New names for old things

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
Method	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:

Exercises

Ruby can tell you about the class of an object. Type these in irb:


12.is_a?(Integer)
12.is_a?(Float)
12.is_a?(String)

'12'.is_a?(Integer)
'12'.is_a?(Float)
'12'.is_a?(String)

12.0.is_a?(Integer)
12.0.is_a?(Float)
12.0.is_a?(String)

What differences do you see?

Type these in:


12 + 12
'12' + '12'

'12'.to_i + 12
'12' + 12.to_s

12 * 12
'12' * 12

Did you get the results you expected?

How would you explain the difference between 12, '12' and 12.0 to a younger sibling?

Strings

Variables