Ruby differentiates between whole numbers, or Integers, and decimal numbers, or Floats.

7.class # => Integer
7.0.class # => Float

(In fact, you can always call the method .class on any object, ever, at any time, to ask it what class it is.)

We’ll learn about integers first.


Let’s experiment with some common methods for Integers:

+ - * / % ** (math)

We, of course, have the standard math methods, like the calculator language. These methods all have the same syntactic sugar that the String versions did, so we can say 12 + 5 rather than 12.+(5) (thankfully).

Try each of the following:

12 + 5
12 - 5
12 * 5
12 / 5

Click here for a REPL to try it.

Whoa! Did you get what you expected for that last one?

It turns out that the Integer version of division will only return another Integer, and so / only returns the whole part (like in elementary school). If you want the remainder, you have to use the % (called the “modulus”) operator. Try this:

12 % 5

Another maybe unexpected thing: raising a number to a power, e.g. 32, is not done using the ^ like in many other computing environments. Instead, use the double-star ** operator:

3 ** 2

odd? and even?

The .odd? and .even? methods return true or false based on whether the number is, well, odd or even. Don’t be thrown off by the question mark at the end of the method name — it’s nothing special, just another letter. Rubyists like to end method names with a question mark when methods return true or false.

p 7.odd?

Click here for a REPL to try it.


There’s another special method like p that we are allowed to call “in space”, i.e. not on the right side of a dot1, called rand. It returns a random number, and is very useful for all kinds of stuff, everything from games to statistical analysis:

rand(6) # => returns a random integer between 0 and 5

Somewhat oddly, rand(n) will return a random integer between 0 and n - 1 rather than between 1 and n. That may seem surprising, but it’s actually pretty handy because a lot of times what we want to do is generate a random offset and it’s convenient for that to include 0 as a possibility.

Give it a try:

# random number between 0 and 9
p rand(9)

Click here for a REPL to try it.


We often will want to combine our Integers with Strings when crafting output for our users. Give it a try:

lucky_number = rand(100)
p "Your lucky number is" + lucky_number

Uh oh! RTEM!

It turns out that String’s + method can only add two strings together, not a string and an object of some other class. So, a lot of times we’ll need to convert an Integer into a String prior to output. Fortunately Integer has a handy method, to_s (or “to string”), that does just that:

p 98.to_s

Click here for a REPL to try it.


Similarly, there’s a to_f (or “to float”) method to convert an Integer to a Float, which is often handy for doing math, as we’ll see next.


That’s it for Integer. Next up, it’s close cousin: Float.

  1. This is another method defined on Kernel, so the longhand would be Kernel.rand(6)