Variables
At their simplest, variables are merely storage containers. They help us hold information. You create your own variables, starting with a lowercase letter followed by an equal sign and its value (usually a number or a string, but sometimes more complex code can be stored in a variable). You’ve seen this before in math.
x = 12
The value of x is?
=> 12
In Ruby, a variable name is defined once you write (almost) anything left of the equal sign. The equal sign (as in most programming languages) is used to assign the value to the right, to the variable. Here are a few more examples of creating variables.
myVar = "my string variable"
a_long_var_name = 42
myCat = "whiskers"
Variable Storage in Memory
With computers, variable storage is slightly different than common math, due to the way memory storage is created. Don’t worry if this section seems complicated, you will start to get the hang of it as you write more code.
In an effort to not waste memory, the computer avoids storing duplicate information. In the following example, each variable (X, Y and Z) will point to the same place in memory because they store the same value: twelve.
x = 12
y = x
z = 12
In order to store the value 12, the computer creates a location in memory with an address, let’s assume the address is ABC12. In this sense, a variable is like an X on a treasure map, guiding you to the right location.
Yet variables don’t actually store their information–they point to it. Or in another way, variables store memory addresses that store information. In the example above, each variable points to the same memory address that stores the number 12. This way the computer doesn’t need to create three different memory locations for one value, it can use ABC12 for all three.
x = 12 => memory address: ABC12
y = x => memory address: ABC12
z = 12 => memory address: ABC12
Now that you know how variables store information (or memory addresses) see if you can figure out the end value of X:
y = 5 => y points to memory address AB1, which contains
the value 5
x = y => x points to memory address AB1
y = 7 => y now points to memory address CD1, which
contains 7
x equals ?
The trick here is to understand that X does not equal Y. Remember, X only POINTS to the value stored in the memory address that Y points to (AB1), at the time it was assigned. No matter what happens later to Y, the only thing X needs to remember is the location of AB1 (which will always be 5 while this program is running).
When we change the value of Y to 7, we are actually telling the computer to create a new memory address that contains the value 7 and point to it. X does not change its original address, so the value of X remains 5.
There is a built-in Ruby method called object_id that shows the id of the Ruby object. This can be correlated to a kind of unique memory address. Let’s look at the following example using this method.
y = "test" => y.object_id => 7021
x = y => x.object_id => 7021
At this point, only one memory address needs to be created, since the stored value (“test”) is the value for both X and Y.
y = "new" => y.object_id => 8333
x equals? => x.object_id => 7021
Once we change the value of Y, we create a new memory location with a new value stored (the string “new”). X does not equal “new”, it still equals the value stored in the original memory address that it pointed to at the time we set X equal to Y (the string “test”).
When we change variables and add new information, Ruby creates more memory locations with specific addresses–if none of the current addresses are holding the same information. And actually, Ruby has already created several object ids to help store common numbers and letters in memory, like the number 12 in our example above.
Don’t worry if this is all too confusing. You don’t actually need to fully understand this concept to program. As we move along through the other core concepts of coding you will begin to see how variables work within the bigger picture.
The key point to remember is that an = equals sign in programming means “assign the value to the right of me, to the name on the left.” Visually, that would look like this:
number12 = 12
(variable name) (is assigned) number 12
number12 = 12
Practice
1) Store the value of 54 / 3 into the variable x. What is
the value of x?
2) Give the value of x (from problem 1) to y, a new
variable. Now make x equal to itself divided by 3. What’s
the value of x? What’s the value of y?
3) What if we performed some math on our variables? If we
set x to 12, what’s x divided by 3?
4) What’s the value of x now?