Default case in select in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of select with default case

What is the output of this Go program?

Go

package main
import (
	"fmt"
	"time"
)
func main() {
	ch := make(chan int)
	select {
	case val := <-ch:
		fmt.Println("Received", val)
	default:
		fmt.Println("No value received")
	}
}

A No value received

B Received 0

C Program deadlocks

D Compilation error

💡 Hint

Think about what happens when the channel has no data and the select has a default case.

The select statement tries to receive from the channel ch. Since ch is empty and no goroutine sends data, the receive would block. But because there is a default case, the select does not block and immediately executes the default case, printing "No value received".

❓ Predict Output

intermediate

Behavior of select without default case

What happens when this Go program runs?

Go

package main
import (
	"fmt"
)
func main() {
	ch := make(chan int)
	select {
	case val := <-ch:
		fmt.Println("Received", val)
	}
}

A Prints "Received 0"

B Prints nothing and exits

C Program deadlocks

D Compilation error

💡 Hint

Consider what happens when select waits on a channel with no sender and no default case.

The select waits to receive from ch, but no goroutine sends data. Without a default case, the select blocks forever, causing a deadlock.

🔧 Debug

advanced

Why does this select with default case print twice?

This Go program prints "default" twice. Why?

Go

package main
import (
	"fmt"
)
func main() {
	ch := make(chan int)
	for i := 0; i < 2; i++ {
		select {
		case val := <-ch:
			fmt.Println("Received", val)
		default:
			fmt.Println("default")
		}
	}
}

A Because the channel sends two zeros automatically

B Because the default case runs immediately both times since channel is empty

C Because the select statement loops twice and receives two values

D Because the program has a race condition causing duplicate prints

💡 Hint

Think about what happens when the channel is empty and select has a default case inside a loop.

Each iteration of the loop runs the select. Since the channel ch is empty and no goroutine sends data, the receive case is not ready. The default case runs immediately both times, printing "default" twice.

🧠 Conceptual

advanced

Purpose of default case in select

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

A To execute code immediately if no other case is ready, preventing blocking

B To close all channels automatically when no case matches

C To prioritize one channel over others in the select

D To catch runtime errors during channel operations

💡 Hint

Think about how select behaves when no channel operation can proceed.

The default case runs immediately if no other channel operation is ready, so the select does not block. This allows the program to continue without waiting.

❓ Predict Output

expert

Output of select with multiple cases and default

What is the output of this Go program?

Go

package main
import (
	"fmt"
	"time"
)
func main() {
	ch1 := make(chan int)
	ch2 := make(chan int)
	go func() {
		time.Sleep(50 * time.Millisecond)
		ch2 <- 42
	}()
	select {
	case val := <-ch1:
		fmt.Println("Received from ch1", val)
	case val := <-ch2:
		fmt.Println("Received from ch2", val)
	default:
		fmt.Println("No channel ready")
	}
}

A Program deadlocks

B Received from ch1 0

C Received from ch2 42

D No channel ready

💡 Hint

Consider the timing of the goroutine sending to ch2 and when select runs.

The select runs immediately before the goroutine sends to ch2 (after 50ms). Since ch1 is empty and ch2 has no value yet, no case is ready. The default case runs, printing "No channel ready".