Sets

Updated Aug 24, 2023 ·

Overview

A set is an unordered collection that guarantees all values are unique. It is useful when you want to avoid duplicates.

In older Ruby versions, you need to add this at the top to use it.

require 'set'

To create a set from an array:

seasons = Set.new(["fall", "winter", "spring", "summer"])
puts seasons

Output:

#<Set: {"fall", "winter", "spring", "summer"}>

As mentioned, sets are used to avoid duplicates. If we try to add duplicate values, they will be ignored:

seasons.add("winter")
seasons.add("fall")

puts seasons

Output:

#<Set: {"fall", "winter", "spring", "summer"}>

Note that sets are case-sensitive, which means values like "Fall" and "fall" are considered different and can both exist in the same set.

seasons.add("Fall")
puts seasons

Output:

#<Set: {"fall", "winter", "spring", "summer", "Fall"}>

Common Set Methods

Sets provide a few familiar methods that help you inspect and work with unique values.

• length returns how many unique elements are in the set
• include? checks whether a value exists in the set
• each iterates over every element in the set

These methods behave similarly to arrays and hashes, except they always respect the set’s uniqueness rules.

Examples:

1. Iterating over a set:

   seasons = Set.new(["fall", "winter", "spring", "summer"])
   
   seasons.each do |season|
     puts season
   end

   Output (order is not guaranteed):

   fall
   winter
   spring
   summer

2. Using length:

   seasons = Set.new(["fall", "winter", "spring", "summer"])
   
   puts seasons.length

   Output:

   4 

3. Using include?:

   seasons = Set.new(["fall", "winter", "spring", "summer"])
   
   puts seasons.include?("winter")
   puts seasons.include?("Winter")

   Output:

   true
   false

Adding Elements

You can add new elements using the add method.

books = Set.new(["1984", "Animal Farm"])

books.add("Brave New World")
p books

Output:

#<Set: {"1984", "Animal Farm", "Brave New World"}>

The new value is added because it did not already exist in the set. The add method also returns the set itself, which shows the updated state immediately.

If you try to add the same value again, nothing changes.

books.add("Brave New World")
p books

Output:

#<Set: {"1984", "Animal Farm", "Brave New World"}>

Duplicate values are ignored because a set always enforce uniqueness.

Removing elements

You can remove values using the delete method.

books = Set.new(["1984", "Animal Farm", "Brave New World"])

books.delete("Animal Farm")
p books

Output:

#<Set: {"1984", "Brave New World"}>

The value is removed if it exists, and the updated set is returned.

If you try to delete a value that is not in the set, Ruby does nothing and does not raise an error.

books.delete("Unknown Book")
p books

Output:

#<Set: {"1984", "Brave New World"}>

Example: Generate unique phone numbers from a file

The files can be found here: Github

In this example, we have a text file of customers who visited a shop. Each line contains a name and a phone number. For example:

Alex Tan,91234567
Jamie Lim,98765432
Alex Tan,91234567

Some entries are repeated. The task is to extract and return only the unique phone numbers.

1. Read the file line by line
2. Remove the newline character
3. Split the line by the comma
4. Add the phone number to a set

Solution:

def gen_uniq_phone_nums
  phone_nums = Set.new 

  File.open('customers.txt').each do |customer|
    newline = customer.chomp 
    elems = newline.split(',')
    nums = elems[1]
    phone_nums.add(nums)
  end

  phone_nums
end

report_page = gen_uniq_phone_nums

puts report_page
puts report_page.length

Running the script:

ruby main.rb  

Output:

#<Set: {"91234567", "98765432", "90112233", "93445566", "95667788", "92334455", "98887766", "94556677", "97778899", "91112222", "92223333", "93334444", "94445555", "95556666", "96667777", "98889900", "90001111", "98990011", "97881234", "96782345", "95683456", "94584567", "93485678", "92386789", "91287890", "90188901"}>

To check the number of unique phone numbers:

puts report_page.length

Output:

26