Variables and
Expressions
CMSC 201
Today we start Python!
Two ways to use python:
 You can write a program, as a series of instructions
in a file, and then execute it.
 You can also test simple python commands in the
python interpreter.
Variables
 Last class we talked about variables being places with
names that store information. In python, you can make a
variable like this:
someVariable = 10
This creates a variable, called someVariable, that has the
value of 10. This value can change later on.
The equals sign is called the assignement operator. It
allows you to change the value of a variable.
Variables
The right hand side of the assignment can be any
mathematical expression. For example!
someVariable = 10
anotherVariable= 5 + 9
aThirdVariable= 10 * anotherVariable
someVariable = someVariable + anotherVariable
Whenever a variable is on the right hand side of an
expression, imagine it being replaced by whatever value
is currently stored in that variable.
Variables
There are many different kinds of variables!
 Numbers (Integers or decimals)
 True / False values (called booleans)
 Strings (collections of characters)
aString = ‘Hello everyone’
decimal = 1.12
bool = True
Rules for Naming Variables
We have certain rules about what a variable can be
named:
 Variable names are case sensitive. A variable
named Hello and a variable named hello are regarded
as different.
 Only may contain alphabetic letters, underscores or
numbers.
 Should not start with a number.
 Cannot be any other python keyword (if, while, def,
etc).
You should always attempt to give your variables
meaningful names!
When do I make variables?
Variables are designed for storing information. Going
back to our averaging example from earlier, when we
were summing up the list we needed a place to put that
sum as it was being generated.
Any piece of information you wish your program to use or
record must be stored in a variable. Think of it as giving
that piece of information a name.
Output
We’d like to see what’s stored in our variables!
Python can print things for us:
print(“Hello world”)
You can also output the contents of variables:
someVariable = 10
print(someVariable)
You can even do combinations!
print(“Your variable is “, someVariable)
Exercise
What will the following code snippet print?
a = 10
b=a*5
c = “Your result is: “
print(c, b)
Exercise
What will the following code snippet print?
a = 10
b=a
a=3
print(b)
There are two possible options for what this could do!
Any guesses?
Exercise
a = 10
b=a
a=3
print(b)
It will print out 10. When you set one variable equal to
another, they don’t become linked; b is set to 10 and no
longer has anything else to do with a.
Input
Sometimes, we’d like the user to participate! We can also
get input from the user.
userInput = input(“Please enter a number “)
print(userInput)
The output will look like this:
Please enter a number 10
10
Input
This line:
userInput = input(“Please enter a number “)
Takes whatever the user entered and stores it in the
variable named “userInput”
You can do this as many times as you like!
userInput1 = input(“Please enter your first number.”)
userInput2 = input(“Please enter your second number.”)
Input
Note: All input will come as a string, a series of
characters. There is a difference between “10” and the
number 10. “10” is composed of two characters stuck
together, while python understands 10 as a number.
To turn an input into a number, you can do the following:
someNumber = input(“Please enter a number: “)
someNumber = int(someNumber)
int stands for integer
Exercise
Write, on paper or on your computer, a program that asks
the user for two numbers a prints out the average.
Exercise
Pretend you’re writing a program to compute someone’s
weight grade. You have so far:
hwWeight = .4
examWeight = .5
discussionWeight = .1
Write a program starting with these three lines that asks
the user for their homework grade, exam grades, and
discussion grades and prints out their total grade in the
class.