Select with multiple channels in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of select with two channels

What is the output of this Go program?

Go

package main

import (
	"fmt"
	"time"
)

func main() {
	ch1 := make(chan string)
	ch2 := make(chan string)

	go func() {
		time.Sleep(100 * time.Millisecond)
		ch1 <- "channel 1"
	}()

	go func() {
		time.Sleep(50 * time.Millisecond)
		ch2 <- "channel 2"
	}()

	select {
	case msg1 := <-ch1:
		fmt.Println(msg1)
	case msg2 := <-ch2:
		fmt.Println(msg2)
	}
}

A channel 2\nchannel 1

B channel 1

C channel 1\nchannel 2

D channel 2

💡 Hint

Remember that select picks the first channel ready to send or receive.

The goroutine sending to ch2 sleeps less (50ms) than the one sending to ch1 (100ms). So ch2 is ready first, and select receives from ch2, printing "channel 2".

❓ Predict Output

intermediate

Select with default case output

What will this Go program print?

Go

package main

import "fmt"

func main() {
	ch := make(chan int)

	select {
	case val := <-ch:
		fmt.Println(val)
	default:
		fmt.Println("no value received")
	}
}

A no value received

B 0

C deadlock

D runtime error

💡 Hint

The channel is empty and no goroutine sends to it.

Since ch has no value and no goroutine sends to it, the receive case blocks. The default case runs immediately, printing "no value received".

🔧 Debug

advanced

Why does this select cause deadlock?

This Go program causes a deadlock. Why?

Go

package main

func main() {
	ch1 := make(chan int)
	ch2 := make(chan int)

	select {
	case <-ch1:
		// do nothing
	case <-ch2:
		// do nothing
	}
}

A The select statement syntax is incorrect and causes compile error.

B Both channels are unbuffered and no goroutine sends to them, so select blocks forever causing deadlock.

C The channels are closed, so select panics at runtime.

D The program runs fine and prints nothing.

💡 Hint

Think about what happens when select waits on channels with no senders.

Both channels are unbuffered and no goroutine sends to them, so the select waits forever for a value that never comes, causing a deadlock.

❓ Predict Output

advanced

Select with multiple ready channels

What output does this Go program produce?

Go

package main

import (
	"fmt"
)

func main() {
	ch1 := make(chan string, 1)
	ch2 := make(chan string, 1)

	ch1 <- "first"
	ch2 <- "second"

	select {
	case msg := <-ch1:
		fmt.Println(msg)
	case msg := <-ch2:
		fmt.Println(msg)
	}
}

A first\nsecond

B second

C first

D second\nfirst

💡 Hint

When multiple channels are ready, select picks one at random but Go's implementation picks the first case in source order.

Both channels have buffered values ready. Select picks one randomly, but Go's implementation picks the first ready case in source order, so it prints "first".

❓ Predict Output

expert

Select with timeout using time.After

What is the output of this Go program?

Go

package main

import (
	"fmt"
	"time"
)

func main() {
	ch := make(chan string)

	select {
	case msg := <-ch:
		fmt.Println(msg)
	case <-time.After(100 * time.Millisecond):
		fmt.Println("timeout")
	}
}

A timeout

B runtime error

C deadlock

D empty string

💡 Hint

The channel ch never receives a value, but time.After sends a value after 100ms.

The channel ch never receives any value, so the receive case blocks. The time.After channel sends a value after 100ms, so the select unblocks and prints "timeout".