Introduction

Variables are a way of assigning data to names in your programs. You can think of a variable as a box with a label on it: it stores something and has a name so that you know what’s inside. This is an imperfect metaphor as you’ll see by the end of this lesson, but it should help with understanding variables for now.

Lesson overview

This section contains a general overview of topics that you will learn in this lesson.

• Describe what a variable is and how to assign it a value or expression.
• Explain what the +=, -=, *=, and /= assignment operators do.
• Describe the naming conventions for variables.

Declaring a variable

This is how to create a variable in Ruby:

age = 18 #=> 18

You can also assign the result of an expression to a variable.

age = 18 + 5 #=> 23

Variable names are reusable, so you can assign a new value to a variable at any point in your program. Naturally, doing so will override the original value.

age = 18
age #=> 18
age = 33
age #=> 33

There will often be scenarios where you want to perform an operation on the original value of a variable and then reassign the result of that operation to the same variable.

age = 18
age #=> 18
age = age + 4
age #=> 22

Because this is a common scenario, Ruby provides a nice shorthand assignment operator for doing this: +=.

age = 18
age += 4 #=> 22

There are similar assignment operators for all the common math operators:

age = 18
age -= 2  #=> 16

cash = 10
cash *= 2 #=> 20

temperature = 40
temperature /= 10 #=> 4

How to name variables

Ruby is a language that aims to be natural to read and easy to write. Remember this when you’re naming your variables. The name should, as clearly as possible, describe what the value of the variable represents.

Naming variables clearly will pay dividends when you review your code months after you’ve written it, when you can no longer remember what that variable was designed to store. From now on, when naming your variables, remember the following quote by John Woods:

Always code as if the person who ends up maintaining your code will be a violent psychopath who knows where you live.

The most basic thing you can do to write clean, maintainable code is to name your variables properly. So get into this habit early to avoid psychopath programmers coming after you.

Variable names should always be lowercase, and multiple words that make up a variable name should be split by an underscore. This is known as snake_case.

# bad
a = 19
string = "John"

# good
age = 19
name = "John"
can_swim = false

Variables are references

The information you name with a variable is stored in memory on your computer, so a variable is effectively a reference or a pointer to that address in memory. This is important to know as it can sometimes be the cause of unexpected behavior from your code.

Let’s look at an example of this unexpected behavior, with two variables: desired_location, which is assigned to the string “Barcelona”, and johns_location, which is assigned to the desired_location variable. Both variables are pointing to where “Barcelona” is stored in memory.

desired_location = "Barcelona"
johns_location = desired_location

desired_location  #=> "Barcelona"
johns_location    #=> "Barcelona"

Unexpected behavior happens if the string “Barcelona” that is stored in memory is modified. One way to modify a string is to use the upcase! method, instead of the safe upcase method. If the string is modified using johns_location.upcase! then desired_location will also reflect that change:

johns_location.upcase!  #=> "BARCELONA"

desired_location        #=> "BARCELONA"
johns_location          #=> "BARCELONA"

This example may be hard to completely understand at this point in the lesson. The important concept is that assigning variables to other variables can have unintended side effects. Just because you can do it, doesn’t mean you should. You will have the opportunity to revisit this example in one of the following assignments.

Assignment

Knowledge check

This section contains questions for you to check your understanding of this lesson. If you’re having trouble answering the questions below on your own, review the material above to find the answer.

• What is a variable?
• How do you assign a value or an expression to a variable?
• What does the += assignment operator do?
• What does the -= assignment operator do?
• What does the *= assignment operator do?
• What does the /= assignment operator do?
• What are the variable naming conventions?

Additional resources

This section contains helpful links to related content. It isn’t required, so consider it supplemental.

• Read the full Variables chapter from The Bastards Book of Ruby if you can’t get enough about variables.
• To dive deeper into how variables point to memory locations on your computer, go through these short sections:
  • Variables as Pointers, from LaunchSchool’s Introduction to Programming With Ruby.
  • A visual guide to variables from the Variables chapter of The Bastards Book of Ruby
• If you want to know more about Ruby’s naming conventions, check out the Ruby Style Guide. Don’t get too deep into it; just know that it’s there.