Pass a result from goroutine to a variable inside the loop

Issue

At the code below how to assign a result from slowExternalFunction to a proper person? It can be done via channels and just for clarity I defined that slowExternalFunction returns int.

type Person struct {
    Id        int
    Name      string
    WillDieAt int
}

func slowExternalAPI(i int) int {
    time.Sleep(10)
    willDieAt := i + 2040
    return willDieAt 
}

func fastInternalFunction(i int) string {
    time.Sleep(1)
    return fmt.Sprintf("Ivan %v", i)
}

func main() {
    var persons []Person

    for i := 0; i <= 100; i++ {
        var person Person
        person.Id = i
        person.Name = fastInternalFunction(i)
        go slowExternalAPI(i)
        person.WillDieAt = 2050 //should be willDieAt from the slowExternalAPI
        persons = append(persons, person)
    }
    fmt.Printf("%v", persons)
}

https://play.golang.org/p/BRBgtH5ryo

Solution

To do it using channels you’ll have to refactor your code quite a bit.

Smallest change would be to do the assignment in the goroutine:

go func(){
    person.WillDieAt = slowExternalFunction(i)
}()

However, to make this work we’d need to make some other changes as well:

  • Use an array of pointers so that you can add the person before the assignment finishes.
  • Implement a wait group so that you wait for all goroutines to finish before printing the results.

Here’s the complete main function with the changes:

func main() {
    var persons []*Person
    var wg sync.WaitGroup

    for i := 0; i <= 100; i++{
        person := &Person{}
        person.Id = i
        person.Name = fastInternalFunction(i)
        wg.Add(1)
        go func(){
            person.WillDieAt = slowExternalFunction(i)
            wg.Done()
        }()

        persons = append(persons,person)
    }
    wg.Wait()
    for _, person := range persons {
        fmt.Printf("%v ", person )
    }
}

Playground: https://play.golang.org/p/8GWYD29inC

Answered By – eugenioy

Answer Checked By – Cary Denson (GoLangFix Admin)

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