正确执行我的goroutines

Question

Background: I am trying to implement a logic which find the smallest positive number that is evenly divisible by all of the numbers from 1 to 20. I have implemented a sequential version and got an answer as 232792560.

Question: When I try to build some concurrency on this problem ( see uncommented block of code), it does run but never shows any result. Can anyone of you guide me on where I am going wrong?

Note: I am very much new to golang; and I am aware that, this is not the best problem for concurrency since there is no guarantee that I will have the smallest positive number as first result. However, I tried it out of my curiosity.

    package main

    import(
        "fmt"
    )

    func divide(num int) bool {
        for i := 1; i <= 20; i++ {
            if num % i != 0 {
                return false
            }
        }
        return true
    }

    func main() {
        num:=0
        //simple function
        /*for {
            num++;
            result := divide(num)
                if result {
                    fmt.Println("Smallest number is: ", num)
                    break
                }
            }*/


        //go-routine
        //go-routine
    var wg sync.WaitGroup
    for {
        num++;
        wg.Add(1)
        go func(x int) {
            result := divide(x)
            if result {
                fmt.Println("Smallest number is: ", x)
                defer wg.Done()
            }

        }(num)

    }
    wg.Wait()
  fmt.Println("End.")

 }

Answer 1

It doesn't make sense to launch unlimited number of goroutines - it will be performing worse, than non-concurrent solution. You can try to use a "worker pool" pattern and batch the calculations.

package main

import (
    "fmt"
    "runtime"
)

func divide(num int) bool {
    for i := 1; i <= 20; i++ {
        if num%i != 0 {
            return false
        }
    }
    return true
}

const step = 100000

func main() {
    result := make(chan int)
    jobs := make(chan int)
    for w := 0; w < runtime.NumCPU(); w++ {
        go func() {
            for n := range jobs {
                for i := n; i < n+step; i++ {
                    if divide(i) {
                        result <- i
                    }
                }
            }
        }()
    }
    num := 1
    for {
        select {
        case jobs <- num:
            num += step
        case x := <-result:
            fmt.Println("Smallest number is:", x)
            return
        }
    }
}