CSE 111: Programming with Functions

W01 Prepare 1: 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.

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 3–4 and 6–8 of example 2 demonstrate that no matter what the user types, the input() function always returns a string. Lines 11 and 12 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 7 and 8. Then at line 10 the program computes the length of a cable from the two numbers. Finally at line 13, 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 11 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 11 like this:

price += toppings_cost

This statement with the shorthand operator is equivalent to the statement on line 11 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 11.

# 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 6. Then we indent all the statements that are in the body of the if statement as shown on lines 7 and 8. The body of the if statement ends with the first line of code that is not indented, like line 10.

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 11. We can combine else and if into the keyword elif as shown on lines 7 and 9.

# 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:

Activity: Python Review

Purpose

Write a Python program that gets input from a user, uses variables, performs arithmetic, and displays results for the user to see.

Problem Statement

When you physically exercise to strengthen your heart, you should maintain your heart rate within a range for at least 20 minutes. To find that range, subtract your age from 220. This difference is your maximum heart rate per minute. Your heart simply will not beat faster than this maximum (220 − age). When exercising to strengthen your heart, you should keep your heart rate between 65% and 85% of your heart’s maximum rate.

Assignment

Write a Python program named heart_rate.py that asks for a person’s age and computes and outputs the slowest and fastest rates necessary to strengthen his heart. To start your program, copy and paste the following code into your program and use it as an outline as you write code. Note that in a Python program, a triple quoted string at the top of the file acts as a comment for the entire program.

 """
When you physically exercise to strengthen your heart, you
should maintain your heart rate within a range for at least 20
minutes. To find that range, subtract your age from 220. This
difference is your maximum heart rate per minute. Your heart
simply will not beat faster than this maximum (220 - age).
When exercising to strengthen your heart, you should keep your
heart rate between 65% and 85% of your heart’s maximum rate.
"""

Testing Procedure

Verify that your program works correctly by following each step in this testing procedure:

  1. Run your program and enter the input shown below. Ensure that your program’s output matches the output below. 
    > python heart_rate.py
Please enter your age: 23
When you exercise to strengthen your heart, you should
keep your heart rate between 128 and 167 beats per minute.
  2. Run your program using your age or the age of one of your parents. Use a calculator to ensure that the output is correct.

Sample Solution

When your program is finished, view the sample solution for this assignment to compare your solution to that one. Before looking at the sample solution, you should work to complete this checkpoint program. However, if you have worked on it for at least an hour and are still having problems, feel free to use the sample solution to help you finish your program.

Ponder

During this assignment, you wrote a Python program that gets input from a user, uses variables, performs arithmetic, and displays results for the user to see. Because you should have learned how to write this type of program in CSE 110, this assignment should have been fairly easy for you. If this assignment was difficult for you, you should review the concepts from CSE 110 and the programs that you wrote in that course.

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