Select/filter for filtering in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using select?

Consider the following Ruby code that filters numbers greater than 3 from an array. What will be printed?

Ruby

numbers = [1, 2, 3, 4, 5]
result = numbers.select { |n| n > 3 }
puts result.inspect

A [1, 2, 3]

B [4, 5]

C []

D [3, 4, 5]

💡 Hint

Remember, select keeps elements where the block returns true.

The select method returns a new array containing all elements for which the block returns true. Here, only 4 and 5 are greater than 3.

❓ Predict Output

intermediate

What does this filter code output?

Given this Ruby code using filter (alias for select), what is the output?

Ruby

words = ['apple', 'banana', 'cherry', 'date']
filtered = words.filter { |w| w.length == 5 }
puts filtered.inspect

A ["apple"]

B ["apple", "cherry"]

C ["banana", "date"]

D ["apple", "banana", "cherry", "date"]

💡 Hint

Check the length of each word carefully.

Only 'apple' has exactly 5 letters. 'cherry' has 6 letters, so it is not included.

❓ Predict Output

advanced

What is the output when filtering hashes with select?

What will this Ruby code print?

Ruby

people = [{name: 'Alice', age: 30}, {name: 'Bob', age: 20}, {name: 'Carol', age: 25}]
adults = people.select { |p| p[:age] >= 25 }
puts adults.map { |p| p[:name] }.inspect

A ["Alice", "Carol"]

B ["Bob", "Carol"]

C ["Alice", "Bob"]

D ["Carol"]

💡 Hint

Select people whose age is 25 or more.

Alice is 30 and Carol is 25, so both are included. Bob is 20, so excluded.

❓ Predict Output

advanced

What error does this code raise?

What error will this Ruby code raise when trying to filter with a wrong block?

Ruby

numbers = [1, 2, 3]
result = numbers.select { |n| n > '2' }
puts result.inspect

A ArgumentError

B No error, outputs [3]

C TypeError

D NoMethodError

💡 Hint

Compare integer with string causes a problem.

Comparing integer n with string '2' using > causes a TypeError in Ruby.

🧠 Conceptual

expert

How many elements are in the result of this chained filter?

Given this Ruby code, how many elements are in the final array?

Ruby

arr = [10, 15, 20, 25, 30]
result = arr.select { |x| x > 10 }.select { |x| x % 10 == 0 }

A 1

B 3

C 4

D 2

💡 Hint

First filter keeps numbers greater than 10, second keeps multiples of 10.

Numbers greater than 10 are [15, 20, 25, 30]. Among these, multiples of 10 are 20 and 30, so 2 elements.

Practice

1. What does the select method do in Ruby when used on an array?

easy

A. It returns a new array with elements that meet the condition in the block.

B. It changes the original array by removing elements that don't meet the condition.

C. It counts how many elements meet the condition in the block.

D. It sorts the array based on the block's return value.

Solution

1. Step 1: Understand the purpose of select

   The select method filters elements from an array based on a condition given in a block.

2. Step 2: Check the behavior of select

   It returns a new array containing only elements for which the block returns true, without changing the original array.

3. Final Answer:

   It returns a new array with elements that meet the condition in the block. -> Option A

4. Quick Check:

   select filters and returns new array [OK]

Hint: Remember: select returns a new filtered array, original stays same [OK]

Common Mistakes:

• Thinking select modifies the original array
• Confusing select with count or sort
• Believing select returns a single element

2. Which of the following is the correct syntax to filter even numbers from an array nums using select?

easy

A. nums.select(n) { n % 2 == 0 }

B. nums.select do n n % 2 == 0 end

C. nums.select { n => n % 2 == 0 }

D. nums.select { |n| n % 2 == 0 }

Solution

1. Step 1: Review Ruby block syntax for select

   The select method requires a block with a parameter and a condition, usually written as { |param| condition }.

2. Step 2: Identify correct syntax

   nums.select { |n| n % 2 == 0 } uses correct block syntax with |n| and condition n % 2 == 0. Other options have syntax errors or wrong block usage.

3. Final Answer:

   nums.select { |n| n % 2 == 0 } -> Option D

4. Quick Check:

   Correct block syntax = nums.select { |n| n % 2 == 0 } [OK]

Hint: Use curly braces with |variable| for select blocks [OK]

Common Mistakes:

• Missing vertical bars around block variable
• Using wrong block delimiters or syntax
• Passing argument inside select parentheses

3. What is the output of this Ruby code?

numbers = [1, 2, 3, 4, 5]
even_numbers = numbers.select { |num| num.even? }
puts even_numbers.inspect

medium

A. [1, 3, 5]

B. [1, 2, 3, 4, 5]

C. [2, 4]

D. []

Solution

1. Step 1: Understand the select block condition

   The block uses num.even? which returns true for even numbers only.

2. Step 2: Filter the array based on condition

   From [1, 2, 3, 4, 5], only 2 and 4 are even, so the new array is [2, 4].

3. Final Answer:

   [2, 4] -> Option C

4. Quick Check:

   select with even? filters evens = [2, 4] [OK]

Hint: Remember even? returns true only for even numbers [OK]

Common Mistakes:

• Confusing even? with odd?
• Expecting original array unchanged output
• Forgetting select returns new filtered array

4. Find the error in this Ruby code that tries to filter strings longer than 3 characters:

words = ["cat", "dog", "elephant", "fox"]
long_words = words.select |word| word.length > 3
puts long_words.inspect

medium

A. Using select instead of filter

B. Missing curly braces or do..end around the block

C. Incorrect method name length instead of size

D. Using puts instead of print

Solution

1. Step 1: Identify block syntax error

   The code uses select |word| word.length > 3 without curly braces or do..end, which is invalid syntax.

2. Step 2: Correct block syntax

   It should be select { |word| word.length > 3 } or select do |word| word.length > 3 end.

3. Final Answer:

   Missing curly braces or do..end around the block -> Option B

4. Quick Check:

   Block syntax requires braces or do..end [OK]

Hint: Always wrap select block with {} or do..end [OK]

Common Mistakes:

• Omitting block delimiters
• Confusing length and size methods (both work but length is correct)
• Thinking select and filter are different methods

5. Given a hash scores = { alice: 50, bob: 75, carol: 40, dave: 90 }, how do you select only the key-value pairs where the score is 50 or more?

hard

A. scores.select { |name, score| score >= 50 }

B. scores.filter { |score| score >= 50 }

C. scores.select { |name| name >= 50 }

D. scores.filter { |name, score| name >= 50 }

Solution

1. Step 1: Understand hash select/filter usage

   When using select on a hash, the block receives two parameters: key and value.

2. Step 2: Apply condition on value (score)

   We want pairs where score >= 50, so the block should check score >= 50.

3. Step 3: Identify correct syntax

   scores.select { |name, score| score >= 50 } correctly uses |name, score| score >= 50. The other options have wrong parameters or conditions.

4. Final Answer:

   scores.select { |name, score| score >= 50 } -> Option A

5. Quick Check:

   Hash select with two block params and value condition [OK]

Hint: Use two block variables for hash select: |key, value| [OK]

Common Mistakes:

• Using only one block variable for hash select
• Checking condition on key instead of value
• Confusing select and filter with wrong parameters