Variables and Expressions
Narrated by Horst
draw_tree(225, 35)
draw_tree(420, 45)
draw_house(720, 60)
draw_snow_person(300, 20)

How to use variables (this chapter)

How to write expressions (this chapter)

How to create our own functions (the next two chapters)
How to Use Variables
Variable and Function Names
Good Variable Names

temperature_in_celsius

tree_position_1

tree_position_2

car_speed

number_of_children

simpson
Bad Names, that still work:

temperatueInCelsius  Uses capital letters. Keep it lower case and use underscores.

x  Too short, and not descriptive.

Smith  Starts with a capital letter.
Names that don't work:

tree position  Can’t use spaces

4runner  Can’t start with a number
Constants

Variables that don't change are constants

They should be in all uppercase

PI = 3.14159

SCREEN_WIDTH = 800
How to Create Expressions
Operator  Description 

+  Addition 
  Subtraction 
*  Multiplication 
**  Exponentiation 
/  Division 
//  Integer division (always rounds down) 
%  Modulus (division remainder) 
Using Operators in Expressions
What's Different?
 Juxtaposition doesn't work  It doesn't multiply
 The '=' isn't an algebraic equality  It is an assignment
Juxtaposition Doesn't Work
Assignment Operators
Operator  Description 

=  Assignment 
+=  Increment 
=  Decrement 
*=  Multiply 
/=  Divide 
Increasing a Variable
Increment/Decrement Operators
Using Expressions In Function Calls
Order of Operations
average = 90 + 86 + 71 + 100 + 98 / 5
average = (90 + 86 + 71 + 100 + 98) / 5
Printing Variables
Review
7 Variables and Expressions
By Paul Craven
7 Variables and Expressions
 288