CSV library basics in Ruby - Mini Project: Build & Apply

CSV library basics

📖 Scenario: You work in a small bookstore. You have a CSV file listing books and their prices. You want to read this file and find books cheaper than a certain price.

🎯 Goal: Build a Ruby program that reads a CSV file of books and prices, filters books cheaper than a set price, and prints their titles.

📋 What You'll Learn

Use Ruby's CSV library to read data

Create a variable for the price limit

Use a loop to filter books cheaper than the limit

Print the titles of the filtered books

💡 Why This Matters

🌍 Real World

Reading and filtering CSV data is common in many jobs like sales, inventory, and data analysis.

💼 Career

Knowing how to use Ruby's CSV library helps you handle real data files and automate tasks in many programming jobs.

1

Create the CSV data as a string

Create a variable called csv_data and set it to this exact string including newlines: "Title,Price\nRuby Basics,25\nJavaScript Guide,30\nPython Intro,20"

Ruby

# Create a variable called csv_data with the exact CSV string
# Your code here

Hint

Use double quotes and include newline characters \n exactly as shown.

2

Set the price limit

Create a variable called price_limit and set it to the integer 25

Ruby

csv_data = "Title,Price\nRuby Basics,25\nJavaScript Guide,30\nPython Intro,20"
# Create a variable called price_limit and set it to 25
# Your code here

Hint

Just assign the number 25 to price_limit.

3

Parse CSV and filter books cheaper than price_limit

Use require 'csv' to load the CSV library. Then create a variable called cheap_books that uses CSV.parse on csv_data with headers: true. Use .select with a block that keeps rows where the Price converted to integer is less than price_limit. Then map the result to get an array of the Title values.

Ruby

csv_data = "Title,Price\nRuby Basics,25\nJavaScript Guide,30\nPython Intro,20"
price_limit = 25
# Require CSV and create cheap_books array with titles of books cheaper than price_limit
# Your code here

Hint

Use CSV.parse with headers: true, then select rows with price less than price_limit, then map to get titles.

4

Print the titles of cheap books

Write a puts statement to print the cheap_books array.

Ruby

require 'csv'
csv_data = "Title,Price\nRuby Basics,25\nJavaScript Guide,30\nPython Intro,20"
price_limit = 25
cheap_books = CSV.parse(csv_data, headers: true).select { |row| row['Price'].to_i < price_limit }.map { |row| row['Title'] }
# Print the cheap_books array
# Your code here

Hint

Use puts to print the array. It will print each title on its own line.

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