Introduction to Python Variables and Expressions
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.
Today we start Python! Learn two ways to use Python: writing programs in files and testing simple commands in the interpreter. Understand how variables work in Python, allowing you to store and change values easily. Dive into different kinds of variables like numbers, booleans, and strings. Explore the rules for naming variables and when to use them. Discover how Python can output the contents of variables for easier understanding and debugging.
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Presentation Transcript
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.
