Reject for inverse filtering in Ruby

Introduction

Rejecting items helps you remove unwanted things from a list based on a rule. Inverse filtering means keeping only those items that do NOT match the rule.

When you want to remove all even numbers from a list and keep only odd numbers.

When you want to filter out all words that contain a certain letter.

When you want to exclude all users under 18 years old from a list.

When you want to remove all empty strings from a list of words.

Syntax

Ruby

array.reject { |item| condition }

The block inside reject returns true for items to remove.

Reject returns a new array without the rejected items.

Examples

Removes even numbers, keeps odd numbers.

Ruby

numbers = [1, 2, 3, 4, 5]
odd_numbers = numbers.reject { |n| n.even? }
puts odd_numbers.inspect

Removes words containing letter 'a'.

Ruby

words = ['apple', 'banana', 'pear']
no_a = words.reject { |w| w.include?('a') }
puts no_a.inspect

Sample Program

This program removes fruits that contain the letter 'a' and shows both lists.

Ruby

fruits = ['apple', 'banana', 'cherry', 'date']
# Remove fruits that have the letter 'a'
filtered = fruits.reject { |fruit| fruit.include?('a') }
puts "Original: #{fruits.inspect}"
puts "Filtered: #{filtered.inspect}"

Important Notes

Reject is the opposite of select, which keeps items matching the condition.

Reject does not change the original array unless you use reject! with an exclamation mark.

Summary

Use reject to remove items matching a condition.

It returns a new array without those items.

Great for filtering out unwanted elements easily.

Practice

1. What does the Ruby method reject do when called on an array?

easy

A. It sorts the array in reverse order.

B. It modifies the original array to keep only elements matching the condition.

C. It returns the first element that matches the condition.

D. It returns a new array excluding elements that match the given condition.

Solution

1. Step 1: Understand the purpose of reject

   The reject method filters elements by removing those that satisfy the condition.

2. Step 2: Compare with other methods

   Unlike select, which keeps matching elements, reject excludes them and returns a new array.

3. Final Answer:

   It returns a new array excluding elements that match the given condition. -> Option D

4. Quick Check:

   reject excludes matching elements = A [OK]

Hint: Reject removes items matching condition, returning a new array [OK]

Common Mistakes:

• Thinking reject modifies the original array
• Confusing reject with select
• Assuming reject returns a single element

2. Which of the following is the correct syntax to use reject on an array nums to remove even numbers?

easy

A. nums.reject { |n| n % 2 == 0 }

B. nums.reject(n % 2 == 0)

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

D. nums.reject do n; n % 2 == 0 end

Solution

1. Step 1: Identify correct block syntax for reject

   The reject method requires a block with { |var| condition } or do |var| ... end.

2. Step 2: Check each option's syntax

   nums.reject { |n| n % 2 == 0 } uses correct block syntax with braces and block variable. nums.reject(n % 2 == 0) misses block. nums.reject => n { n % 2 == 0 } uses invalid syntax. nums.reject do n; n % 2 == 0 end misses block variable in do syntax.

3. Final Answer:

   nums.reject { |n| n % 2 == 0 } -> Option A

4. Quick Check:

   Correct block syntax with |var| = D [OK]

Hint: Use braces with |var| for reject blocks [OK]

Common Mistakes:

• Omitting block variable in block
• Passing condition as method argument
• Using incorrect block syntax

3. What is the output of this Ruby code?

arr = [1, 2, 3, 4, 5]
result = arr.reject { |x| x > 3 }
puts result.inspect

medium

A. [1, 2, 3]

B. [4, 5]

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

D. []

Solution

1. Step 1: Understand the reject condition

   The block removes elements where x > 3 is true, so numbers 4 and 5 are removed.

2. Step 2: Determine remaining elements

   Elements 1, 2, and 3 remain because they do not satisfy the condition.

3. Final Answer:

   [1, 2, 3] -> Option A

4. Quick Check:

   reject removes >3, so [1,2,3] remain = A [OK]

Hint: Reject removes matching items, so keep others [OK]

Common Mistakes:

• Confusing reject with select
• Expecting removed elements as output
• Ignoring block condition logic

4. The following code is intended to remove all nil values from the array data. What is the error?

data = [1, nil, 2, nil, 3]
clean = data.reject do |x|
  x = nil
end
puts clean.inspect

medium

A. The block variable is missing in the block.

B. The reject method cannot remove nil values.

C. The block uses assignment (=) instead of comparison (==).

D. The code should use select instead of reject.

Solution

1. Step 1: Identify the block logic error

   The block uses x = nil which assigns nil to x instead of checking if x equals nil.

2. Step 2: Correct the comparison operator

   It should use x == nil to test if x is nil for rejection.

3. Final Answer:

   The block uses assignment (=) instead of comparison (==). -> Option C

4. Quick Check:

   Use == for comparison inside reject blocks [OK]

Hint: Use == for comparison inside reject blocks [OK]

Common Mistakes:

• Using = instead of == in block condition
• Thinking reject modifies original array
• Confusing reject with select

5. Given a hash scores = { alice: 90, bob: 55, carol: 75, dave: 40 }, how can you use reject to create a new hash excluding all entries with scores below 60?

hard

A. scores.reject { |name| name < 60 }

B. scores.reject { |name, score| score < 60 }

C. scores.reject { |score| score < 60 }

D. scores.reject { |name, score| name < 60 }

Solution

1. Step 1: Understand hash reject block parameters

   When rejecting from a hash, the block receives two parameters: key and value.

2. Step 2: Apply condition on values

   We want to reject entries where the score (value) is less than 60, so the block should check score < 60.

3. Step 3: Verify options

   scores.reject { |name, score| score < 60 } correctly uses both key and value parameters and checks the score. The other options either use only one block parameter or check the key instead of the value.

4. Final Answer:

   scores.reject { |name, score| score < 60 } -> Option B

5. Quick Check:

   Reject hash entries by value condition = B [OK]

Hint: Use two block params for hash reject: |key, value| [OK]

Common Mistakes:

• Using only one block parameter for hash reject
• Checking key instead of value
• Confusing reject with select for filtering