6. Range

Similar to slices and maps, channels can be ranged over.

for item := range ch {
    // item is the next value received from the channel
}

This example will receive values over the channel (blocking at each iteration if nothing new is there) and will exit only when the channel is closed.

Assignment

It's that time again, Textio is hiring and we've been assigned to do the interview. The Fibonacci sequence is Textio's interview problem of choice. We've been tasked with building a small toy program we can use in the interview.

Complete the concurrentFib function. It should:

• Create a new channel of ints

• Call fibonacci concurrently

• Use a range loop to read from the channel and append the values to a slice

• Return the slice

exercise.go

package main

func concurrentFib(n int) []int {
	// ?
}

// don't touch below this line

func fibonacci(n int, ch chan int) {
	x, y := 0, 1
	for i := 0; i < n; i++ {
		ch <- x
		x, y = y, x+y
	}
	close(ch)
}

Solution

solution.go

package main

func concurrentFib(n int) []int {
	// Create new channel of ints
	intChannel := make(chan int)

	// Call fibonacci concurrently
	go fibonacci(n, intChannel)

	// Create slice to store values from channel
	result := []int{}

	// Use range loop to read from channel and append to slice
	for item := range intChannel {
		result = append(result, item)
	}
	return result
}

// don't touch below this line

func fibonacci(n int, ch chan int) {
	x, y := 0, 1
	for i := 0; i < n; i++ {
		ch <- x
		x, y = y, x+y
	}
	close(ch)
}

• The range loop on a channel keeps receiving until the channel is closed.

• fibonacci() is responsible for calling close(ch) after sending n values, so the range loop terminates cleanly.

• Because concurrentFib reads from the channel, the sender won’t deadlock on ch <- x (it’ll block until you receive, which is fine).