IO modes (r, w, a) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using mode 'r'?

Consider this Ruby code that tries to read a file in 'r' mode. What will it output if the file 'example.txt' contains the text 'Hello World'?

Ruby

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

A Hello World

B File not found error

C Empty output

D Permission denied error

💡 Hint

Mode 'r' opens the file for reading only.

The 'r' mode opens the file for reading. Since the file exists and contains 'Hello World', the program prints that text.

❓ Predict Output

intermediate

What happens when opening a file with mode 'w'?

Given this Ruby code, what will be the content of 'output.txt' after running it?

Ruby

File.open('output.txt', 'w') do |file|
  file.write('First line')
end
File.open('output.txt', 'w') do |file|
  file.write('Second line')
end

A The file contains 'First line'

B The file contains 'First lineSecond line'

C The file is empty

D The file contains 'Second line'

💡 Hint

Mode 'w' overwrites the file each time it opens it.

Each time the file is opened with 'w', it clears the previous content. So after the second write, only 'Second line' remains.

❓ Predict Output

advanced

What is the output after appending text with mode 'a'?

Assuming 'log.txt' initially contains 'Start\n', what will be the content after running this Ruby code?

Ruby

File.open('log.txt', 'a') do |file|
  file.puts('Entry 1')
end
File.open('log.txt', 'a') do |file|
  file.puts('Entry 2')
end
content = File.read('log.txt')
puts content

A

Start

B

Entry 1
Entry 2

C

Start
Entry 1
Entry 2

D

Start
Entry 2

💡 Hint

Mode 'a' adds new content to the end without deleting existing content.

The 'a' mode appends text. So the original 'Start\n' stays, and 'Entry 1' and 'Entry 2' are added on new lines.

❓ Predict Output

advanced

What error occurs when opening a non-existent file in 'r' mode?

What happens when this Ruby code runs and 'missing.txt' does not exist?

Ruby

File.open('missing.txt', 'r') do |file|
  puts file.read
end

A Errno::ENOENT (No such file or directory) error

B Empty output

C Permission denied error

D Creates an empty file named 'missing.txt'

💡 Hint

Mode 'r' requires the file to exist.

Opening a non-existent file in 'r' mode raises an error because the file must exist to read it.

🧠 Conceptual

expert

How do 'w' and 'a' modes differ when writing to a file?

Which statement correctly describes the difference between opening a file in 'w' mode versus 'a' mode in Ruby?

A 'w' mode appends to the file, 'a' mode overwrites the file content

B 'w' mode overwrites the file content, 'a' mode appends to the file without deleting existing content

C 'w' mode only writes if the file exists, 'a' mode creates a new file if it does not exist

D 'w' mode opens the file for reading and writing, 'a' mode opens the file for reading only

💡 Hint

Think about what happens to existing content in each mode.

'w' mode clears the file before writing, while 'a' mode adds new content at the end preserving existing data.

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