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Last updated on 5/9/23

Control Your Program Flow Using Conditionals

What Is Program Flow?

Program flow is the order in which lines of code are executed. Some lines will only be read once, others multiple times, and others still may be skipped entirely, depending on how you've programmed them.

In this first chapter on program flow, we will look at how to program your code using conditional statements.

Conditionals are ways to control the logic and flow of your code using different conditions. You make decisions based on conditions in your daily life all the time.

For example: If it's sunny outside, go to the beach. In this scenario, whether or not you go to the beach is dependent on the weather as a condition. 

Let's see how this type of logic works in code.

Define Conditions With If/Else Statements

One of the most important and basic building blocks of a conditional flow is the if statement.

With an if statement, you can run certain lines of code only if a certain condition is true. If that condition is false, the code won't run. 

Here's a weather example:

if sunny:
print("go to the beach!")

Line 2 will only execute if the condition  sunny  is  True .

Sounds simple enough, so where does "else" come in?

Now let's expand the weather example to include an else clause:

If it's sunny outside, go to the beach.
Else (for example, if it's raining), stay inside!

When using if/else statements in Python, the syntax looks like this:

if my_boolean:
# execute code when my_boolean is True
# execute code when my_boolean is False

So to only print a statement under a certain condition, you can do the following:

sunny = True
if sunny:
print("go to the beach!")
print("stay inside!")

In this code snippet, since  sunny  is true,  you will see  go to the beach!  in your terminal. If it were set to false, the code would print  stay inside! .

Define Alternative Conditions by Adding an Elif Clause

An if/elif/else statement allows you to define multiple conditions. The elif keyword allows you to add as many conditions as you’d like. You then end the statement with an else clause.

Let’s say you want to go to the beach if it’s warm outside, build a snowman if it’s snowing, or otherwise stay home. You can code that with the following syntax:

sunny = False
snowing = True
if sunny:
print("go to the beach!")
elif snowing:
print("build a snowman")
print("stay inside!")

This code will first check if  sunny  is true, and because it is false, it will then check if  snowing  is true. Because  snowing  is True, the code will print  build a snowman . But if  snowing  were also false, the program would execute the final else statement and print  stay inside! .

Define Multiple Conditions With Logical Operators

If you want to check multiple conditions for a single outcome in the same if statement, you can use logical operators:

  • and : check if two conditions are both true.

  • or : check if at least one condition is true.

  • not: check if a condition is not true (i.e., false). 

These operators can be mixed and matched for your needs.

Let’s say you want to go to the beach only if it’s sunny and a weekend, but if it’s sunny and a weekday, you need to be at work.

is_sunny = True
is_weekday = False
if is_sunny and not is_weekday:
print("go to the beach!")
elif is_sunny and is_weekday:
print("go to work")
print("stay inside!")

Define Complex Conditions With Comparative Expressions

Comparative expressions let you compare different expressions to each other and evaluate whether that expression is true or false.

If you have two values a and b , you can use the following comparison operators in Python: 

  • Equals: a  ==  b

  • Not Equals: a  !=  b

  • Less than: a  <  b

  • Less than or equal to: a  <=  b

  • Greater than: a  >  b

  • Greater than or equal to: a  >=  b 

For example:

number_of_seats = 30;
number_of_guests = 25;
if number_of_guests < number_of_seats:
# allow more guests
# do not allow any more guests

You can mix and match any number of these tools to have really complex and intricate code functionality.

Level-Up: Control Your Program Flow Using Conditionals

If you do this activity, you will level up. Else, you will miss out on a great opportunity. It's time to practice using conditionals! 😁

Then, check your work by:

  • submitting your project,

  • navigating back to the course team page (click on the exercise title on the top left of your screen, then the team name "OCpythonbasics")

  • clicking on "Fork the solution".

Level-Up, Bonus Round: Fix Indentations in a Code Block

Here’s an extra activity to practice using indentations correctly! 😎 Be sure to check your work on it, as well!

Let’s Recap!

  • If/else statements help you define certain conditions for when code is executed.

  • The elif keyword allows you to use multiple conditions.

  • You can group different conditions together using  and ,  or , and  not .

  • Comparative expressions use comparison operator like  <  and  >  help you compare multiple variables.

In the next chapter, you’ll learn how to repeat code tasks using loops.

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