Reject for inverse filtering in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the reject method do in Ruby?

The reject method returns a new array excluding elements for which the given block returns true. It filters out unwanted elements.

beginner

How is reject related to select in Ruby?

reject is the inverse of select. While select keeps elements where the block is true, reject removes them.

beginner

Example: What is the result of [1, 2, 3, 4].reject { |n| n.even? }?

The result is [1, 3] because reject removes even numbers (2 and 4).

intermediate

Can reject be used with hashes in Ruby?

Yes, reject works with hashes and returns a new hash excluding key-value pairs where the block returns true.

intermediate

What happens if no block is given to reject?

If no block is given, reject returns an Enumerator, allowing chaining or delayed execution.

What does reject do in Ruby?

A Keeps elements where the block returns true

B Removes elements where the block returns true

C Sorts elements in ascending order

D Returns the first element

reject removes elements for which the block returns true, filtering them out.

What is the output of [5, 6, 7].reject { |n| n > 6 }?

A [5, 6]

B [7]

C [5, 7]

D [6, 7]

Elements greater than 6 (which is 7) are rejected, so the result is [5, 6].

Which method is the inverse of reject?

A each

B map

C reduce

D select

select keeps elements where the block returns true, opposite of reject.

What does reject return if no block is given?

A nil

B An empty array

C An Enumerator

D Raises an error

Without a block, reject returns an Enumerator for chaining.

Can reject be used on a hash?

A Yes, it returns a hash excluding pairs where block is true

B No, it only works on arrays

C Yes, but returns an array

D No, it raises an error

reject works on hashes and returns a new hash excluding matching pairs.

Explain how the reject method works in Ruby and how it differs from select.

Think about what elements stay or go based on the block's result.

Write a Ruby code example using reject to remove all odd numbers from an array.

Use n.odd? inside the block.