Slice append from channels

Question

I want to create slice and add values returned from channel. Below is the code I tried but could not able to solve it.

I have to send address of the slice, but I am not able to figure out how :(

package main

import "fmt"
import "time"

func sendvalues(cs chan int){
    for i:=0;i<10;i++{
        cs<-i   
    }   
}

func appendInt(cs chan int, aINt []int)[]*int{
    for {
        select {
        case i := <-cs:
            aINt = append(aINt,i)//append returns new type right ?
            fmt.Println("slice",aINt)
        }   
    }   
}



func main() {
    cs := make(chan int)
    intSlice := make([]int, 0,10)

    fmt.Println("Before",intSlice)
    go sendvalues(cs)
    go appendInt(cs,intSlice)// I have to pass address here

    time.Sleep(999*999999)
    fmt.Println("After",intSlice)
}

Answer 1

Your code won't work for two (in fact three) reasons:

1. append returns a new slice as soon as the capacity is reached. Thus, the assignment in appendInt will do nothing.

2. appendInt runs concurrently, therefore:

   • As long as appendInt does not message main that it is finished, main does not know when the intSlice has all the values you want.
   • You have to wait for all goroutines to return at the end of main

Problem 1: Modifying slices in functions

You may know that in Go every value you pass to a function is copied. Reference values, such as slices, are copied too, but have pointers internally which then point to the original memory location. That means you can modify the elements of a slice in a function. What you can't do is reassigning this value with a new slice as the internal pointer would point to somewhere different. You need pointers for that. Example (Play):

func modify(s *[]int) {
    for i:=0; i < 10; i++ {
        *s = append(*s, i)
    }
}

func main() {
    s := []int{1,2,3}
    modify(&s)
    fmt.Println(s)
}

Problem 2: Synchronizing goroutines

To wait for started goroutines, you can use a sync.WaitGroup. Example (Play):

func modify(wg *sync.WaitGroup, s *[]int) {
    defer wg.Done()
    for i:=0; i < 10; i++ {
        *s = append(*s, i)
    }
}

func main() {
    wg := &sync.WaitGroup{}
    s := []int{1,2,3}

    wg.Add(1)
    go modify(wg, &s)

    wg.Wait()
    fmt.Println(s)
}

The example above waits (using wg.Wait()) for modify to finish (modify calls wg.Done() when finished). If you remove the wg.Wait() call, you will see why not synchronizing is a problem. Comparison of outputs:

• With wg.Wait(): [1 2 3 0 1 2 3 4 5 6 7 8 9]
• Without wg.Wait(): [1 2 3]

The main goroutine returns earlier than the modify goroutine which is why you will never see the modified results. Therefore synchronizing is absolutely necessary.

A good way to communicate the new slice would be to use a channel. You would not need to use pointers and you would have synchronization. Example (Play):

func modify(res chan []int) {
    s := []int{}
    for i:=0; i < 10; i++ {
        s = append(s, i)
    }
    res <- s
}

func main() {
    c := make(chan []int)
    go modify(c)

    s := <-c
    fmt.Println(s)
}