CSV library basics in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby CSV code?

Consider this Ruby code using the CSV library. What will it print?

Ruby

require 'csv'
csv_text = "name,age\nAlice,30\nBob,25"
CSV.parse(csv_text, headers: true) do |row|
  puts "#{row['name']} is #{row['age']} years old"
end

A Alice,30\nBob,25

B name is age years old\nAlice is 30 years old

C Alice is 30 years old\nBob is 25 years old

D 30 is Alice years old\n25 is Bob years old

💡 Hint

Look at how CSV.parse with headers:true lets you access columns by name.

The CSV.parse method with headers:true treats the first line as headers. Each row is a CSV::Row object, so row['name'] and row['age'] access the values by column name. The code prints each person's name and age in the given format.

❓ Predict Output

intermediate

What does this Ruby CSV code output?

Given this Ruby code snippet, what will be printed?

Ruby

require 'csv'
csv_text = "id,value\n1,100\n2,200"
rows = CSV.parse(csv_text, headers: true).map { |r| r['value'].to_i * 2 }
puts rows.join(',')

A 100,200

B 200,400

C id,value

D 2,4

💡 Hint

Remember to convert string values to integers before multiplying.

The code parses CSV with headers, extracts the 'value' column as strings, converts them to integers, doubles them, and prints them joined by commas.

🔧 Debug

advanced

Why does this Ruby CSV code raise an error?

This Ruby code raises an error. What is the cause?

Ruby

require 'csv'
csv_text = "name,age\nAlice,30\nBob,25"
CSV.parse(csv_text) do |row|
  puts row['name']
end

A row is an Array, so row['name'] raises TypeError

B CSV.parse requires headers:true to parse correctly

C The CSV library is not required properly

D The CSV text is malformed and missing commas

💡 Hint

Check the type of row when headers option is not set.

Without headers:true, CSV.parse yields each row as an Array, not a CSV::Row. Arrays do not support string keys, so row['name'] raises a TypeError.

📝 Syntax

advanced

Which option correctly reads CSV with headers and prints ages?

Choose the Ruby code snippet that correctly reads CSV text with headers and prints each age.

A CSV.foreach(csv_text) { |row| puts row['age'] }

B CSV.parse(csv_text) { |row| puts row['age'] }

C CSV.read(csv_text, headers: true) { |row| puts row['age'] }

D CSV.parse(csv_text, headers: true) { |row| puts row['age'] }

💡 Hint

Only CSV.parse with headers:true yields rows with string keys.

Option D uses CSV.parse with headers:true, so each row is a CSV::Row and supports row['age']. Option D lacks headers:true, so row is an Array. Option D is invalid because CSV.read does not take a block. Option D expects a filename, not CSV text.

🚀 Application

expert

How many rows are parsed from this CSV string?

Given this Ruby code, how many rows will be parsed and printed?

Ruby

require 'csv'
csv_text = "name,score\nJohn,85\nJane,90\nDoe,78"
rows = CSV.parse(csv_text, headers: true)
puts rows.size

A 3

B 4

C 2

D 1

💡 Hint

Headers are not counted as rows.

The CSV string has one header row and three data rows. CSV.parse with headers:true returns only the data rows, so rows.size is 3.

Practice

1. What does the Ruby CSV method CSV.foreach do?

easy

A. Reads a CSV file line by line, yielding each row as an array.

B. Writes data to a CSV file line by line.

C. Converts a CSV file into a hash with headers as keys.

D. Deletes a CSV file from the system.

Solution

1. Step 1: Understand CSV.foreach purpose

   The method CSV.foreach is designed to read CSV files line by line.

2. Step 2: Identify the output format

   Each line is yielded as an array of strings representing the cells in that row.

3. Final Answer:

   Reads a CSV file line by line, yielding each row as an array. -> Option A

4. Quick Check:

   CSV.foreach reads lines as arrays [OK]

Hint: Remember: foreach reads rows, open writes rows [OK]

Common Mistakes:

• Confusing reading with writing methods
• Thinking it returns a hash instead of an array
• Assuming it deletes files

2. Which of the following is the correct syntax to open a CSV file for writing in Ruby?

easy

A. CSV.read('file.csv', 'w')

B. CSV.open('file.csv', 'r') do |csv| ... end

C. CSV.foreach('file.csv') do |csv| ... end

D. CSV.open('file.csv', 'w') do |csv| ... end

Solution

1. Step 1: Identify mode for writing

   To write to a CSV file, the mode should be 'w' (write mode).

2. Step 2: Match syntax with CSV.open

   The correct syntax is CSV.open('file.csv', 'w') do |csv| ... end to open for writing.

3. Final Answer:

   CSV.open('file.csv', 'w') do |csv| ... end -> Option D

4. Quick Check:

   Open with 'w' writes CSV [OK]

Hint: Use 'w' mode with CSV.open to write files [OK]

Common Mistakes:

• Using 'r' mode which is for reading
• Using CSV.foreach for writing
• Using CSV.read with mode argument

3. What will be the output of this Ruby code?

require 'csv'
CSV.foreach('data.csv') do |row|
  puts row.inspect
end

Assuming data.csv contains:

name,age
Alice,30
Bob,25

medium

A. Error: CSV.foreach requires a block

B. ["name", "age"]\n["Alice", "30"]\n["Bob", "25"]

C. ["name,age"]\n["Alice,30"]\n["Bob,30"]

D. name,age\nAlice,30\nBob,25

Solution

1. Step 1: Understand CSV.foreach behavior

   It reads each line and yields an array of strings split by commas.

2. Step 2: Analyze puts row.inspect output

   Using puts row.inspect prints the array representation of each row.

3. Final Answer:

   ["name", "age"]\n["Alice", "30"]\n["Bob", "25"] -> Option B

4. Quick Check:

   CSV.foreach yields arrays, inspect shows arrays [OK]

Hint: CSV.foreach yields arrays; inspect shows array format [OK]

Common Mistakes:

• Expecting raw CSV string output
• Confusing puts with print
• Missing block for CSV.foreach

4. Identify the error in this Ruby code snippet:

require 'csv'
CSV.open('output.csv', 'w') do |csv|
  csv << 'name,age'
  csv << ['Alice', 30]
end

medium

A. The first csv << should be an array, not a string.

B. Missing require statement for CSV library.

C. The file mode 'w' is invalid for CSV.open.

D. csv << cannot accept arrays.

Solution

1. Step 1: Check CSV.open usage for writing rows

   CSV expects each row as an array of fields, not a single string.

2. Step 2: Identify the incorrect row format

   The line csv << 'name,age' should be csv << ['name', 'age'] to write two columns.

3. Final Answer:

   The first csv << should be an array, not a string. -> Option A

4. Quick Check:

   CSV rows must be arrays [OK]

Hint: Write rows as arrays, not comma strings [OK]

Common Mistakes:

• Passing a string instead of array for a row
• Forgetting to require 'csv'
• Using wrong file mode

5. You want to read a CSV file and create a hash where the first column is the key and the second column is the value. Which Ruby code correctly does this using the CSV library?

hard

A. hash = CSV.read('file.csv').to_h

B. hash = CSV.open('file.csv', 'w') { |csv| csv.to_h }

C. hash = {}
CSV.foreach('file.csv') do |row|
hash[row[0]] = row[1]
end

D. hash = CSV.foreach('file.csv') { |row| row.to_h }

Solution

1. Step 1: Understand CSV.foreach usage for reading

   CSV.foreach reads each row as an array, allowing manual hash construction.

2. Step 2: Build hash from first and second columns

   Assigning hash[row[0]] = row[1] correctly maps keys to values.

3. Final Answer:

   hash = {} CSV.foreach('file.csv') do |row| hash[row[0]] = row[1] end -> Option C

4. Quick Check:

   foreach + manual hash build = correct [OK]

Hint: Use foreach and assign hash keys manually [OK]

Common Mistakes:

• Using CSV.read.to_h which doesn't convert arrays to hash
• Using CSV.open with 'w' mode for reading
• Expecting row.to_h to work directly