CSV library basics in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the CSV library in Ruby?

The CSV library helps read from and write to CSV (Comma-Separated Values) files easily, making it simple to handle tabular data.

beginner

How do you require the CSV library in a Ruby script?

Use require 'csv' at the top of your Ruby file to load the CSV library.

beginner

How do you read a CSV file line by line in Ruby?

Use CSV.foreach('filename.csv') do |row| ... end. Each row is an array of values from that line.

intermediate

How do you write an array of arrays to a CSV file in Ruby?

Use CSV.open('filename.csv', 'w') do |csv| data.each { |row| csv << row } end where data is an array of arrays.

intermediate

What does CSV.parse do?

CSV.parse reads CSV data from a string instead of a file, returning an array of rows.

Which Ruby command loads the CSV library?

A require 'csv'

B import csv

C include CSV

D load 'csv'

In Ruby, require 'csv' loads the CSV library.

What does CSV.foreach('file.csv') do |row| ... end do?

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

B Writes rows to the CSV file

C Parses CSV data from a string

D Deletes the CSV file

It reads the CSV file line by line and gives each row as an array to the block.

How do you write data to a CSV file in Ruby?

A CSV.delete('file.csv')

B CSV.read('file.csv')

C CSV.open('file.csv', 'w') { |csv| csv << ['data'] }

D CSV.parse('data')

Use CSV.open with write mode and add rows with <<.

What type of object is each row when reading CSV with CSV.foreach?

A String

B Array

C Hash

D Integer

Each row is an array of values from the CSV line.

Which method reads CSV data from a string instead of a file?

A CSV.foreach

B CSV.read

C CSV.open

D CSV.parse

CSV.parse reads CSV data from a string.

Explain how to read a CSV file line by line in Ruby using the CSV library.

Think about how to open and process each line easily.

Describe the steps to write multiple rows of data to a CSV file in Ruby.

Consider how to open a file for writing and add rows one by one.

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