File.readlines for line-by-line in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of reading lines with File.readlines?

Given a file named example.txt with these contents:

apple
banana
cherry

What will this Ruby code output?

lines = File.readlines('example.txt')
puts lines[1]

Ruby

lines = File.readlines('example.txt')
puts lines[1]

A ananab

B apple

C

banana

D cherry

💡 Hint

Remember that File.readlines keeps the newline characters at the end of each line.

File.readlines reads all lines into an array, each line including its newline character. So lines[1] is 'banana\n'. puts prints this string, resulting in the output 'banana' followed by a newline.

❓ Predict Output

intermediate

How many lines does File.readlines return?

If a file data.txt contains 5 lines of text, what will File.readlines('data.txt').size return?

Ruby

count = File.readlines('data.txt').size
puts count

A 5

B 4

C 0

D 6

💡 Hint

File.readlines returns an array with one element per line.

File.readlines returns an array where each element is one line from the file. So the size of this array equals the number of lines in the file.

🔧 Debug

advanced

Why does this code raise an error when reading lines?

Consider this Ruby code:

lines = File.readlines('missing.txt')
puts lines[0]

What error will this code raise and why?

Ruby

lines = File.readlines('missing.txt')
puts lines[0]

A Errno::ENOENT (No such file or directory) because the file does not exist

B NoMethodError because lines is nil

C IndexError because lines[0] is out of range

D SyntaxError due to missing quotes

💡 Hint

Check what happens if you try to read a file that is not present.

File.readlines tries to open the file. If the file does not exist, Ruby raises Errno::ENOENT indicating the file was not found.

🧠 Conceptual

advanced

What is the difference between File.readlines and File.foreach?

Which statement correctly describes the difference between File.readlines and File.foreach in Ruby?

A File.readlines writes to a file; File.foreach reads from a file

B File.readlines reads the entire file into memory as an array; File.foreach reads one line at a time without loading the whole file

C Both methods read the entire file into memory but return different data types

D File.readlines reads one line at a time; File.foreach reads the entire file into memory

💡 Hint

Think about memory usage when reading large files.

File.readlines loads all lines into an array at once, which can use a lot of memory for big files. File.foreach reads one line at a time, which is more memory efficient.

❓ Predict Output

expert

What is the output of this code using File.readlines with chomp?

Given a file fruits.txt with contents:

apple
banana
cherry

What will this code output?

lines = File.readlines('fruits.txt').map(&:chomp)
puts lines.join('-')

Ruby

lines = File.readlines('fruits.txt').map(&:chomp)
puts lines.join('-')

A applebanana cherry

B apple\n-banana\n-cherry\n

C apple banana cherry

D apple-banana-cherry

💡 Hint

The chomp method removes newline characters from each line.

File.readlines reads lines with newlines. Using map(&:chomp) removes newlines from each line. Joining with '-' produces 'apple-banana-cherry'.

Practice

1. What does File.readlines('example.txt') return in Ruby?

easy

A. A hash with line numbers as keys and lines as values

B. A string containing the entire file content

C. An array where each element is a line from the file, including newline characters

D. An integer representing the number of lines in the file

Solution

1. Step 1: Understand File.readlines behavior

   File.readlines reads the whole file and returns an array of lines.

2. Step 2: Check line content format

   Each line includes the newline character at the end unless removed by chomp.

3. Final Answer:

   An array where each element is a line from the file, including newline characters -> Option C

4. Quick Check:

   File.readlines returns array of lines [OK]

Hint: Remember: File.readlines returns an array of lines with newlines [OK]

Common Mistakes:

• Thinking it returns a single string
• Assuming it returns a hash or dictionary
• Believing it returns just the line count

2. Which of the following is the correct syntax to read all lines from a file named data.txt into an array in Ruby?

easy

A. File.read_lines[]('data.txt')

B. File.read('data.txt')

C. File.read_lines('data.txt')

D. File.readlines('data.txt')

Solution

1. Step 1: Recall correct method name

   The correct Ruby method to read lines is File.readlines (no underscore or plural misspelling).

2. Step 2: Check method call syntax

   The method is called with parentheses and the filename as a string argument.

3. Final Answer:

   File.readlines('data.txt') -> Option D

4. Quick Check:

   Correct method name and syntax = File.readlines('data.txt') [OK]

Hint: Use exact method name: File.readlines('filename') [OK]

Common Mistakes:

• Using underscores instead of camel case
• Misspelling the method name
• Omitting parentheses or using wrong brackets

3. Given a file notes.txt with these lines:

Line 1
Line 2
Line 3

What will the following Ruby code output?

lines = File.readlines('notes.txt')
puts lines[1]

medium

A. Line 2

B. Line 1

C. Line 3

D. nil

Solution

1. Step 1: Understand File.readlines output

   The file lines are read into an array: ["Line 1\n", "Line 2\n", "Line 3\n"].

2. Step 2: Access the second element

   Array indexing starts at 0, so lines[1] is "Line 2\n".

3. Final Answer:

   Line 2 -> Option A

4. Quick Check:

   Index 1 = second line = Line 2 [OK]

Hint: Remember arrays start at 0; lines[1] is second line [OK]

Common Mistakes:

• Assuming indexing starts at 1
• Forgetting newline characters affect output
• Expecting nil for valid index

4. What is wrong with this Ruby code snippet?

lines = File.readlines('file.txt')
lines.each do |line|
  puts line.chomp
end

Assuming file.txt exists and is readable.

medium

A. It will skip the first line of the file

B. Nothing is wrong; it correctly prints each line without newlines

C. It will print lines with extra newlines

D. It will raise an error because chomp is not a method

Solution

1. Step 1: Analyze File.readlines usage

   File.readlines reads all lines into an array, each ending with a newline.

2. Step 2: Check iteration and chomp usage

   Using each to iterate and chomp removes trailing newline before printing.

3. Final Answer:

   Nothing is wrong; it correctly prints each line without newlines -> Option B

4. Quick Check:

   chomp removes newline, so output is clean [OK]

Hint: Use chomp to remove newlines when printing lines [OK]

Common Mistakes:

• Thinking chomp is invalid method
• Expecting extra newlines despite chomp
• Assuming lines are skipped

5. You want to read a file data.txt and create a new array containing only lines that include the word "error" (case insensitive). Which Ruby code correctly does this using File.readlines?

hard

A. File.readlines('data.txt').select { |line| line.downcase.include?('error') }

B. File.readlines('data.txt').map { |line| line.include?('error') }

C. File.readlines('data.txt').filter { |line| line.include?('Error') }

D. File.readlines('data.txt').reject { |line| line.downcase.include?('error') }

Solution

1. Step 1: Use File.readlines to get all lines

   This returns an array of lines from the file.

2. Step 2: Filter lines case-insensitively

   Use select to keep lines where line.downcase.include?('error') is true.

3. Step 3: Check other options

   File.readlines('data.txt').map { |line| line.include?('error') } uses map which returns booleans, not lines. File.readlines('data.txt').filter { |line| line.include?('Error') } checks case-sensitive 'Error'. File.readlines('data.txt').reject { |line| line.downcase.include?('error') } rejects lines with 'error', opposite of requirement.

4. Final Answer:

   File.readlines('data.txt').select { |line| line.downcase.include?('error') } -> Option A

5. Quick Check:

   select + downcase + include? = correct filter [OK]

Hint: Use select with downcase and include? for case-insensitive filtering [OK]

Common Mistakes:

• Using map instead of select
• Ignoring case sensitivity
• Using reject instead of select