meet it

Ruby comes with a toolbox. Gems are tools others share.

You have used json already. It is part of the : tools that come with Ruby, ready for you to require. gives you Set, a collection that never holds the same item twice. gives you Date, which knows the calendar.

A is a package of Ruby code that someone else wrote and shared. On your own computer you install gems with the gem install command, then require them. Gwecode runs Ruby in your browser with no gems, so these lessons use the standard library only.

require "set"
require "date"
seen = Set.new(["fox", "owl", "fox"])
seen << "owl"
p seen.size
day = Date.new(2026, 12, 25)
puts day.strftime("%A %d %B %Y")
puts day + 7
gap = day - Date.new(2026, 12, 1)
puts gap.to_i

strftime works a bit like format, with %A for the weekday and %B for the month.

guess first

No doubles allowed. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

require "set"
stickers = Set.new
["star", "cat", "star", "moon", "cat"].each { |sticker| stickers << sticker }
p stickers.size
p stickers.include?("moon")

your turn

Your turn. Count down to a big day.

Fill in a month and a day in 2027, as numbers. The program counts how many days that is after New Year.

Your code

press run to see what happens

fix it

Christmas will not fit in the calendar. Fix it!

Broken code to fix

press run to see what happens

Read the error. Ruby says the date is not valid. In what order does Date.new want its numbers?
Date.new takes the year first, then the month, then the day.
Change line 2 to xmas = Date.new(2026, 12, 25), then press run.

nailed it

you know where the tools come from. :D

The standard library comes with Ruby. require loads it.
A Set never holds the same item twice.
Gems are shared packages you install on your own computer.

new words in your collection

standard library
Set
Date
gem

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