Thread creation and execution in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a thread in Ruby?

A thread is a lightweight unit of execution within a Ruby program that allows multiple tasks to run at the same time.

beginner

How do you create a new thread in Ruby?

Use Thread.new { ... } to create and start a new thread that runs the code inside the block.

beginner

What does thread.join do in Ruby?

It makes the main program wait until the thread finishes its work before continuing.

intermediate

Can Ruby threads run truly in parallel?

MRI Ruby threads are native threads and run concurrently but not truly in parallel because of the Global Interpreter Lock (GIL).

intermediate

What happens if you don't join a thread in Ruby?

The main program may finish before the thread completes, causing the thread to be killed prematurely.

How do you start a new thread in Ruby?

A start_thread()

B Thread.start()

C Thread.new { code }

D new Thread()

In Ruby, Thread.new { ... } creates and starts a new thread running the code inside the block.

What method waits for a thread to finish before continuing?

A wait()

B join()

C sleep()

D stop()

join() pauses the main thread until the called thread completes.

Which Ruby implementation has a Global Interpreter Lock (GIL) affecting threads?

A MRI Ruby

B Rubinius

C JRuby

D TruffleRuby

MRI Ruby uses a GIL, so threads run concurrently but not in true parallel.

What happens if the main program ends before a thread finishes?

A Thread is killed

B Thread is paused

C Thread continues running in background

D Thread restarts

If the main program ends, any running threads are killed immediately.

Which of these is a correct way to create and run a thread that prints 'Hello'?

A Thread.run { print 'Hello' }

B Thread.start { puts 'Hello' }

C Thread.create { echo 'Hello' }

D Thread.new { puts 'Hello' }

Thread.new { puts 'Hello' } creates and runs a thread that prints 'Hello'.

Explain how to create and execute a thread in Ruby and why you might want to use join.

Think about starting a task in the background and waiting for it to finish.

Describe the effect of the Global Interpreter Lock (GIL) on Ruby threads and how it impacts parallel execution.

Consider how threads share the CPU in MRI Ruby.

Practice

1. What does Thread.new do in Ruby?

easy

A. It creates a new thread to run code concurrently.

B. It pauses the current thread until another finishes.

C. It terminates all running threads immediately.

D. It converts a method into a thread-safe version.

Solution

1. Step 1: Understand the purpose of Thread.new

   Thread.new starts a new thread that runs the code inside its block concurrently with the main program.

2. Step 2: Compare options with the behavior

   Only It creates a new thread to run code concurrently. correctly describes creating a new thread to run code at the same time as other threads.

3. Final Answer:

   It creates a new thread to run code concurrently. -> Option A

4. Quick Check:

   Thread.new = create thread [OK]

Hint: Remember: Thread.new starts a new thread block [OK]

Common Mistakes:

• Confusing thread creation with thread termination
• Thinking Thread.new pauses threads
• Assuming Thread.new modifies methods

2. Which of the following is the correct syntax to create and start a thread in Ruby?

easy

A. Thread.start puts 'Hello from thread'

B. Thread.run { puts 'Hello from thread' }

C. Thread.create do puts 'Hello from thread' end

D. Thread.new { puts 'Hello from thread' }

Solution

1. Step 1: Recall correct thread creation syntax

   The correct way is Thread.new { ... } where the block contains the code to run.

2. Step 2: Check each option

   Thread.new { puts 'Hello from thread' } uses Thread.new with a block correctly. Options A, C, and D use invalid or non-existent methods or syntax.

3. Final Answer:

   Thread.new { puts 'Hello from thread' } -> Option D

4. Quick Check:

   Use Thread.new { } for threads [OK]

Hint: Use Thread.new with a block to start threads [OK]

Common Mistakes:

• Using Thread.start without a block
• Mistyping method names like Thread.create
• Using Thread.run which does not exist

3. What will be the output of this Ruby code?

thread = Thread.new { 5.times { |i| puts i; sleep 0.1 } }
thread.join
puts 'Done'

medium

A. 0 1 2 3 4 Done (each number on its own line)

B. Done 0 1 2 3 4 (each number on its own line)

C. Only 'Done' is printed

D. No output because thread.join blocks forever

Solution

1. Step 1: Understand thread execution and join

   The thread prints numbers 0 to 4 with a short pause. The main thread waits for it to finish because of thread.join.

2. Step 2: Determine output order

   The numbers 0 to 4 print first, each on its own line, then after the thread finishes, 'Done' prints.

3. Final Answer:

   0 1 2 3 4 Done (each number on its own line) -> Option A

4. Quick Check:

   Thread prints numbers before main prints 'Done' [OK]

Hint: join waits for thread to finish before continuing [OK]

Common Mistakes:

• Assuming 'Done' prints before numbers
• Thinking join blocks forever
• Ignoring sleep delays in thread

4. Identify the error in this Ruby code snippet:

thread = Thread.new do
  puts 'Start'
  sleep 1
  puts 'End'
end
puts 'Waiting'
thread.join

medium

A. The thread block is missing a closing end.

B. The main thread does not wait for the new thread to finish.

C. There is no error; the code runs correctly.

D. The sleep method cannot be used inside a thread.

Solution

1. Step 1: Check thread block syntax

   The thread block is properly closed with end. Syntax is correct.

2. Step 2: Verify thread execution and join

   The main thread prints 'Waiting' then calls thread.join to wait for the thread to finish. This is correct usage.

3. Final Answer:

   There is no error; the code runs correctly. -> Option C

4. Quick Check:

   Thread syntax and join usage are correct [OK]

Hint: Check block ends and join usage carefully [OK]

Common Mistakes:

• Thinking sleep is invalid in threads
• Assuming join is missing
• Miscounting block ends

5. You want to create 3 threads that each print their thread number (1, 2, 3) and then print 'Finished' after all threads complete. Which code correctly achieves this?

hard

A. 3.times do |i|
Thread.new { puts i + 1 }
end
puts 'Finished'

B. threads = []
3.times do |i|
threads << Thread.new { puts i + 1 }
end
threads.each(&:join)
puts 'Finished'

C. threads = []
3.times do |i|
threads << Thread.new { puts i }
end
puts 'Finished'
threads.each(&:join)

D. threads = []
3.times do |i|
threads << Thread.new { puts i + 1 }
end
puts 'Finished'
threads.each(&:join)

Solution

1. Step 1: Create threads storing them in an array

   threads = [] 3.times do |i| threads << Thread.new { puts i + 1 } end threads.each(&:join) puts 'Finished' correctly creates 3 threads, each printing i + 1 (1, 2, 3), and stores them in threads.

2. Step 2: Wait for all threads to finish before printing 'Finished'

   threads = [] 3.times do |i| threads << Thread.new { puts i + 1 } end threads.each(&:join) puts 'Finished' calls threads.each(&:join) to wait for all threads, then prints 'Finished'. This ensures 'Finished' prints last.

3. Final Answer:

   threads = [] 3.times do |i| threads << Thread.new { puts i + 1 } end threads.each(&:join) puts 'Finished' -> Option B

4. Quick Check:

   Store threads, join all, then print [OK]

Hint: Join all threads before printing final message [OK]

Common Mistakes:

• Not storing threads to join later
• Printing 'Finished' before threads complete
• Using wrong thread index values