File.open with block (auto-close) in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does File.open with a block do in Ruby?

It opens a file, runs the code inside the block using the file, and then automatically closes the file when the block finishes.

beginner

Why is using File.open with a block safer than opening and closing files manually?

Because it ensures the file is always closed properly, even if an error happens inside the block.

beginner

Show the syntax of File.open with a block to read a file named 'example.txt'.

File.open('example.txt', 'r') do |file|
  content = file.read
  puts content
end

intermediate

What happens if you try to use the file outside the block when using File.open with a block?

The file is already closed, so trying to read or write will cause an error.

beginner

Can you write to a file using File.open with a block? How?

Yes. Open the file with write mode ('w') and use the block to write inside it. Example:

File.open('output.txt', 'w') do |file|
  file.puts 'Hello!'
end

What does File.open('data.txt', 'r') do |f| ... end do?

A Opens 'data.txt' for reading and closes it automatically after the block.

B Opens 'data.txt' for writing and never closes it.

C Creates a new file named 'data.txt'.

D Deletes 'data.txt' after reading.

Using File.open with a block opens the file and closes it automatically after the block finishes.

What happens if an error occurs inside the block passed to File.open?

A The file remains open and causes a leak.

B The program crashes without closing the file.

C The file is still closed automatically.

D The file is deleted.

Ruby ensures the file is closed even if an error happens inside the block.

Which mode should you use with File.open to write new content to a file?

A 'w'

B 'a'

C 'r'

D 'x'

'w' mode opens the file for writing and overwrites existing content.

What will happen if you try to read from the file outside the block after File.open with a block?

A It will read the file normally.

B It will raise an error because the file is closed.

C It will create a new file.

D It will append data to the file.

The file is closed after the block, so reading outside causes an error.

Which of these is a benefit of using File.open with a block?

A It deletes the file after use.

B You must manually close the file.

C The file stays open for the whole program.

D The file is automatically closed after the block.

The block form automatically closes the file, making code safer and cleaner.

Explain how File.open with a block works and why it is useful.

Think about what happens before, during, and after the block.

Write a simple Ruby code snippet using File.open with a block to read and print a file's content.

Use 'r' mode and a block with |file| parameter.

Practice

1. What happens to the file after the block finishes when you use File.open('file.txt') do |f| ... end in Ruby?

easy

A. The file remains open until you manually close it.

B. The file is automatically closed after the block ends.

C. The file is deleted after the block ends.

D. The file is locked and cannot be accessed again.

Solution

1. Step 1: Understand File.open with block behavior

   When you open a file with a block, Ruby automatically manages the file resource.

2. Step 2: Recognize automatic closing

   After the block finishes, Ruby closes the file automatically to prevent resource leaks.

3. Final Answer:

   The file is automatically closed after the block ends. -> Option B

4. Quick Check:

   File.open with block = auto-close [OK]

Hint: File.open with block always auto-closes the file [OK]

Common Mistakes:

• Thinking the file stays open after the block
• Assuming the file is deleted automatically
• Believing the file locks after block ends

2. Which of the following is the correct syntax to open a file named data.txt for reading using a block in Ruby?

easy

A. File.open('data.txt') |file| puts file.read end

B. File.open('data.txt') puts file.read end

C. File.open('data.txt') { |file| puts file.read }

D. File.open('data.txt') do puts file.read end

Solution

1. Step 1: Identify correct block syntax

   Ruby uses either curly braces or do...end with a block parameter like |file|.

2. Step 2: Check each option

   File.open('data.txt') { |file| puts file.read } correctly uses { |file| ... }. Others have syntax errors or missing block parameters.

3. Final Answer:

   File.open('data.txt') { |file| puts file.read } -> Option C

4. Quick Check:

   Correct block syntax = File.open('data.txt') { |file| puts file.read } [OK]

Hint: Use {|var| ...} or do |var| ... end for blocks [OK]

Common Mistakes:

• Missing block parameter |file|
• Incorrect block delimiters
• Omitting do or braces

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

File.open('test.txt', 'w') do |f|
  f.puts 'Hello'
end

content = File.read('test.txt')
puts content

medium

A. Hello

B. test.txt

C. nil

D. Error: file not found

Solution

1. Step 1: Write 'Hello' to file inside block

   The block writes the string 'Hello' followed by a newline to 'test.txt'.

2. Step 2: Read and print file content

   After the block, the file is closed automatically. Then File.read reads the content, which is 'Hello\n'. Printing it outputs 'Hello'.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   File.write + read = Hello [OK]

Hint: File.open with block writes and auto-closes file [OK]

Common Mistakes:

• Expecting file to be empty
• Thinking file is not saved before reading
• Confusing file name with content

4. Identify the error in this Ruby code snippet:

file = File.open('log.txt', 'w') do |f|
  f.puts 'Log entry'
end
file.puts 'Another entry'

medium

A. file is nil because File.open with block returns the block's value, not the file object.

B. The file is not opened in write mode.

C. Missing block parameter |f| in File.open.

D. File is not closed automatically.

Solution

1. Step 1: Understand return value of File.open with block

   When using a block, File.open returns the block's last expression, not the file object.

2. Step 2: Analyze variable 'file'

   Here, 'file' is assigned the block's return value, which is nil because f.puts returns nil. So calling file.puts causes an error.

3. Final Answer:

   file is nil because File.open with block returns the block's value, not the file object. -> Option A

4. Quick Check:

   File.open block returns block value, not file [OK]

Hint: File.open with block returns block result, not file object [OK]

Common Mistakes:

• Expecting File.open with block to return file object
• Trying to use file after block ends
• Ignoring file mode argument

5. You want to read a file and count how many lines contain the word 'error'. Which Ruby code correctly uses File.open with a block to do this safely?

hard

A. count = 0
File.open('log.txt') { |f| f.each { |line| count += 1 if line.include?('error') } }
puts count

B. count = 0
f = File.open('log.txt', 'r')
f.each_line { |line| count += 1 if line.include?('error') }
f.close
puts count

C. count = 0
File.open('log.txt', 'r') do
each_line { |line| count += 1 if line.include?('error') }
end
puts count

D. count = 0
File.open('log.txt', 'r') do |f|
f.each_line { |line| count += 1 if line.include?('error') }
end
puts count

Solution

1. Step 1: Use File.open with block and read lines

   count = 0 File.open('log.txt', 'r') do |f| f.each_line { |line| count += 1 if line.include?('error') } end puts count opens the file with a block and iterates each line safely.

2. Step 2: Count lines containing 'error'

   Inside the block, it increments count if the line includes 'error'. The file auto-closes after the block.

3. Final Answer:

   count = 0 File.open('log.txt', 'r') do |f| f.each_line { |line| count += 1 if line.include?('error') } end puts count -> Option D

4. Quick Check:

   File.open block + each_line + count = count = 0 File.open('log.txt', 'r') do |f| f.each_line { |line| count += 1 if line.include?('error') } end puts count [OK]

Hint: Use File.open with block and each_line for safe file reading [OK]

Common Mistakes:

• Forgetting to close file when not using block
• Missing block parameter or using wrong syntax
• Using methods outside block scope