learn-co-curriculum / phase-3-method-scope

Method Scope

Learning Goals

• Explain how scope prevents you from accidentally overwriting previous work.
• Differentiate between the functionality of local variables created inside and outside of methods.
• Define methods that pass variables in as arguments.

Introduction

In this lesson, we'll introduce the concepts of method scope and variable scope.

Variable Names and Scope

Naming variables is hard, but it is important. We need our code to be as descriptive as possible. Any other developer reading over one of our programs should be able to understand what our code does. A big part of this is having variable (and method) names that are sensible and descriptive.

Our variable names should be descriptive of what they hold, but they should also be concise. This can be difficult to achieve. You might even be worrying that you'll run out of unique, descriptive and concise variable names. Don't worry though, all programming languages have what is called scope. Scope means that not all variables exist everywhere in a program. If they did, then a variable that you'd be writing in file A could accidentally overwrite a variable made by your friend in file B. Having a scope for variables allows you to have greater confidence that you aren't overwriting someone else's work when you create variables.

What is Scope?

Methods in ruby create their own scope. "Scope" refers to the areas of your program in which certain data is available to you. Any local variable created outside of a method will be unavailable inside of a method. In addition, local variables created inside of a method (i.e. in between the def and end keywords of a method) 'fall out of scope' once you're outside the method.

Let's take a look at the following example:

name = "Joe"

def greeting(name)
   puts "Hello, #{name}"
end

In this code snippet, we have a variable, name, set equal to a string, "Joe". Then we define a method, #greeting, that takes in an argument of name. Are the name variable that is set equal to "Joe" and the name variable we are using as a parameter (or argument) for our #greeting method definition the same? Let's see.

If we call our greeting method in the following way:

greeting("Sophie")

What do we expect to be outputted to the terminal?

In this case, the above method invocation would puts out Hello, Sophie to the terminal. It is true that we are setting a variable, name, equal to "Joe" in this code snippet. But we are not using that variable anywhere else in our code. The name argument of the #greeting method is just a placeholder. It means: when we call the #greeting method with an argument of, say, "Sophie", set the variable name inside of the method equal to that string argument.

The name variable inside of the #greeting method is different from the name variable that we set equal to "Joe" outside of the method. The #greeting method has its own scope, and variables inside of it don't know about variables outside of it and vice versa.

Method Scope in Ruby

Think of a method as a castle. The def and end keywords are like the gates that keep out the barbarian hordes, dragons, etc. Let's take a look:

evil_monster = "Bowser"

def princess_peaches_castle
  puts "#{evil_monster} is trying to kidnap Princess Peach!"
end

We've defined the variable evil_monster outside of the method, princess_peaches_castle. Then, we try to call on the evil_monster variable inside that method. Watch what happens when we invoke the method:

princess_peaches_castle
#=> NameError: undefined local variable or method `evil_monster' for main:Object

The evil_monster variable is out of scope for this method. The method can't access it unless we pass it in as an argument.

If we define our method to accept an argument, we can pass our variable into the method and the method will be able to operate on/use that variable. Let's take a look:

evil_monster = "Bowser"

def princess_peaches_castle(evil_monster)
  puts "#{evil_monster} is trying to kidnap Princess Peach!"
end

princess_peaches_castle(evil_monster)
#=> "Bowser is trying to kidnap Princess Peach!"

The method call, which happens outside the method itself, has access to our evil_monster variable. We can therefore make the value stored in evil_monster available to princess_peaches_castle by passing it as an argument when we call the method. So now Mario can start his adventure.

So far, we've seen that variables defined outside of methods are not available inside methods (unless we pass them in as arguments). This works the other way around as well: variables defined inside of methods are not available outside of those methods. Let's take a look.

If we define the following method to include a local variable:

def practicing_method_scope
  im_trapped_in_the_method = "You can't access me outside this method!"
end

Trying to access that variable elsewhere in our program, outside of this method, will raise the following error:

im_trapped_in_the_method
#=> NameError: undefined local variable or method `im_trapped_in_the_method' for main:Object

Conclusion

Remember: A variable defined inside a method can't leave that method. It is not available to your program outside of the method. A variable defined outside of a method can only be made available to the code inside the method if you pass that variable in to the method as an argument.