File.readlines for line-by-line in Ruby - Mini Project: Build & Apply

Reading a File Line-by-Line with File.readlines in Ruby

📖 Scenario: You have a text file named colors.txt that contains a list of colors, one color per line. You want to read this file line-by-line to process each color.

🎯 Goal: Build a Ruby program that reads the colors.txt file line-by-line using File.readlines and prints each color in uppercase.

📋 What You'll Learn

Create a variable called colors that reads all lines from colors.txt using File.readlines

Create a variable called count and set it to 0

Use a for loop with variable color to iterate over colors

Inside the loop, print each color in uppercase

After the loop, print the total number of colors read

💡 Why This Matters

🌍 Real World

Reading files line-by-line is common when processing logs, configuration files, or data lists.

💼 Career

Many programming jobs require reading and processing text files efficiently, especially in scripting and automation.

1

Read all lines from the file

Create a variable called colors that reads all lines from the file colors.txt using File.readlines.

Ruby

# Your code here

Hint

Use File.readlines('colors.txt') to read all lines into an array.

2

Create a counter variable

Create a variable called count and set it to 0 to count the number of colors.

Ruby

colors = File.readlines('colors.txt')
# Your code here

Hint

Initialize count to zero before counting.

3

Loop through each color and print uppercase

Use a for loop with variable color to iterate over colors. Inside the loop, print each color in uppercase and increase count by 1.

Ruby

colors = File.readlines('colors.txt')
count = 0
# Your code here

Hint

Use for color in colors to loop. Use color.chomp.upcase to print uppercase without extra newlines.

4

Print the total number of colors

After the loop, print the total number of colors read using puts and the count variable.

Ruby

colors = File.readlines('colors.txt')
count = 0
for color in colors
  puts color.chomp.upcase
  count += 1
end
# Your code here

Hint

Use puts "Total colors: #{count}" to print the count after the loop.

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