Default case in select in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the default case in a Go select statement?

The default case runs immediately if no other case in the select is ready. It prevents the select from blocking and allows the program to continue.

beginner

Can a select statement in Go block if it has a default case?

No. If a select has a default case, it will never block. It will run the default immediately if no other channel operation is ready.

intermediate

What happens if multiple channel operations in a select are ready at the same time?

Go picks one of the ready cases at random to run. If there is a default case, it only runs if no other case is ready.

beginner

Write a simple Go select statement with a default case that prints "No message received" if no channel is ready.

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

intermediate

Why might you use a default case in a select statement in Go?

To avoid blocking when waiting on channels, allowing the program to do other work or check conditions repeatedly without getting stuck.

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

A Blocks until a channel is ready

B Runs immediately if no other case is ready

C Closes all channels

D Terminates the program

The default case runs immediately if no other channel operation is ready, preventing blocking.

If a select statement has a default case, can it ever block?

A No, never

B Yes, always

C Only if channels are closed

D Only if the default case is empty

Having a default case means the select will never block; it runs the default immediately if no other case is ready.

What happens if multiple channel cases in a select are ready at the same time?

A The program panics

B The first case is always chosen

C One case is chosen at random

D The default case runs

Go randomly picks one of the ready cases to run when multiple are ready.

Which of these is a reason to use a default case in a select?

A To close channels automatically

B To wait indefinitely for a message

C To restart the program

D To avoid blocking when no channels are ready

The default case lets the program continue without blocking if no channels are ready.

What will this code print if channel ch has no message ready?

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

A It prints "No message"

B It prints the message from ch

C It blocks forever

D It causes a runtime error

Since ch has no message ready, the default case runs and prints "No message".

Explain how the default case works in a Go select statement and why it is useful.

Think about what happens when no channels are ready to send or receive.

Write a Go select statement with a default case that handles receiving from a channel and prints a message if no data is available.

Use <code>select</code> with a receive case and a <code>default</code> that prints.

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