Default case in select in Go - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a default case in the select statement.

Go

select {
case msg := <-ch:
    fmt.Println("Received:", msg)
[1]:
    fmt.Println("No message received")
}

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

Adefault

Bcase

Celse

Dfallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'case' instead of 'default' for the default case.

Using 'else' or 'fallback' which are not valid in select.

The default case in a select runs if no other case is ready.

2fill in blank

medium

Complete the code to print a message when no channel is ready using the default case.

Go

select {
case <-ch1:
    fmt.Println("ch1 ready")
case <-ch2:
    fmt.Println("ch2 ready")
[1]:
    fmt.Println("No channels ready")
}

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

Acase else

Bcase default

Celse

Ddefault

Attempts:

3 left

💡 Hint

Common Mistakes

Writing 'case default:' which is invalid syntax.

Using 'else' which is not a Go keyword in select.

The default keyword alone defines the default case in a select.

3fill in blank

hard

Fix the error in the select statement by completing the default case correctly.

Go

select {
case msg := <-ch:
    fmt.Println("Got message:", msg)
[1]:
    fmt.Println("No message received")
}

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

Acase default

Bdefault

Ccase else

Delse

Attempts:

3 left

💡 Hint

Common Mistakes

Writing 'case default:' which causes a syntax error.

Using 'else' which is not valid in select.

The default case in Go's select uses only the keyword default without 'case'.

4fill in blank

hard

Fill both blanks to create a select with two channel cases and a default case.

Go

select {
case msg1 := <-ch1:
    fmt.Println("ch1:", msg1)
case msg2 := <-ch2:
    fmt.Println("ch2:", msg2)
[1]:
    fmt.Println("[2]")
}

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

Adefault

Bcase

CNo channels ready

DNo message

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'case' before 'default' keyword.

Printing a wrong message.

The default case starts with default: and the print statement shows the message.

5fill in blank

hard

Fill all three blanks to write a select that reads from two channels and uses a default case with a message.

Go

select {
case val1 := <-chan1:
    fmt.Println("chan1 value:", [1])
case val2 := <-chan2:
    fmt.Println("chan2 value:", [2])
[3]:
    fmt.Println("No data received")
}

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

Aval1

Bval2

Cdefault

Dval3

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names in print statements.

Writing 'case default' instead of 'default'.

Use the variables val1 and val2 to print channel values, and default for the default case.

Practice

1.

What does the default case do in a Go select statement?

easy

A. Closes all channels automatically

B. Blocks until a channel is ready

C. Runs immediately if no other channel operations are ready

D. Sends data to all channels simultaneously

Solution

1. Step 1: Understand the role of select

   The select statement waits on multiple channel operations.

2. Step 2: Identify the purpose of default

   The default case runs immediately if no channels are ready, preventing blocking.

3. Final Answer:

   Runs immediately if no other channel operations are ready -> Option C

4. Quick Check:

   default runs when no channels ready = A [OK]

Hint: Default runs instantly if no channels are ready [OK]

Common Mistakes:

• Thinking default blocks until a channel is ready
• Assuming default closes channels
• Believing default sends data to all channels

2.

Which of the following is the correct syntax for using a default case in a Go select?

select {
  case msg := <-ch1:
    fmt.Println(msg)
  // What goes here?
}

easy

A. default:
fmt.Println("No message received")

B. else:
fmt.Println("No message received")

C. fallback:
fmt.Println("No message received")

D. otherwise:
fmt.Println("No message received")

Solution

1. Step 1: Recall Go select syntax

   The default keyword is used for the fallback case in select.

2. Step 2: Match correct keyword

   Only default is valid; else, fallback, and otherwise are invalid in Go select.

3. Final Answer:

   default: fmt.Println("No message received") -> Option A

4. Quick Check:

   Use default keyword in select = C [OK]

Hint: Default is the only valid fallback keyword in select [OK]

Common Mistakes:

• Using else instead of default
• Trying fallback or otherwise keywords
• Omitting the colon after default

3.

What will be the output of this Go program?

package main
import (
  "fmt"
  "time"
)
func main() {
  ch := make(chan string)
  select {
  case msg := <-ch:
    fmt.Println(msg)
  default:
    fmt.Println("No message received")
  }
}

medium

A. Compilation error

B. Program blocks forever

C. Empty string printed

D. No message received

Solution

1. Step 1: Analyze channel state

   The channel ch is created but no data is sent, so it's empty and no receive is ready.

2. Step 2: Understand select with default

   Since no data is ready on ch, the default case runs immediately, printing "No message received".

3. Final Answer:

   No message received -> Option D

4. Quick Check:

   No data ready triggers default = A [OK]

Hint: Default runs if channel has no data ready [OK]

Common Mistakes:

• Expecting program to block waiting for channel
• Thinking empty string is printed
• Assuming compilation error due to empty channel

4.

Identify the error in this Go code snippet using select with a default case:

select {
case msg := <-ch:
  fmt.Println(msg)
default
  fmt.Println("No message")
}

medium

A. Default case must be last

B. Missing colon after default keyword

C. Cannot use variable msg in select

D. Channel ch is not initialized

Solution

1. Step 1: Check default syntax

   The default case requires a colon (:) after the keyword.

2. Step 2: Identify missing colon

   The code has default without a colon, causing a syntax error.

3. Final Answer:

   Missing colon after default keyword -> Option B

4. Quick Check:

   Default needs colon (:) = D [OK]

Hint: Always put colon after default in select [OK]

Common Mistakes:

• Forgetting colon after default
• Assuming default must be last case
• Confusing variable scope in select

5.

Consider this Go program snippet:

ch1 := make(chan int)
ch2 := make(chan int)
go func() {
  time.Sleep(100 * time.Millisecond)
  ch1 <- 1
}()
go func() {
  time.Sleep(200 * time.Millisecond)
  ch2 <- 2
}()
select {
case val := <-ch1:
  fmt.Println("Received from ch1:", val)
case val := <-ch2:
  fmt.Println("Received from ch2:", val)
default:
  fmt.Println("No channels ready")
}

What is the most likely output when this select runs?

hard

A. No channels ready

B. Received from ch1: 1

C. Received from ch2: 2

D. Program deadlocks

Solution

1. Step 1: Understand goroutine timing

   The goroutines send to channels after 100ms and 200ms delays, but the select runs immediately.

2. Step 2: Check channel readiness at select

   At the moment select runs, neither channel has data ready, so no case is ready.

3. Step 3: Default case triggers

   Because no channels are ready, the default case runs, printing "No channels ready".

4. Final Answer:

   No channels ready -> Option A

5. Quick Check:

   Default runs if no channel ready = B [OK]

Hint: Select runs immediately; default if no channel ready [OK]

Common Mistakes:

• Assuming select waits for channels
• Expecting output from ch1 or ch2 immediately
• Thinking program deadlocks without default