Select with multiple channels in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the select statement in Go?

The select statement lets a Go program wait on multiple channel operations. It blocks until one of the channels is ready to send or receive, then executes the corresponding case.

intermediate

How does Go decide which select case to run if multiple channels are ready?

If multiple channels are ready, Go picks one case at random to run. This prevents starvation and ensures fairness.

beginner

What happens if none of the channels in a select statement are ready?

The select statement blocks and waits until one of the channels becomes ready, unless there is a default case, which runs immediately if no channels are ready.

beginner

Write a simple Go select statement that listens on two channels ch1 and ch2 and prints which channel received data.

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

intermediate

What is the role of the default case in a select statement?

The default case runs immediately if no other channel operations are ready. It prevents blocking and can be used to perform non-blocking checks.

What does the select statement do in Go?

A Waits on multiple channel operations and executes one that is ready

B Creates a new goroutine

C Declares a new channel

D Locks a mutex

The select statement waits on multiple channels and runs the case for the first ready channel.

If multiple channels are ready in a select, how does Go choose which case to run?

A Runs the first case declared

B Picks one case at random

C Runs all ready cases simultaneously

D Returns an error

Go picks one ready case at random to avoid starvation.

What happens if no channels are ready and there is no default case in a select?

A The <code>select</code> blocks until a channel is ready

B The program panics

C The <code>select</code> skips all cases

D The <code>select</code> runs the last case

Without a default, select waits (blocks) until a channel is ready.

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

A To handle errors

B To close all channels

C To run code if no channels are ready, avoiding blocking

D To create new channels

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

Which of these is a valid use of select with multiple channels?

A select { for ch1: ... }

B select { ch1 <- 1; ch2 <- 2 }

C select { if ch1 == ch2: ... }

D select { case <-ch1: ... case <-ch2: ... }

Option D shows correct syntax waiting to receive from multiple channels.

Explain how the select statement works with multiple channels in Go.

Think about how Go handles multiple channel operations at once.

Describe a real-life situation where using select with multiple channels would be helpful.

Imagine you are waiting for messages from different friends on separate phones.

Practice

1.

What does the select statement do when used with multiple channels in Go?

easy

A. It waits until one of the channels is ready to send or receive and executes that case.

B. It executes all channel operations simultaneously regardless of readiness.

C. It blocks forever until all channels are ready at the same time.

D. It randomly picks a channel to operate on without checking readiness.

Solution

1. Step 1: Understand select behavior

   The select statement waits for one of its channel cases to be ready.

2. Step 2: Identify how select chooses a case

   It picks one of the channels that is ready to send or receive and runs that case.

3. Final Answer:

   It waits until one of the channels is ready to send or receive and executes that case. -> Option A

4. Quick Check:

   Select waits for ready channel = A [OK]

Hint: Select picks the first ready channel to act on [OK]

Common Mistakes:

• Thinking select runs all channels at once
• Believing select waits for all channels simultaneously
• Assuming select picks channels randomly without readiness check

2.

Which of the following is the correct syntax to use select with two channels ch1 and ch2 for receiving values?

?

easy

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

B. select {
case <-ch1:
fmt.Println(val)
case <-ch2:
fmt.Println(val)
}

C. select {
case val := ch1:
fmt.Println(val)
case val := ch2:
fmt.Println(val)
}

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

Solution

1. Step 1: Check channel receive syntax

   Receiving from a channel uses val := <-ch to assign the received value.

2. Step 2: Verify select case syntax

   Each case must have a receive operation with assignment like val := <-ch1.

3. Final Answer:

   select { case val := <-ch1: fmt.Println(val) case val := <-ch2: fmt.Println(val) } -> Option D

4. Quick Check:

   Correct receive syntax uses := <-channel [OK]

Hint: Use := <-channel to receive inside select case [OK]

Common Mistakes:

• Using = instead of := for assignment
• Omitting the <- operator for receiving
• Trying to assign channel itself instead of received value

3.

What will be the output of the following Go program?

package main
import (
    "fmt"
    "time"
)
func main() {
    ch1 := make(chan string)
    ch2 := make(chan string)
    go func() {
        time.Sleep(100 * time.Millisecond)
        ch1 <- "hello"
    }()
    go func() {
        time.Sleep(50 * time.Millisecond)
        ch2 <- "world"
    }()
    select {
    case msg1 := <-ch1:
        fmt.Println(msg1)
    case msg2 := <-ch2:
        fmt.Println(msg2)
    }
}

medium

A. hello

B. world

C. hello
world

D. No output (deadlock)

Solution

1. Step 1: Analyze goroutine sleep durations

   The goroutine sending to ch2 sleeps 50ms, and ch1 sleeps 100ms.

2. Step 2: Determine which channel is ready first

   Since ch2 sends first, the select will receive from ch2 and print "world".

3. Final Answer:

   world -> Option B

4. Quick Check:

   Channel with shorter sleep sends first = world [OK]

Hint: Select picks channel ready first (shorter sleep) [OK]

Common Mistakes:

• Assuming ch1 sends first because it is listed first
• Expecting both messages to print
• Thinking select waits for all channels

4.

Identify the error in this Go code using select with multiple channels:

package main
import "fmt"
func main() {
    ch1 := make(chan int)
    ch2 := make(chan int)
    select {
    case val := <-ch1:
        fmt.Println(val)
    case val := <-ch2:
        fmt.Println(val)
    }
}

medium

A. Channels must be buffered to use select.

B. The select statement syntax is invalid without a loop.

C. The program will deadlock because channels are never written to.

D. The default case is not allowed in select statements.

Solution

1. Step 1: Analyze channel creation

   Channels ch1 and ch2 are unbuffered with no goroutines to send values.

2. Step 2: Select behavior without senders

   The select blocks forever since no channels become ready, causing a deadlock.

3. Final Answer:

   The program will deadlock because channels are never written to. -> Option C

4. Quick Check:

   No senders + no default = deadlock [OK]

Hint: No senders and no default causes deadlock [OK]

Common Mistakes:

• Thinking channels must be buffered to use select
• Believing select syntax is invalid without a loop
• Assuming a default case is required or invalid

5.

You want to write a Go function that waits on two channels chA and chB and returns the first value received from either. If neither channel is ready immediately, it should return "timeout" after 100 milliseconds. Which select structure correctly implements this?

hard

A. select {
case val := <-chA:
return val
case val := <-chB:
return val
case <-time.After(100 * time.Millisecond):
return "timeout"
}

B. select {
case val := <-chA:
return val
case val := <-chB:
return val
default:
time.Sleep(100 * time.Millisecond)
return "timeout"
}

C. select {
case val := <-chA:
return val
case val := <-chB:
return val
case <-time.Tick(100 * time.Millisecond):
return "timeout"
}

D. select {
case val := <-chA:
return val
case val := <-chB:
return val
case <-time.After(100):
return "timeout"
}

Solution

1. Step 1: Use time.After for timeout

   The time.After function returns a channel that sends a value after the specified duration, perfect for timeout.

2. Step 2: Check select cases

   Cases for receiving from chA and chB return the first received value; the timeout case returns "timeout" after 100ms.

3. Step 3: Validate other options

   select { case val := <-chA: return val case val := <-chB: return val default: time.Sleep(100 * time.Millisecond) return "timeout" } uses default with sleep which blocks select; select { case val := <-chA: return val case val := <-chB: return val case <-time.Tick(100 * time.Millisecond): return "timeout" } uses time.Tick which repeats; select { case val := <-chA: return val case val := <-chB: return val case <-time.After(100): return "timeout" } uses incorrect duration type.

4. Final Answer:

   select { case val := <-chA: return val case val := <-chB: return val case <-time.After(100 * time.Millisecond): return "timeout" } -> Option A

5. Quick Check:

   Use time.After channel for timeout in select [OK]

Hint: Use time.After channel for timeout in select [OK]

Common Mistakes:

• Using default with sleep instead of time.After
• Using time.Tick which repeats instead of time.After
• Passing wrong duration type to time.After