Ruby uses special names for things that we already know. For instance, it uses the word Float to mean “decimals”. Here are more definitions:

• Object: That’s just any piece of data. Like the number 3 or the string ‘hello’.
• Class: Ruby separates everything into classes (or categories), such as integers, floats and strings.
• Method: These are the things that you can do with an object. Think of it as the object’s “behaviour”. For example, you can add integers together, so + is an integer method. You can capitalize strings, so upcase is a string method.

You’ve already seen three classes for things that you already know (note that Ruby class names are capitalized):

Old name	Ruby class
integer	Integer
decimals	Float
text	String

You have also seen several methods:

Ruby class	Some methods
Integer	+ – / * % **
Float	+ – / * % **
String	capitalize, reverse, length, upcase

Classes vs. objects¶

Make sure you understand the difference between classes and objects. An object is a unit of data. A class is what kind of data it is.

For example, 3 and 5 are different numbers. They are not the same object. But they are both integers, so they belong to the same class. Here are more examples:

Object	Class
2	Integer
-5	Integer
7.2	Float
3.14	Float
‘hello’	String
‘world’	String

Class#method notation

Remember, different classes have different methods. Here are some differences that you have already seen.

• Division (/) doesn’t work the same with integers and floats.
• Addition (+) doesn’t work the same with strings as it does with integers.
• Strings have several methods that integers and floats don’t have (e.g., capitalize, length, and upcase).

For this reason, we will use the notation Class#method to state exactly which method we mean. For instance, I will say Integer#+ to differentiate it from Float#+ and String#+. I can also say that String#upcase exists, but Integer#upcase does not exist.

Converting between classes¶

Ruby has some methods for converting between classes:

Method	Converts
	From	To
String#to_i	String	Integer
String#to_f	String	Float
Float#to_i	Float	Integer
Float#to_s	Float	String
Integer#to_f	Integer	Float
Integer#to_s	Integer	String

Examples:

shell> irb --simple-prompt
>> 34.to_s
=> "34"
>> "12".to_i
=> 12
>> "hello".to_i
=> 0
>> 27.2.to_i
=> 27
>> 3.to_f
=> 3.0

Exercises¶

Finding Accurate Averages A quick exercise to practice using floats.