Channel closing behavior in Go - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to close the channel.

Go

ch := make(chan int)
// Close the channel
[1](ch)

Drag options to blanks, or click blank then click option'

Aclose

BcloseChan

CchanClose

Dshutdown

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-existent function like closeChan or chanClose.

Trying to assign nil to the channel instead of closing it.

The close function is used to close a channel in Go.

2fill in blank

medium

Complete the code to receive from a closed channel without blocking.

Go

ch := make(chan int)
go func() {
    close(ch)
}()

value, [1] := <-ch

Drag options to blanks, or click blank then click option'

Aclosed

Bdone

Cok

Dreceived

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name other than 'ok' which is conventional.

Not using the two-value receive syntax.

The second value when receiving from a channel indicates if the channel is open (true) or closed (false).

3fill in blank

hard

Fix the error in the code that tries to send on a closed channel.

Go

ch := make(chan int)
go func() {
    close(ch)
}()

// This will cause a panic
[1] ch <- 5

Drag options to blanks, or click blank then click option'

A// do not send

Bclose(ch)

Cselect

Dch <- 5

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to close the channel again.

Using select without proper case to avoid sending.

Sending on a closed channel causes a panic. To fix it, do not send after closing.

4fill in blank

hard

Fill both blanks to create a map comprehension that stores lengths of words longer than 3 characters.

Go

words := []string{"go", "code", "chat", "bot"}
lengths := map[string]int{}
for _, word := range words {
    if len(word) [1] 3 {
        lengths[[2]] = len(word)
    }
}

Drag options to blanks, or click blank then click option'

A>

Bword

C<

Dlen

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than instead of greater than.

Using len instead of word as the map key.

The condition checks if the word length is greater than 3, and the map key is the word itself.

5fill in blank

hard

Fill all three blanks to create a select statement that handles sending, receiving, and default case on a channel.

Go

select {
case [1] <- 10:
    fmt.Println("Sent 10")
case val := <-[2]:
    fmt.Println("Received", val)
default:
    fmt.Println([3])
}

Drag options to blanks, or click blank then click option'

Ach

C"No communication"

D"Waiting"

Attempts:

3 left

💡 Hint

Common Mistakes

Using different channels for send and receive.

Not using a string literal in the default case.

The select sends 10 on channel ch, receives from ch, and prints "No communication" if neither is ready.

Practice

1. What happens when you close a channel in Go?

easy

A. The channel is deleted from memory immediately

B. Senders can still send values without error

C. Receivers know no more values will be sent

D. The channel blocks all operations permanently

Solution

1. Step 1: Understand channel closing purpose

   Closing a channel signals receivers that no more values will be sent.

2. Step 2: Effects on senders and receivers

   Senders cannot send after closing; receivers get zero value and ok=false.

3. Final Answer:

   Receivers know no more values will be sent -> Option C

4. Quick Check:

   close(channel) signals receivers [OK]

Hint: Closing signals receivers no more data comes [OK]

Common Mistakes:

• Thinking senders can send after close
• Believing channel is deleted immediately
• Assuming channel blocks forever after close

2. Which of the following is the correct way to close a channel named ch in Go?

easy

A. ch.Close()

B. ch.close()

C. close channel ch

D. close(ch)

Solution

1. Step 1: Recall Go syntax for closing channels

   The built-in function close() is used with the channel as argument.

2. Step 2: Identify correct syntax

   Only close(ch) is valid; methods like ch.close() or ch.Close() do not exist.

3. Final Answer:

   close(ch) -> Option D

4. Quick Check:

   Use close(channel) syntax [OK]

Hint: Use built-in close(channel) function [OK]

Common Mistakes:

• Trying to call close as a method
• Using incorrect keywords like 'close channel'
• Capitalizing close as Close

3. What is the output of this Go program?

package main
import "fmt"
func main() {
  ch := make(chan int, 2)
  ch <- 10
  ch <- 20
  close(ch)
  for v := range ch {
    fmt.Print(v, " ")
  }
}

medium

A. 10 20

B. 10 20 0

C. Runtime panic: send on closed channel

D. No output

Solution

1. Step 1: Understand channel buffering and closing

   Channel has buffer size 2, two values sent, then channel closed.

2. Step 2: Loop over channel with range

   Range reads all values until channel is closed and drained, printing 10 and 20.

3. Final Answer:

   10 20 -> Option A

4. Quick Check:

   Range over closed channel prints buffered values [OK]

Hint: Range prints all buffered values before stopping [OK]

Common Mistakes:

• Expecting panic on reading closed channel
• Thinking zero value prints after buffered values
• Assuming no output because channel is closed

4. Identify the error in this Go code snippet:

ch := make(chan int)
go func() {
  for i := 0; i < 3; i++ {
    ch <- i
  }
  close(ch)
}()
for {
  v, ok := <-ch
  if !ok {
    break
  }
  fmt.Println(v)
}

medium

A. Channel is unbuffered but sender is in goroutine, no deadlock

B. No error, code runs and prints 0 1 2

C. Closing channel inside goroutine causes panic

D. Receiver loop never ends, causing infinite loop

Solution

1. Step 1: Analyze channel usage and goroutine

   Unbuffered channel with sender in goroutine sends 3 values then closes channel.

2. Step 2: Check receiver loop behavior

   Receiver reads values until channel closes, then breaks loop correctly.

3. Final Answer:

   No error, code runs and prints 0 1 2 -> Option B

4. Quick Check:

   Proper close and ok check ends loop [OK]

Hint: Use ok to detect closed channel and break loop [OK]

Common Mistakes:

• Thinking closing in goroutine causes panic
• Assuming infinite loop without break
• Ignoring need to check ok when receiving

5. Consider this Go program:

package main
import "fmt"
func main() {
  ch := make(chan int)
  go func() {
    for i := 1; i <= 3; i++ {
      ch <- i
    }
    close(ch)
  }()
  for {
    v, ok := <-ch
    if !ok {
      break
    }
    if v%2 == 0 {
      fmt.Println(v)
    }
  }
}

What is the output and why?

hard

A. 2
Only even numbers printed before channel closes

B. 1
2
3
All numbers printed despite channel closing

C. No output because channel is closed before reading

D. Runtime panic due to sending on closed channel

Solution

1. Step 1: Understand sending and closing in goroutine

   Values 1, 2, 3 sent then channel closed properly.

2. Step 2: Receiver loop filters even numbers

   Loop reads until closed; prints only values where v%2 == 0, which is 2.

3. Final Answer:

   2 Only even numbers printed before channel closes -> Option A

4. Quick Check:

   Filter with condition inside loop after close [OK]

Hint: Filter values after reading until channel closes [OK]

Common Mistakes:

• Expecting all values printed
• Thinking channel closes before sending
• Assuming panic on close with goroutine sender