Go Slices Slide - HackMD

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!important; width: 34px !important; height: 21px !important; border: none !important; float: right; margin-top: -7px; margin-right: -10px; } </style> # Go Slices Slide ###### tags: `Golang`, `簡報` --- ## Agenda - 起因 - slice in GO - 回到問題上(起因) - 延伸閱讀 - slice 上的 "gotcha" - 延伸閱讀 - slice 當參數傳遞 - 延伸閱讀 - Method receivers - 延伸閱讀 - slice 相關豆知識 --- ## 起因 ---- ### 看到了一篇 medium [Go’s append is not always thread safe](https://medium.com/@cep21/gos-append-is-not-always-thread-safe-a3034db7975) ---- <div class="textleft"> 這邊舉了兩個例子，在使用 goroutine 對同一個 slice append 會發生 race 的情況 p.s. 使用 `go test -race` 可以檢查是否會發生 race </div> ---- ### 範例一 ```go= package main import ( "sync" "testing" ) func TestAppend(t *testing.T) { x := []string{"start"} wg := sync.WaitGroup{} wg.Add(2) go func() { defer wg.Done() y := append(x, "hello", "world") t.Log(cap(y), len(y)) }() go func() { defer wg.Done() z := append(x, "goodbye", "bob") t.Log(cap(z), len(z)) }() wg.Wait() } ``` ---- ### 結果 ```shell > go test -race . ok _/Users/kais.lin/go/goTest/slice 1.015s > go test -v z=> [start goodbye bob] y=> [start hello world] [start] --- PASS: TestAppend (0.00s) app_test.go:24: 3 app_test.go:18: 3 PASS ok _/Users/kais.lin/go/goTest/slice 0.006s ``` ---- ### 範例二 ```go= package main import ( "testing" "sync" ) func TestAppend(t *testing.T) { x := make([]string, 0, 6) wg := sync.WaitGroup{} wg.Add(2) go func() { defer wg.Done() y := append(x, "hello", "world") t.Log(len(y)) }() go func() { defer wg.Done() z := append(x, "goodbye", "bob") t.Log(len(z)) }() wg.Wait() } ``` ---- ### 結果 ```shell > go test -race . ================== WARNING: DATA RACE Write at 0x00c4200be060 by goroutine 8: _/tmp.TestAppend.func2() /tmp/main_test.go:20 +0xcb Previous write at 0x00c4200be060 by goroutine 7: _/tmp.TestAppend.func1() /tmp/main_test.go:15 +0xcb Goroutine 8 (running) created at: _/tmp.TestAppend() /tmp/main_test.go:18 +0x14f testing.tRunner() /usr/local/Cellar/go/1.10.2/libexec/src/testing/testing.go:777 +0x16d Goroutine 7 (running) created at: _/tmp.TestAppend() /tmp/main_test.go:13 +0x105 testing.tRunner() /usr/local/Cellar/go/1.10.2/libexec/src/testing/testing.go:777 +0x16d ================== ================== WARNING: DATA RACE Write at 0x00c4200be070 by goroutine 8: _/tmp.TestAppend.func2() /tmp/main_test.go:20 +0x11a Previous write at 0x00c4200be070 by goroutine 7: _/tmp.TestAppend.func1() /tmp/main_test.go:15 +0x11a Goroutine 8 (running) created at: _/tmp.TestAppend() /tmp/main_test.go:18 +0x14f testing.tRunner() /usr/local/Cellar/go/1.10.2/libexec/src/testing/testing.go:777 +0x16d Goroutine 7 (finished) created at: _/tmp.TestAppend() /tmp/main_test.go:13 +0x105 testing.tRunner() /usr/local/Cellar/go/1.10.2/libexec/src/testing/testing.go:777 +0x16d ================== --- FAIL: TestAppend (0.00s) main_test.go:16: 2 main_test.go:21: 2 testing.go:730: race detected during execution of test FAIL FAIL _/tmp 0.901s > go test -v === RUN TestAppend z=> [hello world] y=> [hello world] [] --- PASS: TestAppend (0.00s) app_test.go:24: 2 app_test.go:18: 2 PASS ok _/Users/kais.lin/go/goTest/slice 0.005s ``` ---- SO WHY? --- ## slice in GO ---- ### 要瞭解 slice 前必須先了解 array > The slice type is an abstraction built on top of Go’s array type, and so to understand slices we must first understand arrays. ---- ### Array in GO <div class="textleft"> Array 宣告時會指定了長度和元素類型，且不需初始化，預設值為 0 </div> ```go var a [4]int a[0] = 1 i := a[0] // i == 1 a[2] == 0 // true, the zero value of the int type ``` <div class="textleft"> 在記憶體中會長這樣 </div> ![](https://blog.golang.org/go-slices-usage-and-internals_slice-array.png) ---- ### 那 slice 呢? <div class="textleft"> 為了可以更彈性的利用 Array，所以 slice 長這樣: </div> ```go type sliceHeader struct { Length int Capacity int ZerothElement *byte } ``` <div class="textleft"> - Length: slice 長度<br> - Capacity: 底層 array 的長度<br> - ZerothElement: 參考(指向)的底層 array </div> ---- 所以當你從 slice 擷取部分值時: s = s[2:4] ![](https://blog.golang.org/go-slices-usage-and-internals_slice-2.png) []byte 上到下分別為: ZerothElement, len, cap ---- 所以對 `slice` `append` 值進去時，其實是進了底層 array，但如過底層 array 不夠大時怎麼辦? ---- ```go func AppendByte(slice []byte, data ...byte) []byte { m := len(slice) n := m + len(data) if n > cap(slice) { // if necessary, reallocate // allocate double what's needed, for future growth. newSlice := make([]byte, (n+1)*2) copy(newSlice, slice) slice = newSlice } slice = slice[0:n] copy(slice[m:n], data) return slice } ``` --- ## 回到問題上(起因) ---- 回到範例一 ```go ... x := []string{"start"} ... go func() { ... y := append(x, "hello", "world") ... } go func() { ... z := append(x, "hello", "world") ... } ``` ----  ![](https://cdn-images-1.medium.com/max/1600/1*MoOXXCuFEbYag5ZYIjgx5g.png) ----  ![](https://cdn-images-1.medium.com/max/1600/1*VsEQuGIq9YE1__suevGPlg.png) ---- 範例二 ```go ... x := make([]string, 0, 6) ... go func() { ... y := append(x, "hello", "world") ... } go func() { ... z := append(x, "hello", "world") ... } ``` ----  ![](https://cdn-images-1.medium.com/max/1600/1*oOJ9t3KPpbihDnMSF_pLfA.png) ----  ![](https://cdn-images-1.medium.com/max/1600/1*jaT9M8OvuYFzyf29Qu8cbA.png) ---- ### 作者提供的解決方法 ```go package main import ( "sync" "testing" ) func TestAppend(t *testing.T) { x := make([]string, 0, 6) x = append(x, "start") wg := sync.WaitGroup{} wg.Add(2) go func() { defer wg.Done() y := make([]string, 0, len(x)+2) y = append(y, x...) y = append(y, "hello", "world") t.Log(cap(y), len(y), y[0]) }() go func() { defer wg.Done() z := make([]string, 0, len(x)+2) z = append(z, x...) z = append(z, "goodbye", "bob") t.Log(cap(z), len(z), z[0]) }() wg.Wait() } ``` --- ## 延伸閱讀 - slice 上的 "gotcha" ---- ### 還記得我們說過<br>slice 指向(參考)著底層的 array ---- <div class="textleft"> 所以如果存了一大堆資料進 slice 而最後只為了取其中一小部分值的話，就會發生: </div> ```go var digitRegexp = regexp.MustCompile("[0-9]+") func FindDigits(filename string) []byte { b, _ := ioutil.ReadFile(filename) // b 為 slice return digitRegexp.Find(b) // 回傳由 b(slice) 做正規畫後得到的資料 } ``` <div class="textleft"> 會導致其他的 data 依然存在底層 array 中~ </div> ---- ### 解決方法 ```go func CopyDigits(filename string) []byte { b, _ := ioutil.ReadFile(filename) b = digitRegexp.Find(b) c := make([]byte, len(b)) copy(c, b) return c } ``` --- ## 延伸閱讀 - slice 當參數傳遞 ---- ```go func AddOneToEachElement(slice []int) { for i := range slice { slice[i]++ } } func main() { buffer := []int {0,0,0,0,0,0,0,0,0,0,0,1,2,3,4,5,6,7,8,9,10} slice := buffer[10:20] fmt.Println("slice before ", slice) // slice before [0 1 2 3 4 5 6 7 8 9] fmt.Println("buffer before", buffer) // buffer before [0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9 10] AddOneToEachElement(slice) fmt.Println("slice after ", slice) // slice after [1 2 3 4 5 6 7 8 9 10] fmt.Println("buffer after ", buffer) // buffer after [0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9 10 10] } ``` ##### 儘管 slice 傳遞看起來像 passed by value，但因為 slice header 內含指標，所以會更新到原始參照的 array --- ## 延伸閱讀 - Method receivers ---- 想將這功能改用 receivers 的方式實作 ```go func main() { pathName := path("/usr/bin/tso") // Conversion from string to path. i := bytes.LastIndex(pathName, []byte("/")) if i >= 0 { pathName = (pathName)[0:i] } fmt.Printf("%s\n", pathName) // /usr/bin } ``` ---- > It is idiomatic to use a pointer receiver for a method that modifies a slice. > ```go type path []byte func (p *path) TruncateAtFinalSlash() { i := bytes.LastIndex(*p, []byte("/")) if i >= 0 { *p = (*p)[0:i] } } func main() { pathName := path("/usr/bin/tso") // Conversion from string to path. pathName.TruncateAtFinalSlash() fmt.Printf("%s\n", pathName) // /usr/bin } ``` ---- 但如果這邊改用 value receiver 的話 ```go type path []byte func (p path) TruncateAtFinalSlash() { i := bytes.LastIndex(p, []byte("/")) if i >= 0 { p = (p)[0:i] } fmt.Printf("%s\n", p) // /usr/bin } func main() { pathName := path("/usr/bin/tso") // Conversion from string to path. pathName.TruncateAtFinalSlash() fmt.Printf("%s\n", pathName) // /usr/bin/tso } ``` ---- 範例二: 要將 path 全轉大寫 ```go type path []byte func (p path) ToUpper() { for i, b := range p { if 'a' <= b && b <= 'z' { p[i] = b + 'A' - 'a' } } } func main() { pathName := path("/usr/bin/tso") pathName.ToUpper() fmt.Printf("%s\n", pathName) // /USR/BIN/TSO } ``` ---- ```go type path []byte func (p *path) ToUpper() { // for i, b := range *p { for _, b := range *p { if 'a' <= b && b <= 'z' { // (*p)[i] = b + 'A' - 'a' b = b + 'A' - 'a' } } fmt.Printf("%s\n", *p) // /usr/bin/tso } func main() { pathName := path("/usr/bin/tso") pathName.ToUpper() fmt.Printf("%s\n", pathName) // /usr/bin/tso } ``` ---- <div class="textleft"> ※ go 在 range 時所遍歷的值是複製的值而不是元素本身，而如果使用`＆遍歷值`這樣拿到的也是臨時變數的位置而範例二的重點我覺得是 range 的小細節，而非將重點放在 slice 上 </div> --- ## 延伸閱讀 - slice 相關豆知識 ---- ### 空 slice = nil 當你 `make` 一個空 slice 時 ```go sliceHeader{ Length: 0, Capacity: 0, ZerothElement: nil, } ``` ---- ```go var a []int fmt.Println(a == nil) // true b := make([]int,0) fmt.Println(b == nil) // flase ``` ---- ### String 也是 slice > Strings are actually very simple: they are just read-only slices of bytes with a bit of extra syntactic support from the language. ---- 所以你也可以直接做互相轉換 ```go str := string(slice) // reverse slice := []byte(usr) ``` ---- <div class="textleft"> 這種 slice-like 的設計，可以輕鬆的建立子字串，並且跟改後不會影響父子串 </div> ```go a := "1234567890" b := a[1:5] fmt.Println(b) // 12345 b = "54321" fmt.Println(a) // 1234567890 fmt.Println(b) // 54321 ```