CODEWITHSUNDEEP
goconcurrency
3rdeye7
3rdeye7

Reputation: 536

communicating with n goroutines

I am trying to implement fibonacci recursion in golang using n goroutines with communicating via channels.

I am returning an integer from the function, but i am actually just sending the sum of f(n-1) +f(n-2) over channel c but this is not working correctly. It prints the first two values correct, and every value after is just 1. 

package main

import "fmt"

// Fibonacci in a recursive version
func fiboR(n int, c chan int ) int {

  if(n == 0){
      c <- 0
      return 0
  } else if n == 1 {
      c <- 1
      return 1
  } else{
      c <- fiboR(n-1,c) + fiboR(n-2,c)
      return fiboR(n-1,c) + fiboR(n-2,c)
  }

}


func main() {
  for i := 0; i < 10; i++ {
    procchan := make(chan int)
    go fiboR(i,procchan)
    fmt.Println(i,<-procchan )
}
}

Also is it possible to use channels for receiving the two recursive calls?

Upvotes: 1

Views: 301

Answers (2)

Nandu Kalidindi
Nandu Kalidindi

Reputation: 6280

Adding to @nissefors answer, the main process there is most likely a sequential one because in the for loop you would be waiting on the channel to return and then proceed to the next iteration.

A minor modification in the main function could fire all the fibonaccis at once and then in a separate for loop the channels that are corresponding to each go routine can be accessed

Playground URL: https://play.golang.org/p/7e3JnWeSp6

package main

import "fmt"

// Fibonacci in a recursive version
func fiboRchan(n int, c chan int) {
        fmt.Println("PROCESSING FOR %d", n)
    c <- fiboR(n)
}

func fiboR(n int) int {
    if n == 0 {
        return 0
    } else if n == 1 {
        return 1
    } else {
        return fiboR(n-1) + fiboR(n-2)
    }

}

func main() {
    var arr[10]chan int
    for i := 0; i < 10; i++ {
        procchan := make(chan int)
        arr[i] = procchan
        go fiboRchan(i, procchan)
    }
    // By now all the go routines are fired

    // Now iterate through the channel array and read from the
    // respective channel
    for i:=0; i< 10; i++ {
        fmt.Println(i, <-arr[i])
    }

}

Upvotes: 1

nissefors
nissefors

Reputation: 440

Your solution will try to output more than the one value you extract from the channel as you increase the value of i. What your code will try to send to the channel for each i:

0: 0
1: 1
2: 1,0,1
3: 1,0,1,1,2
4: 1,0,1,1,2,1,0,1,3
...

Since you create a new channel for each i and then only extract one value you will always get the first value in the line above.

If you try to run it with these modifications it will output what you wanted (https://play.golang.org/p/_mn3l5x8iZ).

package main

import "fmt"

// Fibonacci in a recursive version
func fiboRchan(n int, c chan int) {
    c <- fiboR(n)
}

func fiboR(n int) int {

    if n == 0 {
        return 0
    } else if n == 1 {
        return 1
    } else {
        return fiboR(n-1) + fiboR(n-2)
    }

}

func main() {

    for i := 0; i < 10; i++ {
        procchan := make(chan int)
        go fiboRchan(i, procchan)
        fmt.Println(i, <-procchan)
    }

}

Upvotes: 2

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