go 迭代string数组操作 go for string[]

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go 迭代string数组操作 go for string[]

作者：EasternUnbeaten 时间：2021-02-04 09:01

go 迭代string数组,直接拷贝去用即可

package main
import (
"fmt"
)
func main() {
  subsCodes := []string{"aaaa", "vvvvv", "dddd", "eeeee", "gfgggg"}
  for _, s := range subsCodes {
    fmt.Println(s)
  }
}

补充：golang字符串string与字符数组[]byte高效转换

string与[]byte的直接转换是通过底层数据copy实现的

var a = []byte("hello boy")

var b = string(a)

这种操作在并发量达到十万百万级别的时候会拖慢程序的处理速度

通过gdb调试来看一下string和[]byte的数据结构

(gdb) l main.main
import (
  "fmt"
)
func main() {
  s := "hello, world!"
  b := []byte(s)
  fmt.Println(s, b)
(gdb) b 11
Breakpoint 1 at 0x487cd9: file /export/home/machao/src/test/strbytes.go, line 11.
(gdb) r
Starting program: /export/home/machao/src/test/test1 
Breakpoint 1, main.main () at /export/home/machao/src/test/strbytes.go:11
  fmt.Println(s, b)
(gdb) info locals
s = {
str = 0x4b8ccf "hello, world!level 3 resetload64 failednil stackbaseout of memorys.allocCount=srmount errorstill in listtimer expiredtriggerRatio=unreachable: value method xadd64 failedxchg64 failed nmidlelocked= on "..., len = 13}
b = {array = 0xc4200140e0 "hello, world!", len = 13, cap = 16}
(gdb) ptype s
type = struct string {
 uint8 *str;
 int len;
}
(gdb) ptype b
type = struct []uint8 {
 uint8 *array;
 int len;
 int cap;
}

转换后 [ ]byte 底层数组与原 string 内部指针并不相同，以此可确定数据被复制。那么，如不修改数据，仅转换类型，是否可避开复制，从而提升性能？

从 ptype 输出的结构来看，string 可看做 [2]uintptr，而 [ ]byte 则是 [3]uintptr，这便于我们编写代码，无需额外定义结构类型。如此，str2bytes 只需构建 [3]uintptr{ptr, len, len}，而 bytes2str 更简单，直接转换指针类型，忽略掉 cap 即可。

通过unsafe.Pointer（指针转换）和uintptr（指针运算）实现转换

package main
import (
  "fmt"
  "strings"
  "unsafe"
)
func str2bytes(s string) []byte {
  x := (*[2]uintptr)(unsafe.Pointer(&s))
  h := [3]uintptr{x[0], x[1], x[1]}
  return *(*[]byte)(unsafe.Pointer(&h))
}
func bytes2str(b []byte) string {
  return *(*string)(unsafe.Pointer(&b))
}
func main() {
  s := strings.Repeat("abc", 3)
  b := str2bytes(s)
  s2 := bytes2str(b)
  fmt.Println(b, s2)
}

没有出现逃逸现象

package main
import (
 "testing"
 "io/ioutil"
 "time"
 "fmt"
)
var s, _ = ioutil.ReadFile("mydata4vipday.720.datx")
func test() {
 b := string(s)
 _ = []byte(b)
}
func test2() {
 b := bytes2str(s)
 _ = str2bytes(b)
}
func BenchmarkTest(b *testing.B) {
 t1 := time.Now()
 for i := 0; i < b.N; i++ {
   test()
 }
 fmt.Println("test", time.Now().Sub(t1), b.N)
}
func BenchmarkTestBlock(b *testing.B) {
 t1 := time.Now()
 for i := 0; i < b.N; i++ {
   test2()
 }
 fmt.Println("test block", time.Now().Sub(t1), b.N)
}

对比一下优化前后的性能差异