Fibonacci generator with Golang

Question

This is my Fibonacci generator:

package main

import "fmt"

func main() {
    for i, j := 0, 1; j < 100; i, j = i+j,i {
        fmt.Println(i)
    }
}

It's working, but I don't know how I can improve it, so I'd like more expert approaches to solving it.

Answer 1

Your code is quite nice, and can't really be improved. However, let's put the “generator” back into the code.

Go has channels which can be used to write elegant generators/iterators. We spawn of a goroutine that fills the channel with the fibonacci sequence. The main thread then takes as many fibonacci numbers as it needs.

So let's write a function fib_generator that returns a channel to the fibonacci sequence:

func fib_generator() chan int {
  c := make(chan int)

  go func() { 
    for i, j := 0, 1; ; i, j = i+j,i {
        c <- i
    }
  }()

  return c
}

We return a chan int. Here, we use an unbuffered channel. You may want to introduce a bit of buffering, e.g. make(chan int, 7).

Next, we spawn a goroutine. This contains your code, but instead of printing the numbers, we fill the channel with them. Note that the generator does not have a termination condition.

The unusual syntax

go func() { ... }()

is the standard way to start a goroutine. Because the function literal is a closure over the channel c, we can run multiple generators concurrently.

Our main will look like

func main() {
    c := fib_generator()
    for n := 0; n < 12 ; n++ {
        fmt.Println(<- c)
    }
}

That is, we create a new generator, and then pull the first 12 values from the channel. We cannot write for i := range c { ... }, because the fibonacci generator does not terminate.

As I said, we can have multiple generators concurrently:

func main() {
    c1 := fib_generator()
    c2 := fib_generator()

    // read first 12 numbers from 1st channel
    for n := 0; n < 10 ; n++ { fmt.Print(" ", <- c1) }
    fmt.Println()

    // read first 12 numbers from 2nd channel. The same.
    for n := 0; n < 10 ; n++ { fmt.Print(" ", <- c2) }
    fmt.Println()

    // read next   5 numbers from 1st channel.
    for n := 0; n <  5 ; n++ { fmt.Print(" ", <- c1) }
    fmt.Println()
}

Output:

 0 1 1 2 3 5 8 13 21 34
 0 1 1 2 3 5 8 13 21 34
 55 89 144 233 377

Answer 2

Your Fibonacci generator works (and looks) great but only for values below 2147483647 (i.e. 2³¹-1).

To work with larger numbers, we can use the math/big package:

package main

import (
    "fmt"
    "math/big"
)

func main() {
    F1 := big.NewInt(0)
    F2 := big.NewInt(1)
    for {
        F1.Add(F1, F2)
        F1, F2 = F2, F1

        // print to standard output
        fmt.Printf("%v\n", F1)
    }
}

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