unit-2.md

Unit 02

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Table of Contents

• variables
• f-strings
• input()
• Integers / Floats
• Arithmetic Operators

Variables

Variables are used to store data that the program will use later.

# a string, "Jim", is assigned to a variable called name
name = "Jim"
# we later use that variable in a print statement
print("Hello " + name + "!") # Hello Jim!

A variable's value can be changed after assignment.

# a string, "Jim", is assigned to a variable called name
name = "Jim"

# a string, "Pam", is assigned to a variable called name which OVERWRITES the previous data
name = "Pam"

# the string, "Jim", no longer exists!
print("Hello " + name + "!") # Hello Pam!

Variable names:

• must start with a letter or the underscore character
• cannot start with a number
• can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
• are case-sensitive (age, Age and AGE are three different variables)

Valid variable names:

username = 'Athena'
user_name = 'Dionysus'
username_2 = 'Artemis'
_user_name = 'Aphrodite'
userName = 'Poseidon'
USERNAME = 'Hera'

Invalid variable names:

2username = 'Nyx'  # cannot begin with a number
user-name = 'Hades'  # no hyphens
u$ern&me = 'Cerberus'  # no special characters other than underscores
user name = 'Persephone'  # no spaces

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f-strings

Let's say we want to print a message to the user with some information that we have about them:

# Variables
name = "Lisa"
city = "Portland"

# print statement with the concatenation
print("Hello " + name + "! Today in " + city + ", it is warm and sunny!")  # Hello Lisa! Today in Portland, it is warm and sunny!

Doesn't that look cumbersome? An easier and cleaner way would be to use f-strings! We use f-strings to format Python expressions (bits of code) into our strings!

f-strings start with an f before the opening quotation mark and code can be placed into the string using curly brackets {}. Any code inside an f-string's curly brackets is evaluated and its result is used.

# Variables
city = "Portland"
weather = "warm and sunny"

# print statement with an f-string
print(f"Today in {city}, it is {weather}!")  # Today in Portland, it is warm and sunny!

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input()

Example 1:

# this line has three actions!
user_name = input("What is your name?")
# prints a question to the user
# then allows the user to type an input
# saves user input to a variable


print(f"Hello {user_name}!")
# prints "Hello" with the user's name that they typed in!
# outcome: "Hello <user name>"

• input() code demo (Type in your name after the programs asks you for it!)
• read more here

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Numbers

Integers are whole numbers. They can positive or negative.

Let's use the example below to calculate how old we are this year.

current_year = 2020  # integer (int)
year_of_birth = 1988  # integer (int)
age = current_year - year_of_birth

print(age)  # 32

Floats are one way that Python represents decimal numbers. They can also be positive or negative, but will always contain a decimal.

Error:

# Error! Cannot concatenate string and integer
print(4 + "4")  # TypeError: unsupported operand type(s) for +: 'int' and 'str'

Arithmetic Operators

Operator	Name	Example
+	Addition	x + y
-	Subtraction	x - y
*	Multiplication	x * y
/	Division	x / y
//	Floor Division	x // y
**	Exponentiation	x ** y
%	Modulus	x % y

Arithmetic Example

The adventure club you're in has decided to check out Powell Butte Nature Park. When the idea comes up, 20 people are interested in going. On the day before the hike, 6 people drop out. Let's update the headcount.

num_of_interested = 20
drop_outs = 6
num_of_attendees = num_of_interested - drop_outs

print(f"There were {num_of_interested} people who were interested in going hiking but {drop_outs} have changed their mind. The total number of people going is now {num_of_attendees}.")

Output

There were 20 people who were interested in going hiking but 6 have changed their mind. The total number of people going is now 14.

We can also calculate the attendance rate as a percentage

num_of_interested = 20
drop_outs = 6
num_of_attendees = num_of_interested - drop_outs

attendance_rate = (num_of_attendees / num_of_interested) * 100

print(f"There were {num_of_interested} people who were interested in going hiking but {drop_outs} have changed their mind. The total number of people going is now {num_of_attendees}.")

print(f"That's an attendance rate of {attendance_rate}%")

Output

There were 20 people who were interested in going hiking but 6 have changed their mind. The total number of people going is now 14.
That's an attendance rate of 70.0%.

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Lab 02: Mad Libs

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