01 Prepare: Review Python

The concepts in CSE 111 build on the concepts that you learned in CSE 110. In order to be successful in CSE 111, it is important that you remember and understand the concepts from CSE 110. To help you remember those concepts, during lesson 1 of CSE 111, you will review programming concepts that you learned in CSE 110.

Concepts

Here is a list of the Python programming concepts and topics from CSE 110 that you should review during this lesson.

Comments

A comment in a computer program is a note or description that is supposed to help a programmer understand the program. Computers ignore comments in a program. In Python, a comment begins with the hash symbol (#) and extends to the end of the current line.

# This is a comment because it has
# hash symbols at the beginning.

Variables

A variable is a location in a computer’s memory where a program stores a value. A variable has a name, a data type, and a value. In Python, we assign a value to a variable by using the assignment operator, which is the equals symbol (=). A computer may change the value and data type of a variable while executing a program.

# Here are four Python variables.
length = 5
time = 7.2
in_flight = True
first_name = "Cho"

Data Types

Python has many data types including str, bool, int, float, list, and dict. Most of the data types that you will use in your programs in CSE 111 are shown in the following list.

It is possible to convert between many of the data types. For example, to convert from any data type to a string, we use the str() function. To convert from a string to an integer, we use the int() function, and to convert from a string to a float, we use the float() function. The int() and float() functions are especially useful to convert user input, which is always a string, to a number. See the program in example 2 below.

User Input

In a Python program, we use the input() function to get input from a user in a terminal window. The input function always returns a string of characters.

text = input("Please enter your name: ")
color = input("What is your favorite color? ")

Displaying Results

In a Python program, we use the print() function to display results to a user. The easiest way to print both text and numbers together is to use a formatted string literal (also known as an f-string).

print(f"Heart rate: {rate}")

The Python program in example 1 creates ten different variables. Some of the variables are of type str, some of type bool, some of type int, and some of type float. The program uses f-strings to print the name, data type, and value of each variable.

# Example 1

# Create variables of different data types and then
# print the variable names, data types, and values.

a = "Her name is "  # string
b = "Isabella"      # string
c = a + b           # string plus string makes string
print(f"a: {type(a)} {a}")
print(f"b: {type(b)} {b}")
print(f"c: {type(c)} {c}")
print()

d = False  # boolean
e = True   # boolean
print(f"d: {type(d)} {d}")
print(f"e: {type(e)} {e}")
print()

f = 15     # int
g = 7.62   # float
h = f + g  # int plus float makes float
print(f"f: {type(f)} {f}")
print(f"g: {type(g)} {g}")
print(f"h: {type(h)} {h}")
print()

i = "True"   # string because of the surrounding quotes
j = "2.718"  # string because of the surrounding quotes
print(f"i: {type(i)} {i}")
print(f"j: {type(j)} {j}")
> python example_1.py
a: <class 'str'> Her name is
b: <class 'str'> Isabella
c: <class 'str'> Her name is Isabella

d: <class 'bool'> False
e: <class 'bool'> True

f: <class 'int'> 15
g: <class 'float'> 7.62
h: <class 'float'> 22.62

i: <class 'str'> True
j: <class 'str'> 2.718

The Python program in example 2 creates six different variables, some of type string, some of type int, and some of type float. Lines 4–5 and 7–8 of example 2 demonstrate that no matter what the user types, the input() function always returns a string. Lines 13 and 14 show how to use the int() and float() functions to convert a string to a number so that the numbers can be used in calculations.



# Example 2

# The input function always returns a string.
k = input("Please enter a number: ")        # string
m = input("Please enter another number: ")  # string
n = k + m          # string plus string makes string
print(f"k: {type(k)} {k}")
print(f"m: {type(m)} {m}")
print(f"n: {type(n)} {n}")
print()

# The int and float functions convert a string to a number.
p = int(input("Please enter a number: "))          # int
q = float(input("Please enter another number: "))  # float
r = p + q                     # int plus float makes float
print(f"p: {type(p)} {p}")
print(f"q: {type(q)} {q}")
print(f"r: {type(r)} {r}")



> python example_2.py
Please enter a number: 6
Please enter another number: 4
k: <class 'str'> 6
m: <class 'str'> 4
n: <class 'str'> 64

Please enter a number: 5
Please enter another number: 3
p: <class 'int'> 5
q: <class 'float'> 3.0
r: <class 'float'> 8.0




    
Arithmetic

    
Python has many arithmetic operators including power
    (**), negation (-), multiplication (*), division (/), floor division
    (//), modulo (%), addition (+), and subtraction (-).


    
The power operator (**) is sometimes called exponentiation and
    causes a computer to raise a base number to an exponent. For
    example, the following Python code causes the computer to calculate
    and store 125 in the variable result because the value in
    x is 5 and the value in y is 3 and
    5 ** 3 or 53 or 5 raised to the
    exponent 3 is 125.


x = 5
y = 3
result = x ** y


    
The modulo operator (%), sometimes called the modulus operator,
    causes a computer to calculate the remainder after division. For
    example, the following Python code causes the computer to store 2 in
    the variable result. The value in x is 17 and
    the value in y is 3. 17 % 3
    causes the computer to divide 17 by 3. 17 divided by 3 is 5 with a
    remainder of 2.


x = 17
y = 3
result = x % y


    
Here are a few more examples of the result of the modulo
    operator.


print(f"21 % 5 == {21 % 5}")
print(f"45 % 5 == {45 % 5}")
print(f"5 % 1 == {5 % 1}")
print(f"3 % 8 == {3 % 8}")
print(f"-3 % 8 == {-3 % 8}")
print(f"3 % -8 == {3 % -8}")
print(f"-3 % -8 == {-3 % -8}")


21 % 5 == 1
45 % 5 == 0
5 % 1 == 0
3 % 8 == 3
-3 % 8 == 5
3 % -8 == -5
-3 % -8 == -3


    
For more information, read the answers in this Stack Overflow
    question about the
    Python modulo operator and negative numbers.


    
Operator Precedence

    
When we write an arithmetic expression that contains more than
    one operator, the computer executes the operators according to their
    precedence, also known as the order of
    operations. This table shows the precedence for the arithmetic
    operators.


    

        
Operators Description Precedence

    
        
( ) parentheses
            highest

        
** exponentiation (power)
            ↑

        
- negation
            |

        
*  /  //  %
            multiplication, division, floor division, modulo
            |

        
+  - addition, subtraction
            ↓

        
= assignment
            lowest

    


    
When an arithmetic expression includes two operators
    with the same precedence, the computer evaluates the operators from
    left to right. For example, in the arithmetic expression
    
x / y * c
 the computer will first
    divide x by y and then multiply that result by
    c. If you need the computer to evaluate a lower
    precedence operator before a higher precedence one, you can add
    parentheses to the expression to change the evaluation order. The
    computer will always evaluate arithmetic that is inside parentheses
    first because parentheses have the highest precedence of all the
    arithmetic operators.


    
If this is the first time that you have encountered arithmetic
    operator precedence, you should watch this Khan Academy video:
    Introduction to Order of Operations
    (10 minutes).


    
The Python program in example 3 gets input from the user and
    converts the user input into two numbers on lines
    9 and 10. Then
    at line 13 the
    program computes the length of a cable from the two numbers. Finally
    at line 17, the
    program uses an f-string to print the length rounded to two places
    after the decimal point.





# Example 3

# Given the distance that a cable will span and the distance
# it will sag or dip in the middle, this program computes the
# length of the cable.

# Get user input and convert it from
# strings to floating point numbers.
span = float(input("Distance the cable must span in meters: "))
dip = float(input("Distance the cable will sag in meters: "))

# Use the numbers to compute the cable length.
length = span + (8 * dip**2) / (3 * span)

# Print the cable length in the
# console window for the user to see.
print(f"Length of cable in meters: {length:.2f}")



> python example_3.py
Distance the cable must span in meters: 500
Distance the cable will sag or dip in meters: 18.5
Length of cable in meters: 501.83




    
In example 3, the arithmetic that is written on
    line 13
    comes from a well known formula. Given the distance that a cable
    must span and the vertical distance that the cable will be allowed
    to sag or dip in the middle of the cable, the formula for
    calculating the length of the cable is:


    
length = span + 
8 dip2
3 span


    
Shorthand Operators

    
The Python programming language includes many augmented
    assignment operators, also known as shorthand
    operators. All the shorthand operators have the same
    precedence as the assignment operator (=). Here is a list of some of
    the Python shorthand operators:



**=  *=  /=  //=  %=  +=  -=



    
To understand what the shorthand operators do and why Python
    includes them, imagine a program that computes the price of a pizza.
    The price of a large pizza with cheese and no other toppings is
    $10.95. The price of each topping, such as ham, pepperoni, olives,
    and pineapple is $1.45. Here is a short example program that asks
    the user for the number of toppings and computes the price of a
    pizza:





# Example 4

# Compute the total price of a pizza.

# The base price of a large pizza is $10.95
price = 10.95

# Ask the user for the number of toppings.
number_of_toppings = int(input("How many toppings? "))

# Compute the cost of the toppings.
price_per_topping = 1.45
toppings_cost = number_of_toppings * price_per_topping

# Add the cost of the toppings to the price of the pizza.
price = price + toppings_cost

# Print the price for the user to see.
print(f"Price: ${price:.2f}")



> python example_4.py
How many toppings? 3
Price: $15.30




    
The statement at
    line 16 in
    example 4 causes the computer to get the value in the
    price variable which is 10.95, then add the cost of the
    toppings to 10.95, and then store the sum back into the
    price variable. Python includes a shorthand operator that
    combines addition (+) and assignment (=) into one operator (+=). We
    can use this shorthand operator to rewrite line 16 like
    this:


price += toppings_cost



    
This statement with the shorthand operator is equivalent to the
    statement on
    line 16
    of example 4, meaning the two statements cause the computer to
    do the same thing. Example 5 contains the same program as
    example 4 but uses the shorthand operator += at
    line 16.





# Example 5

# Compute the total price of a pizza.

# The base price of a large pizza is $10.95
price = 10.95

# Ask the user for the number of toppings.
number_of_toppings = int(input("How many toppings? "))

# Compute the cost of the toppings.
price_per_topping = 1.45
toppings_cost = number_of_toppings * price_per_topping

# Add the cost of the toppings to the price of the pizza.
price += toppings_cost

# Print the price for the user to see.
print(f"Price: ${price:.2f}")



> python example_5.py
How many toppings? 3
Price: $15.30




    
if Statements

    
In Python, we use if statements to cause the
    computer to make decisions; if statements are also
    called selection statements because the computer selects
    one group of statements to execute and skips the other group of
    statements.


    
There are six comparison operators that we can use in an
    if statement:


    

        
< less than

        
<= less than or equal

        
> greater than

        
>= greater than or equal

        
== equal to

        
!= not equal to

    


    
Example 6 contains Python code that checks if a number is
    greater than 500.





# Example 6

# Get an account balance as a number from the user.
balance = float(input("Enter the account balance: "))

# If the balance is greater than 500, then
# compute and add interest to the balance.
if balance > 500:
    interest = balance * 0.03
    balance += interest

# Print the balance.
print(f"balance: {balance:.2f}")



> python example_6.py
Enter the account balance: 350
balance: 350.0

> python example_6.py
Enter the account balance: 525
balance: 540.75




    
If you have written programs in other programming languages such
    as JavaScript, Java, or C++, you always used curly braces to mark
    the start and end of the body of an if statement.
    However, notice in example 6 that if statements in
    Python do not use curly braces. Instead, we type a colon (:) after
    the comparison of the if statement as shown on
    line 8.
    Then we indent all the statements that are in the body of the
    if statement as shown on lines
    9 and 10.
    The body of the if statement ends with the first line
    of code that is not indented, like
    line 13.


    
It may seem strange to not use curly braces to mark the start and
    end of the body of an if statement. However, the Python
    way forces us to write code where the indentation matches the
    functionality or in other words, the way we indent the code matches
    the way that the computer will execute the code.


    
if … elif … else Statements

    
Each if statement may have an else
    statement as shown in example 7 on
    line 13.
    We can combine else and if into the
    keyword elif as shown on lines
    9 and 11.




# Example 7

# Get the cost of an item from the user.
cost = float(input("Please enter the cost: "))

# Determine a discount rate based on the cost.
if cost < 100:
    rate = 0.10
elif cost < 250:
    rate = 0.15
elif cost < 400:
    rate = 0.18
else:
    rate = 0.20

# Compute the discount amount
# and the discounted cost.
discount = cost * rate
cost -= discount

# Print the discounted cost for the user to see.
print(f"After the discount, you will pay {cost:.2f}")



> python example_7.py
Please enter the cost: 300
After the discount, you will pay 246.00




    
Logical Operators

    
Python includes two logic operators which are the
    keywords and, or that we can use to
    combine two comparisons. Python also includes the logical
    not operator. Notice in Python that the logical
    operators are literally the words: and,
    or, not and not symbols as in other
    programming languages:

if driver >= 54 or (driver >= 32 and passenger >= 54):
    message = "Enjoy the ride!"



    
Videos

    
If any of the concepts or topics in the previous list seem
    unfamiliar to you, you should review them. To review the unfamiliar
    concepts, you could rewatch some of the Microsoft videos about
    Python that you watched for CSE 110:


    

    
Tutorials

    
Reading these tutorials may help you recall programming concepts
    from CSE 110.


    

    
You could also read some of the Python tutorials at
    W3 Schools.
    Or you could search for "Python" in the
    BYU-Idaho library online catalog
    and read one of the online books from the result set.


    
Summary

    
During this lesson, you are reviewing the programming concepts
    that you learned in CSE 110. These concepts include how to do
    the following: