Select/filter for filtering in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the select method do in Ruby?

The select method filters elements from a collection based on a condition. It returns a new array with elements for which the condition is true.

beginner

How is select different from reject in Ruby?

select keeps elements where the condition is true, while reject keeps elements where the condition is false.

beginner

Given numbers = [1, 2, 3, 4, 5], what does numbers.select { |n| n.even? } return?

It returns [2, 4] because it selects only the even numbers from the array.

intermediate

Can select be used on hashes in Ruby? How?

Yes, select can be used on hashes. It returns a new hash with key-value pairs where the block condition is true.

beginner

What is the return type of select when used on an array?

It returns a new array containing the filtered elements.

What does select return when no elements match the condition?

A nil

B The original array

C An empty array

D An error

select returns an empty array if no elements satisfy the condition.

Which method would you use to get elements that do NOT match a condition?

A reject

B select

C map

D each

reject returns elements where the condition is false, opposite of select.

What will [1, 2, 3].select { |n| n > 3 } return?

A [1, 2, 3]

B []

C [3]

D nil

No elements are greater than 3, so select returns an empty array.

Can select change the original array?

A Only for hashes

B Yes, it modifies the original array

C Only if used with <code>!</code>

D No, it returns a new array

select returns a new array and does not modify the original.

Which of these is a valid use of select on a hash {a: 1, b: 2}?

A <code>.select { |k, v| v.even? }</code>

B <code>.select { |k| k == :a }</code>

C <code>.select { |v| v > 1 }</code>

D <code>.select { |k| k > 1 }</code>

When selecting on a hash, the block receives key and value. Checking if value is even is valid.

Explain how the select method works in Ruby with an example.

Think about choosing items that match a rule.

Describe the difference between select and reject methods in Ruby.

One chooses what you want, the other what you don't want.

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