Week 2 Day 1 - If/Else

Find the code for class here.


By the end of the class, you should be able to answer the following:

  • How do you use the boolean operators and, or, and not? What do they do?
  • What is the purpose of an if statement?
  • What is the difference between elif and else?
  • How do you write a program that can choose between a variety of possible options and inputs?

Questions from the Reading

These are questions or themes compiled from Perusall to help guide our discussion before jumping into course content.

Basics/Book Questions

Why is the book formatted like that/what are the triple arrows?

How do you take user input?

Boolean Values

What is the difference between boolean and binary?

Can I take more than one argument for and? What about or?

Does the order of operations apply to and/or?


Can you use an elif statement by itself?

Lecture/Live Coding

Data types

There are two very basic types of data in Python that we’re interested in: Numbers and Strings.


Numbers come in two basic formats: integers and floating points.

Integers are numbers without any decimals. E.g. 5, 7, 8.

Floating point numbers, or floats, are numbers with decimal places. E.g. 5.4, 2.0, -1.4

In most languages, integers and floating point numbers are treated very differently. For example, dividing two integers will always produce an integer. In Python, this isn’t the case. They act more like you usually assume numbers to act; they are interchangeable in most contexts.


Strings are basically words or collections of words.

Strings can have spaces, numbers, and can be added together (called concatenation)

my_string = 'Python is cool'
my_string = 'cmsc140'
my_string = 'hello ' + 'friend'

You can use either single or double quotes to note strings, but they have to match. If you want to put a single or double quote inside a string, you can use an escape character like this:

my_string = 'What\'s the temperature?`

This tells Python to interpret the apostrophe as a literal character, not a command.

Interactions between Types

Try this yourself.

Can you add together an int and a float?

num1 = 5
num2 = 4.2
num3 = num1 + num2

What about a float and a string?

apples = 20
sentence = "There are " + apples + "apples."

Strings and integers don’t add together because the operation + isn’t well defined. The program isn’t sure whether you want it to return a number or a sequence of letters, so it does neither.

Conversions Between Type

In Python, it’s easily possible to convert between types. For example, to change an integer or float into a string, simply use str().

my_variable = str(4.5)

It may seem initially as though nothing has changed. But try the following:

apples = 20
sentence = "There are " + str(apples) + "apples."

Now you’re able to add them together, because you’re being very clear about what you want.

What about something like this?

apples = int("apples")

This kind of conversion isn’t well-defined in Python. You could certainly create a conversion, but Python doesn’t have one by default because it’s not obvious what that would be.

Some Basic Functions

Here are some basic built-in functions that will help you complete various tasks:

User input can be handled by

a = input()

By default inputs come in as strings, even if they are numbers.

As the program is executing, it will stop when it gets to an input line. But it won’t tell you that by default, so it’s important that you let the user know that that’s why the program is idling.

We have already used print a few itmes, but to be very explicit, you can do the following for pretty much any structure in Python.

my_list = [3, 4, 5]
my_string = "My string!"
my_dict = {1: "Hello", 2: ["another", "example"], 3: 4}

This is one of the most popular and powerful features of Python. In other languages you will often bang your head against the wall just getting something to print in a way that makes intuitive sense to you.


Boolean expressions are a type of expression in computer science. Just like an entity can be a number or a string, it can be a boolean.

my_num = 7 # a numeric value
my_string = "CMSC 140" # a string
my_bool = True # a boolean

Unlike numbers and strings though, which can take on an infinite number of values, booleans can only take on two values: True or False.

This may not sound very useful, but it’s really the key cornerstone of programming! Many, many, many times, you will be checking whether something is True or False and using that information to make choices in your program.

Boolean Operators

There are three main boolean operators:

  • and: Evaluates to True if both values are true
  • or: Evalutes to True if either value is true
  • not: Flips the value (True becomes False, False becomes True)

Comparison Operators

You can also create boolean values out of comparison operators.

  • <, <=, >, and >= act as you expect them to from mathematics
  • == checks equality
  • != checks not-equal

Go to Exercise 1.

Control Flow

Control flow is basically a term for things the program does that is not just reading and evaluating expressions as they are typed in. There is some input that triggers the evaluation of certain pieces of the code, called blocks.

This is powerful because it means the same code can be made to do many different things depending on the input, which makes it reusable for many more cases.

In today’s class we’ll be looking at if statements and match statements. Both of these basically use the boolean value of something to decide whether or not to execute.

If Statements

if statements are lines of code with the following structure

if {statement is true}:
    {do something}

Here is a very basic example that will always execute:

if True:

Here is an example that will never execute:

if False:
    print("Secret message that you won't see.")

Note that this is effectively no different than just writing print("Hello") in the first case, and writing nothing in the second case. So here is a more realistic example.

i = 0
if i == 0:
    print("i is equal to 0")

Anything you could do outside an if statement, you can do inside an if statement.

i = 7
if i < 10:
    i = 10
    print(2 + 2)
    y = 25
    if 7 == 7:
        print("7 is 7")

Else Clauses

You can choose to have your program do more than one thing based on the value of your if statement.

You can add an else clause, which will only evaluate if the if clause is False.

example = 10
if example < 10:
    print("Small number.")
    print("Big number.")

What will print if I change < to <=?

Elif Clauses

You can similarly chain together these statements with elif to create a long chain of flow if there is more than one value you could take as input.

class_code = 140
if class_code == 140:
    print("This is Python Programming.")
elif class_code == 150:
    print("This is Java Programming.")
elif class_code == 270:
    print("This is Data Structures.")
elif class_code >= 200:
    print("This is an upper-level class.")
elif class_code >= 400: 
    print("This is an advanced class.")

You can add an else clause at the end to catch all other cases, but this is not required.

What will print if I set class_code to 500?

Go to in-class exercises.

In-Class Exercises

Exercise 1: Booleans

On your whiteboards with your group:

  1. Write an expression that uses < and evalutes to True.
  2. Write an expression that uses or and != and evalutes to True.
  3. Write an expression that uses and, >=, and == and evaluates to False.

Exercise 2: Calculator

With your groups:

We’re going to write a very simple caluclator in Python. This calculator takes two numbers from the user and asks them whether they want to add, multiply, divide, or subtract them. Then it performs the correct operation and prints the value.

Below is what your program’s output should be, including user input. It’s up to you to write the internals!

Welcome to the calculator. 
Enter number 1: 8
Enter number 2: 4
Enter operation [a, s, d, m]: d
8 divided by 4 is 2.

Hint: Start small and build up.

You know how to print something stored in a variable, and you know how to take input and store it in a variable. Can you take two numbers from the user and just print them out?

Can you ask what operation they want and store that in a variable? Take it one step at a time.