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'.