IO modes (r, w, a) in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'r' mode do when opening a file in Ruby?

The 'r' mode opens a file for reading only. The file must exist, or Ruby will raise an error.

beginner

What happens when you open a file with 'w' mode in Ruby?

The 'w' mode opens a file for writing. If the file exists, it is cleared (emptied). If it doesn't exist, a new file is created.

beginner

Explain the 'a' mode in Ruby file handling.

The 'a' mode opens a file for appending. Data written is added to the end of the file. If the file doesn't exist, it is created.

beginner

What will happen if you try to open a non-existent file with 'r' mode?

Ruby will raise an error because 'r' mode requires the file to exist for reading.

beginner

How does 'w' mode affect existing file content?

Opening a file with 'w' mode deletes all existing content, making the file empty before writing new data.

Which mode should you use to add text to the end of an existing file without deleting its content?

A 'r' (read)

B 'a' (append)

C 'w' (write)

D 'x' (exclusive creation)

The 'a' mode appends data to the end of the file without deleting existing content.

What happens if you open a file with 'w' mode and the file already exists?

A An error is raised.

B The file is opened for reading only.

C The file is opened without changing content.

D The file is cleared (emptied) before writing.

'w' mode clears the file content before writing new data.

If you want to read a file, which mode should you use?

A 'w'

B 'a'

C 'r'

D 'x'

'r' mode opens the file for reading only.

What will happen if you try to open a file that does not exist with 'r' mode?

A An error is raised.

B The file is appended.

C The file is opened for writing.

D The file is created.

'r' mode requires the file to exist; otherwise, Ruby raises an error.

Which mode creates a new file if it does not exist and opens it for writing?

A 'w'

B 'r'

C 'a'

D 'r+'

'w' mode creates a new file if it does not exist and opens it for writing.

Describe the differences between the 'r', 'w', and 'a' modes when opening files in Ruby.

Think about what happens to the file content and whether the file must exist.

What errors or behaviors should you expect when opening files with these modes if the file does not exist?

Consider which modes require the file beforehand and which create it.

Practice

1. What does the 'r' mode do when opening a file in Ruby?

easy

A. Opens the file for reading only, without changing its content.

B. Opens the file for writing and erases existing content.

C. Opens the file for appending new content at the end.

D. Creates a new file if it doesn't exist and writes to it.

Solution

1. Step 1: Understand the purpose of 'r' mode

   The 'r' mode is used to open a file only for reading, so the file content is not changed.

2. Step 2: Compare with other modes

   'w' mode writes and erases content, 'a' appends, but 'r' only reads.

3. Final Answer:

   Opens the file for reading only, without changing its content. -> Option A

4. Quick Check:

   'r' = read only [OK]

Hint: Remember 'r' stands for read only, no changes made [OK]

Common Mistakes:

• Confusing 'r' with 'w' and thinking it erases content
• Assuming 'r' can write to the file
• Mixing 'r' with 'a' mode functionality

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

easy

A. File.open('data.txt', 'x')

B. File.open('data.txt', 'w')

C. File.open('data.txt', 'r')

D. File.open('data.txt', 'a')

Solution

1. Step 1: Identify the mode for appending

   The mode 'a' is used to open a file for appending new content at the end.

2. Step 2: Check the syntax

   The correct syntax is File.open('filename', 'a') to append.

3. Final Answer:

   File.open('data.txt', 'a') -> Option D

4. Quick Check:

   Append mode = 'a' [OK]

Hint: Use 'a' to add content without erasing [OK]

Common Mistakes:

• Using 'r' which is read-only mode
• Using 'w' which erases file content
• Using 'x' which creates a new file and errors if exists

3. What will be the output of this Ruby code if example.txt initially contains "Hello"?

File.open('example.txt', 'w') { |f| f.puts 'World' }
content = File.read('example.txt')
puts content

medium

A. Hello\nWorld

B. Hello

C. World

D. Error: file not found

Solution

1. Step 1: Understand 'w' mode effect

   Opening with 'w' erases existing content and writes new content.

2. Step 2: Analyze the code output

   The file is overwritten with "World" and then read back, so output is "World" with a newline.

3. Final Answer:

   World -> Option C

4. Quick Check:

   'w' erases old content, so output is new content with newline [OK]

Hint: Writing with 'w' erases old content, so only new text shows [OK]

Common Mistakes:

• Thinking 'w' appends instead of overwriting
• Expecting old content to remain
• Confusing output with combined old and new text

4. The following Ruby code is intended to add the line "Goodbye" to the end of log.txt. What is the error?

File.open('log.txt', 'w') do |file|
  file.puts 'Goodbye'
end

medium

A. The file name should be in double quotes, not single quotes.

B. Using 'w' mode erases the file instead of appending.

C. Missing a block parameter for the file object.

D. The method 'puts' cannot be used inside File.open.

Solution

1. Step 1: Identify the mode used

   The code uses 'w' mode which overwrites the file content.

2. Step 2: Understand the intended action

   To add content without erasing, 'a' mode should be used instead of 'w'.

3. Final Answer:

   Using 'w' mode erases the file instead of appending. -> Option B

4. Quick Check:

   Append needs 'a', not 'w' [OK]

Hint: Use 'a' to add lines, 'w' erases file [OK]

Common Mistakes:

• Confusing 'w' with append mode
• Thinking 'w' adds content without erasing
• Believing quotes style affects file opening

5. You want to create a Ruby script that reads a file notes.txt, appends the line "New note added", and then reads the updated content to print it. Which sequence of file modes should you use?

hard

A. Open with 'r' to read, then 'a' to append, then 'r' to read again.

B. Open with 'a' to append, then 'r' to read the updated content.

C. Open with 'w' to write, then 'a' to append, then 'r' to read.

D. Open with 'r' to read, then 'w' to write, then 'r' to read again.

Solution

1. Step 1: Read the original file

   Use 'r' mode to read the existing content without changing it.

2. Step 2: Append the new line

   Use 'a' mode to add "New note added" at the end without erasing.

3. Step 3: Read the updated content

   Open again with 'r' mode to read and print the updated file content.

4. Final Answer:

   Open with 'r' to read, then 'a' to append, then 'r' to read again. -> Option A

5. Quick Check:

   Read, append, read = 'r', 'a', 'r' [OK]

Hint: Read first, append with 'a', then read again [OK]

Common Mistakes:

• Using 'w' which erases content before appending
• Trying to append before reading original content
• Not reopening file to read updated content