looping in parallel

goroutine 也可以運用在 for loop 裡面，讓 for loop 裡面的工作也可以同時進行．
下面的範例沒有使用 goroutine 時，迴圈會逐一取得 names 的名字，然後算出加總，每個迴圈都讓它停 1 秒．

package main

import (
	"fmt"
	"time"
)

func main() {
	start := time.Now()

	names := []string{"Allen", "Jack", "Daniel", "Sam", "Lucas"}
	namesLength := len(names)

	var totalLength int

	for i := 0; i < namesLength; i++ {
		name := names[i]
		totalLength += len(name)
		time.Sleep(1 * time.Second)
		fmt.Printf("index : %v, name : %v\n", i, name)
	}
	fmt.Printf("total length is %d \n", totalLength)

	end := time.Now()
	executeTime := end.Sub(start)
	fmt.Printf("executeTime : %v ", executeTime)
}

執行結果

index : 0, name : Allen
index : 1, name : Jack
index : 2, name : Daniel
index : 3, name : Sam
index : 4, name : Lucas
total length is 23
executeTime : 5.021016241s %

改使用 goroutine 的方式讓 for loop 每個要做的事都變成每一個 goroutine 變成非同步的方式，
並把每個 goroutine 取得到的 name 長度加到 channel 裡，等每個 goroutine 都工作完後，
再取出 channel 所有的長度做加總．

package main

import (
	"fmt"
	"sync"
	"time"
)

func main() {
	start := time.Now()

	names := []string{"Allen", "Jack", "Daniel", "Sam", "Lucas"}
	namesLength := len(names)

	totalLengthChen := make(chan int, namesLength)
	var wg sync.WaitGroup
	wg.Add(namesLength)

	for i := 0; i < namesLength; i++ {
		go func(i int) {
			defer wg.Done()
			name := names[i]
			totalLengthChen <- len(name)
			time.Sleep(1 * time.Second)
			fmt.Printf("index : %v, name : %v\n", i, name)
		}(i)
	}
	wg.Wait()
	var totalLength int
	for i := 0; i < namesLength; i++ {
		totalLength += <-totalLengthChen
	}
	fmt.Printf("total length is %d \n", totalLength)
	end := time.Now()
	executeTime := end.Sub(start)
	fmt.Printf("executeTime : %v ", executeTime)
}

執行結果只花了 1 秒，比沒有使用 goroutine 快了 4 秒

index : 2, name : Daniel
index : 1, name : Jack
index : 3, name : Sam
index : 0, name : Allen
index : 4, name : Lucas
total length is 23
executeTime : 1.000596334s %

select case

select case 可以針對不同的 channel，接收該 channel 的值．
繼續利用上面的例子，但不用 sync.WaitGroup 等待 goroutine 了，
在最後跑個 for 的無窮迴圈利用 select case 如果 channel 有值的話就取出來做加總．

package main

import (
	"fmt"
	"time"
)

func main() {
	names := []string{"Allen", "Jack", "Daniel", "Sam", "Lucas"}
	namesLength := len(names)

	totalLengthChen := make(chan int, namesLength)
	namesChen := make(chan string, namesLength)

	for i := 0; i < namesLength; i++ {
		go func(i int) {
			name := names[i]
			totalLengthChen <- len(name)
			namesChen <- name
			time.Sleep(1 * time.Second)
			fmt.Printf("index : %v, name : %v\n", i, name)
		}(i)
	}

	var totalLength int
	var totalNames string

	for {
		time.Sleep(500 * time.Millisecond)
		select {
		case len := <-totalLengthChen:
			totalLength += len
			fmt.Printf("current totalLength is %d \n", totalLength)
		case name := <-namesChen:
			totalNames += name + ","
			fmt.Printf("Get names %v \n", totalNames)
		}
	}
}

如果 select case 沒有給 default 在 channel 都被取完後會發生 deadlock．

Get names Sam,
Get names Sam,Daniel,
current totalLength is 3
current totalLength is 9
current totalLength is 14
Get names Sam,Daniel,Lucas,
current totalLength is 18
current totalLength is 23
Get names Sam,Daniel,Lucas,Jack,
index : 1, name : Jack
index : 2, name : Daniel
index : 3, name : Sam
index : 4, name : Lucas
index : 0, name : Allen
Get names Sam,Daniel,Lucas,Jack,Allen,
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [select]:
main.main()
        /Volumes/Transcend/golang/goHello/src/practice/example.go:29 +0x1f9
exit status 2

所以最後再加上 default 可以不做任何事，在兩個 channel 都沒有值時，就會一直跑 default

package main

import (
	"fmt"
	"time"
)

func main() {
	names := []string{"Allen", "Jack", "Daniel", "Sam", "Lucas"}
	namesLength := len(names)

	totalLengthChen := make(chan int, namesLength)
	namesChen := make(chan string, namesLength)

	for i := 0; i < namesLength; i++ {
		go func(i int) {
			name := names[i]
			totalLengthChen <- len(name)
			namesChen <- name
			time.Sleep(1 * time.Second)
			fmt.Printf("index : %v, name : %v\n", i, name)
		}(i)
	}

	var totalLength int
	var totalNames string

	for {
		time.Sleep(1 * time.Second)
		select {
		case len := <-totalLengthChen:
			totalLength += len
			fmt.Printf("current totalLength is %d \n", totalLength)
		case name := <-namesChen:
			totalNames += name + ","
			fmt.Printf("Get names %v \n", totalNames)
		default:
			fmt.Println("nothing")
		}
	}
}

執行結果

index : 0, name : Allen
index : 3, name : Sam
Get names Jack,
index : 2, name : Daniel
index : 4, name : Lucas
index : 1, name : Jack
current totalLength is 4
Get names Jack,Lucas,
current totalLength is 9
current totalLength is 15
current totalLength is 20
Get names Jack,Lucas,Daniel,
current totalLength is 23
Get names Jack,Lucas,Daniel,Allen,
Get names Jack,Lucas,Daniel,Allen,Sam,
nothing
nothing
nothing
nothing