Group_by for categorization in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the group_by method do in Ruby?

It groups elements of a collection based on the result of a block, returning a hash where keys are the group criteria and values are arrays of elements in each group.

beginner

How do you use group_by to categorize numbers as even or odd?

You pass a block that returns :even or :odd based on each number. For example: <br>numbers.group_by { |n| n.even? ? :even : :odd }

beginner

What type of object does group_by return?

It returns a Hash where each key is a group label and each value is an array of elements belonging to that group.

intermediate

Can group_by be used on arrays of strings? Give an example.

Yes! For example, grouping words by their first letter: <br>words.group_by { |word| word[0] } groups words by their starting character.

beginner

What happens if the block passed to group_by returns the same value for multiple elements?

All those elements are grouped together under the same key in the resulting hash.

What does group_by return when called on an array?

A An array sorted by the block's return value

B A hash grouping elements by the block's return value

C A single element from the array

D A boolean indicating if grouping was successful

group_by returns a hash where keys are group labels and values are arrays of grouped elements.

How would you group an array of numbers into 'positive' and 'negative' using group_by?

A numbers.group_by { |n| n > 0 ? :positive : :negative }

B numbers.group_by(&:positive?)

C numbers.group_by { |n| n.abs }

D numbers.group_by { |n| n < 0 }

Option A correctly uses a block returning symbols :positive or :negative based on the number's sign.

If you group words by their length using group_by, what will the keys of the resulting hash be?

A The length of each word

B The first letter of each word

C The words themselves

D The last letter of each word

Grouping by length means the keys are integers representing word lengths.

What happens if the block passed to group_by returns nil for some elements?

A Those elements are ignored

B Those elements are grouped under an empty string key

C An error is raised

D Those elements are grouped under the <code>nil</code> key

Elements with block result nil are grouped under the nil key in the hash.

Which of these is a valid use of group_by?

A [1,2,3,4].group_by

B [1,2,3,4].group_by(2)

C [1,2,3,4].group_by { |n| n % 2 }

D [1,2,3,4].group_by(&:to_s)

Options C and D are valid uses of group_by: C uses a block to group numbers by remainder when divided by 2 (even/odd), and D uses a symbol to proc to group by string representation.

Explain how the group_by method works in Ruby and give a simple example.

Think about how you can sort items into labeled boxes based on a rule.

Describe a real-life situation where you could use group_by to organize data.

Imagine sorting your books or clothes into categories.