slice of channels and concurrent function execution - go

Question

How to create slice of channels and run function double(i) concurrently inside slice iteration:
package main
import (
"fmt"
"time"
)
func double(i int) int {
result := 2 * i
fmt.Println(result)
time.Sleep(500000000)
return result
}
func notParallel(arr []int) (outArr []int) {
for _, i := range arr {
outArr = append(outArr, double(i))
}
return
}
// how to do the same as notParallel func in parallel way.
// For each element of array double func should evaluate concuruntly
// without waiting each next element to eval
func parallel(arr []int) (outArr []int) {
var chans []chan int
for i := 0; i < len(arr); i++ {
chans[i] = make(chan int) // i = 0 : panic: runtime error: index out of range
}
for counter, number := range arr {
go func() {
chans[counter] <- double(number)
}()
}
return
}
func main() {
arr := []int{7, 8, 9}
fmt.Printf("%d\n", notParallel(arr))
fmt.Printf("%d\n", parallel(arr))
}
playground
As function double(i) sleeps for 500 ms function notParallel(arr []int) works for 1500 ms for 3 elements of arr []int but function parallel(arr []int) would work about 500 ms.
In my implementation have error...
panic: runtime error: index out of range
... on line ...
chans[i] = make(chan int) // i = 0

Answer 1

In this case, you don't need to use chan.
package main
import (
"fmt"
"sync"
"time"
)
func double(i int) int {
result := 2 * i
fmt.Println(result)
time.Sleep(500000000)
return result
}
func notParallel(arr []int) (outArr []int) {
for _, i := range arr {
outArr = append(outArr, double(i))
}
return
}
// how to do the same as notParallel func in parallel way.
// For each element of array double func should evaluate concuruntly
// without waiting each next element to eval
func parallel(arr []int) (outArr []int) {
outArr = make([]int, len(arr))
var wg sync.WaitGroup
for counter, number := range arr {
wg.Add(1)
go func(counter int, number int) {
outArr[counter] = double(number)
wg.Done()
}(counter, number)
}
wg.Wait()
return
}
func main() {
arr := []int{7, 8, 9}
fmt.Printf("%d\n", notParallel(arr))
fmt.Printf("%d\n", parallel(arr))
}
Because parallel must wait all of finish of goroutine(s).
And I notice your code doesn't work because you refer counter, number in same function scope.