Ensure for cleanup in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using ensure?

Consider the following Ruby code snippet. What will it print when run?

Ruby

def test_ensure
  begin
    puts "Start"
    raise "Error occurred"
  ensure
    puts "Cleanup"
  end
end

test_ensure

A Start\nRuntimeError: Error occurred

B Start\nCleanup

C Cleanup\nStart\nRuntimeError: Error occurred

D Start\nCleanup\nRuntimeError: Error occurred

💡 Hint

Remember that ensure runs no matter what, even if an error is raised.

The code prints "Start" first, then raises an error. The ensure block runs next, printing "Cleanup". After that, the error is propagated, causing the RuntimeError message.

❓ Predict Output

intermediate

What value does the method return with ensure?

What will the following Ruby method return when called?

Ruby

def return_test
  begin
    return 10
  ensure
    return 20
  end
end

return_test

A 20

B 10

C nil

D RuntimeError

💡 Hint

The ensure block can override the return value if it has a return statement.

Even though the begin block returns 10, the ensure block's return 20 overrides it, so the method returns 20.

❓ Predict Output

advanced

What is the output and final value of x?

Analyze this Ruby code. What is printed and what is the final value of x after execution?

Ruby

x = 0
begin
  x = 1
  raise "fail"
rescue
  x = 2
ensure
  x = 3
end
puts x

A 1

B 3

C 2

D RuntimeError

💡 Hint

Ensure runs after rescue and can change variable values.

The begin block sets x=1 then raises an error. Rescue sets x=2. Ensure runs last and sets x=3. So x is 3 when printed.

❓ Predict Output

advanced

What error does this code raise?

What error, if any, will this Ruby code raise?

Ruby

def test
  begin
    raise "Oops"
  ensure
    raise "Cleanup error"
  end
end

test

A No error, method returns normally

B RuntimeError: Oops

C RuntimeError: Cleanup error

D SyntaxError

💡 Hint

An error in ensure overrides any previous error.

The first raise triggers "Oops" error, but the ensure block raises "Cleanup error" which overrides the first error. So the final error is "Cleanup error".

🧠 Conceptual

expert

Why use ensure instead of rescue for cleanup?

Which statement best explains why Ruby programmers use ensure blocks for cleanup instead of rescue blocks?

A Ensure runs regardless of whether an exception occurs, guaranteeing cleanup code always executes.

B Rescue runs only if no exception occurs, so it is unreliable for cleanup.

C Rescue blocks run before the main code, so they cannot clean up after it.

D Ensure blocks catch exceptions and prevent program crashes.

💡 Hint

Think about when cleanup code must run.

Ensure blocks always run after the begin block, whether or not an exception was raised, making them ideal for cleanup tasks like closing files or releasing resources.

Practice

1. What is the main purpose of the ensure block in Ruby?

easy

A. To run code that must execute regardless of errors

B. To catch exceptions and handle them

C. To define a method inside a class

D. To start a loop that repeats code

Solution

1. Step 1: Understand the role of ensure

   The ensure block runs code no matter what happens in the begin block, even if an error occurs.

2. Step 2: Compare with other blocks

   Unlike rescue which handles errors, ensure always runs for cleanup tasks.

3. Final Answer:

   To run code that must execute regardless of errors -> Option A

4. Quick Check:

   ensure = always runs cleanup [OK]

Hint: Remember: ensure always runs, even after errors [OK]

Common Mistakes:

• Confusing ensure with rescue
• Thinking ensure only runs on errors
• Using ensure to catch exceptions

2. Which of the following is the correct syntax to use ensure in Ruby?

easy

A. if
# code
else
# cleanup code
end

B. try
# code
finally
# cleanup code
end

C. begin
# code
rescue
# cleanup code
end

D. begin
# code
ensure
# cleanup code
end

Solution

1. Step 1: Identify Ruby's syntax for ensure

   Ruby uses begin ... ensure ... end to guarantee cleanup code runs.

2. Step 2: Eliminate other language or incorrect syntax

   Options B and D use non-Ruby keywords; C uses rescue which is for error handling, not guaranteed cleanup.

3. Final Answer:

   begin # code ensure # cleanup code end -> Option D

4. Quick Check:

   Ruby ensure syntax = begin ... ensure ... end [OK]

Hint: Use begin ... ensure ... end for guaranteed cleanup [OK]

Common Mistakes:

• Using try/finally (not Ruby syntax)
• Confusing rescue with ensure
• Using if/else for cleanup

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

def test
  begin
    puts "Start"
    raise "Error!"
  ensure
    puts "Cleanup"
  end
end

test

medium

A. Start\nCleanup\nRuntimeError: Error!

B. Start\nRuntimeError: Error!\nCleanup

C. Cleanup\nStart\nRuntimeError: Error!

D. Start\nCleanup

Solution

1. Step 1: Trace the code execution

   The method prints "Start", then raises an error. The ensure block runs next, printing "Cleanup".

2. Step 2: Understand error propagation

   After ensure, the error is not rescued, so it propagates and shows the RuntimeError message.

3. Final Answer:

   Start\nCleanup\nRuntimeError: Error! -> Option A

4. Quick Check:

   ensure runs before error propagates [OK]

Hint: Ensure runs before error stops program [OK]

Common Mistakes:

• Thinking ensure stops error
• Assuming cleanup prints after error message
• Ignoring error propagation

4. Identify the error in this Ruby code using ensure:

begin
  puts "Processing"
  # missing end for begin
ensure
  puts "Cleaning up"
end

medium

A. Cannot use ensure without rescue

B. Missing end for begin block

C. The puts statements are invalid inside ensure

D. The ensure block must come before begin

Solution

1. Step 1: Check block structure

   The begin block is missing its closing end before ensure.

2. Step 2: Understand ensure placement

   ensure must be inside the begin ... end block, so the missing end causes a syntax error.

3. Final Answer:

   Missing end for begin block -> Option B

4. Quick Check:

   Every begin needs matching end [OK]

Hint: Count your begin and end keywords carefully [OK]

Common Mistakes:

• Thinking ensure needs rescue
• Placing ensure outside begin-end
• Assuming puts invalid in ensure

5. You want to open a file, write data, and always close it even if an error occurs. Which Ruby code correctly uses ensure for this cleanup?

hard

A. begin
file = File.open('data.txt', 'w')
file.write('Hello')
rescue
file.close
end

B. file = File.open('data.txt', 'w')
file.write('Hello')
file.close

C. file = File.open('data.txt', 'w')
begin
file.write('Hello')
ensure
file.close
end

D. begin
file = File.open('data.txt', 'w')
file.write('Hello')
ensure
file = nil
end

Solution

1. Step 1: Identify correct use of ensure for cleanup

   file = File.open('data.txt', 'w') begin file.write('Hello') ensure file.close end opens the file before begin, writes inside, and closes in ensure, guaranteeing closure.

2. Step 2: Check other options for errors

   begin file = File.open('data.txt', 'w') file.write('Hello') rescue file.close end closes in rescue, which only runs on errors, not always. file = File.open('data.txt', 'w') file.write('Hello') file.close doesn't handle errors. begin file = File.open('data.txt', 'w') file.write('Hello') ensure file = nil end sets file to nil but doesn't close it.

3. Final Answer:

   file = File.open('data.txt', 'w') begin file.write('Hello') ensure file.close end -> Option C

4. Quick Check:

   Ensure always closes file [OK]

Hint: Put cleanup like file.close inside ensure [OK]

Common Mistakes:

• Closing file only in rescue block
• Not closing file on error
• Setting variable to nil instead of closing