Ractor for true parallelism in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ractor communication code?

Consider this Ruby code using Ractors. What will it print?

Ruby

r = Ractor.new do
  (0...5).map { |i| i * 2 }
end

result = r.take
puts result.inspect

A [0, 2, 4, 6, 8]

B nil

C 0

D RactorError

💡 Hint

Remember that Ractor.new runs the block in parallel and take receives the result.

The Ractor runs the block which returns an array of doubled numbers from 0 to 4. The take method receives this array, so the output is [0, 2, 4, 6, 8].

🧠 Conceptual

intermediate

Which statement about Ractors is true?

Choose the correct statement about Ruby Ractors.

A Ractors share all instance variables between them by default.

B Ractors can directly modify each other's local variables.

C Ractors communicate only by message passing and do not share memory.

D Ractors run on the same thread and do not provide parallelism.

💡 Hint

Think about how Ractors achieve true parallelism safely.

Ractors do not share memory to avoid race conditions. They communicate only by passing messages, which makes them safe for parallel execution.

🔧 Debug

advanced

What error does this Ractor code raise?

What error will this code produce when run?

Ruby

r1 = Ractor.new do
  @counter = 0
  @counter += 1
  @counter
end

puts r1.take

A NameError

B Ractor::IsolationError

C NoMethodError

D No error, outputs 1

💡 Hint

Instance variables inside a Ractor are local to it.

The instance variable @counter is local to the Ractor and can be used normally. The code runs without error and outputs 1.

❓ Predict Output

advanced

What is the output of this Ractor message passing example?

What will this Ruby code print?

Ruby

r = Ractor.new do
  msg = Ractor.receive
  msg * 3
end

r.send(7)
puts r.take

A 21

B 7

C Ractor::ClosedError

D nil

💡 Hint

Look at how the Ractor receives and processes the message.

The Ractor receives the number 7, multiplies it by 3, and returns 21. The take method prints 21.

🚀 Application

expert

How many items are in the resulting array after this Ractor code runs?

Given this Ruby code using multiple Ractors, how many items will the final array contain?

Ruby

ractors = (0...3).map do |i|
  Ractor.new(i) do |index|
    Array.new(index + 2) { index }
  end
end

results = ractors.map(&:take)
final = results.flatten
puts final.size

A 6

B 9

C 3

D 7

💡 Hint

Count how many elements each Ractor creates and sum them.

The Ractors create arrays of sizes 2, 3, and 4 (because index goes 0,1,2). So total elements = 2 + 3 + 4 = 9. But index starts at 0, so arrays are of sizes 2,3,4 respectively. Actually, 0+2=2, 1+2=3, 2+2=4. So total is 2+3+4=9. The code prints 9.