Variables

A VARIABLE is a NAME for an object. You give an object a name using the ASSIGNMENT operator (it looks like an equal sign).

color = "blue"
fruit = "berry"

Anywhere you can use an object, you can use a variable instead.

color + fruit
fruit.upcase

The Warehouse Metaphor

Think of memory as a giant warehouse.

Like this warehouse from the movie Raiders of the Lost Ark, computer memory is vast and filled with boxes of various sizes.

The Warehouse Metaphor Explained

If memory is a giant warehouse...

...and objects are boxes in that warehouse

...then a value is the contents of a box

...and a variable is a label you stick on the outside of the box

Variables are documentation

Which is clearer, this:

60 * 60 * 24

or this:

seconds_per_minute = 60
minutes_per_hour = 60
hours_per_day = 24
seconds_per_day = seconds_per_minute * minutes_per_hour * hours_per_day

?

Lab: Play In IRB

Let's spend a few minutes just playing around in IRB. Some things to try:

• write a poem
• YELL THE POEM
• calculate 2 + 2 and more complicated things
• assign your best friend to a variable
• reverse your best friend's name
• get a new best friend and reverse their name too

The Pointer Metaphor

snack = "Apple"

Think of a variable as pointing to an object.

Changing Variables

You can assign and reassign variables at will.

color = "blue"
fruit = "berry"
color + fruit

color = "black"
color + fruit

Changing a variable (using ASSIGNMENT) just changes the name of an object. It does not change the data inside the object.

This is analogous to a label being removed from one box and placed on a different box.

Many pointers can point to the same thing

fruit = "Apple"
snack = fruit

After this both snack and fruit...

• are pointing to the same object
• have the same value

This is analogous to two labels being placed on the same box.

Return values are new

most messages return new values

fruit = "banana"
snack = fruit.upcase

"banana" and "BANANA" are two different objects in memory

Changing Values

Most messages do not change the data inside the object.

color.upcase
color

But some messages do change the data!

color.upcase!
color

This can be dangerous so sometimes those messages end with a BANG (exclamation point).

Variables summary

• variables are names for objects
• you can change which object a variable points to
• you can have many names for the same object
• sometimes values can change inside an object but the names (variables) stay the same