点表示展开fruits切片成一个个元素
5: fmt.Println(food)
1: func modifySlice(a []int) { 2: a[0] = 1000
3: }
4: func main(){ 5: var a [3]int = [3]int{1, 2, 3} 6: modifySlice(a[:])
7: fmt.Println(a)
8: }
1: >>> [1000, 2, 3]
切片的拷贝
1: func main() { 2: veggies := []string{"potatoes", "tomatoes", "brinjal"} 3: fruits := []string{"oranges", "apples"} 4: copy(veggies, fruits)
5: fmt.Println(veggies, fruits)
6: }
1: >>> [oranges apples brinjal] [oranges apples]
1: func main() { 2: var a := []int{1, 2} 3: var a = []int{1, 2} 4: var b := []int{4, 5, 6} 5: copy(a, b)
6: fmt.Println(a, b)
7: }
1: >>> [4 5] [4 5 6]
make和new的区别
make为内建类型为slice, map和channel分配内存
new用于各种类型的内存分配, new返回的是一个指针
几个小练习
1.下面程序输出什么
1: func main() { 2: var a = make([]string, 5, 10);
3: for i:=0;i<10;i++{ 4: a = append(a, fmt.Sprintf("%d", i)) 5: }
6: fmt.Println(a)
7: }
1: >>> a= [ ]
2: a= [ 0 1 2 3 4 5 6 7 8 9]
2.使用golang标准包"sort"对数组进行排序
1: func main() { 2: a := []int{5, 4, 3, 2, 1} 3: sort.Ints(a[:])
4: fmt.Println("a=",a) 5: b := []string{"f", "e", "c", "b", "a"} 6: sort.Strings(b[:])
7: fmt.Println("b=",b) 8: c := []float64{58.554,88.23,998.255,48.25,566.66} 9: sort.Float64s(c[:])
10: fmt.Println("c=",c) 11: }
3.实现一个密码生成工具，支持以下功能
PS:标准包"flag"解析命令行参数 a.用户可以通过-l指定生成让那个密码的长度 b.用户可以通过-t指定生成密码的字符集,比如-t num生成全数字的密码-t char生成包含全英文字符的密码, -t mix包含生成数字和英文的密码,-t advance生成包含数字,英文以及特殊字符的密码1: package main
2:
3: import (
4: "fmt"
5: "flag"
6: "math/rand"
7: "time"
8: )
9: var (
10: length int
11: charset string
12: )
13: const (
14: NumStr = "0123456789"
15: CharStr = "abcdefghjkmnopqrstuvwxyzABCDEFGHJKMNOPQRSTUVWXYZ"
16: SpecStr = "@#$%^&*_+=/*-"
17: )
18: func parseArgs() { 19: flag.IntVar(&length, "l", 16, "-l 生成密码长度")
20: flag.StringVar(&charset, "t", "num",
21: `-t 制定密码生成的字符集,
22: num:只使用数字[0-9],
23: char:只使用英文字母[a-zA-Z],
24: mix:使用数字和字母,
25: advance：使用数字字母以及特殊字符`)
26: flag.Parse()
27: }
28: func generatePasswd() string { 29: var password []byte = make([]byte, length, length)
30: var sourceStr string
31: if charset == "num" { 32: sourceStr = NumStr
33: }else if charset == "char"{ 34: sourceStr = CharStr
35: }else if charset == "mix" { 36: sourceStr = fmt.Sprintf("%s%s", NumStr, CharStr) 37: }else if charset == "advance" { 38: sourceStr = fmt.Sprintf("%s%s%s", NumStr, CharStr, SpecStr) 39: }else { 40: sourceStr = NumStr
41: }
42: fmt.Println("sourceStr:", sourceStr) 43: for i:=0;i
44: index := rand.Intn(len(sourceStr))
45: password[i] = sourceStr[index]
46: }
47: return string(password)
48: }
49: func main() { 50: rand.Seed(time.Now().UnixNano())
51: parseArgs()
52: fmt.Printf("length:%d charset:%s\n", length, charset) 53: passwd := generatePasswd()
54: fmt.Println(passwd)
55: }
编译后输入：
1: main.exe -l 45 -t num
结果
1: length:45 charset:num
2: sourceStr: 0123456789
3: 263415358329572320363459838031515468782761014