We just finished a thorough overview of the major data structures in Python, including:

• Integers
• Strings
• Lists
• Tuples
• Dictionaries
• and Sets

However, all of the data structure knowledge in the world is not enough to build real-world applications. There is a common saying in computer programming:

"Data Structures + Algorithms = Applications"

In this lesson, we'll begin learning about logical operators, which are the backbone of the algorithms you'll need to build full applications later in your career.

This will be one of the longest lessons in this course, but it is also one of the most important, so please make sure you understand these concepts thoroughly before proceeding.

A Quick Review of Booleans

I briefly introduced the concept of boolean variables in an earlier lesson of this course. Understanding boolean variables is extremely important for learning the content of this lesson, so a quick review is merited.

Boolean variables are a special data structure that can only be assigned two values: True or False. The values are not wrapped in quotes, and the first letter must be capitalized.

Examples of each boolean variable value are below:

trueBool = True

falseBool = False

Throughout this lesson, we will see how boolean variables can be used to control whether or not a block of code is executed.

We need to understand how to generate boolean variables first. Let's cover that next.

Equality Operators

As you might imagine by the nature of boolean variables, they are used to represent whether a certain statement is true or not. The most basic form of these statements is an equality statement: are two things equal to one another?

Python has a built-in way to check this statement for two variables. It is called the equality operator and is represented by ==. The equality operator returns True if two variables are equal and False if they are not equal.

Here are five examples of the equality operator in action:

'Nick' == 'Not Nick'

#Returns False

"Nick" == 'Nick'

#Returns True

a = 1

b = 2

a == b

#Returns False

a + 1 == b

#Returns True

boolean_variable = True

boolean_variable == False

#Returns False

Please make sure to understand all of these examples before proceeding. Specifically, be sure to understand the difference between =, the assignment operator, and ==, the equality operator.

Inequality Operators

The inequality operator is the opposite of the equality operator. It tests whether two variables are not equal to each other. The inequality operator is represented by != in Python.

Five examples of the inequality operator are listed below:

trueBoolean = True

falseBoolean = False

trueBoolean != falseBoolean

#Returns True

trueBoolean != True

#Returns False

falseBoolean != False

#Returns False

1 != two

#Returns True

'String' != 'string'

#Returns True (notice the difference in capitalization on the 's')

Given that the equality operator and the inequality operator are polar opposites of one another, you might be wondering if there is any way to relate the two. There is, and it is called the 'not' operator.

The 'Not' Operator

The word 'not' is a special keyword in Python that modifies the value of a boolean variable. Specifically, it changes True values to False and False values to True.

Two examples are below:

isNickCool = True

not isNickCool

#Returns False

Chaining Equality Operators Together

You can easily chain together equality and inequality variables in Python. Here is an example, which tests for whether someone is over 65, likes to dance, and has grey hair:

isOver65 = True
likesToDance = True
hasGreyHair = False

True == isOver65 == likesToDance == hasGreyHair
#Returns False

Storing Equality Tests in Variables

Testing for equality or inequality is useful, but it becomes significantly more useful when you start to store the outputs of these tests in variables. You can do this using the assignment operator = that we have been relying on throughout this course. Let's peruse three examples to understand this properly.

As you can tell, I find it useful to write my boolean variable names as quasi-questions so that I can easily remember the statement that they're storing a True or False value for. Variable names like isNickCool or hasGreyHair allow us to easily recognize what statement they're representing.

Here are a few examples of storing equality tests in variables:

int1 = 1
int2 = 2
areTheIntegersEqual = int1 == int2
areTheIntegersEqual 		#To print its value
#Returns False, since they are not equal

myName = 'Nick'
myFriendsName = 'Levi'
doWeHaveTheSameName = myName == myFriendsName
doWeHaveTheSameName 		#To print its value
#Returns False, since the names are different

So far, we have discussed how to test for equality and inequality, and how to store those tests' outputs in variables. We have also explored how to change the value of a boolean variable using the 'not' operator.

The and and or Operators

It is often useful to be able to test boolean logic in an all-or-nothing fashion. More specifically, we often would like to return True if all conditions are met and False if one or more conditions are not met.

The and operator is the best way to do this. An example of the and operator is below:

True and True and True
#Returns True

True and False and True

#Returns False

Having a more realistic example is helpful for understanding how the and operator is used in practice:

sunnyDay = True

rainyDay = False

sunnyDay and rainyDay

#Returns False

In a slightly different scenario, it can be useful to return True if even a single value is true, and return False only if all values are false. The or operator allows us to do this.

An example of the or operator is below:

True or False
#Returns True

False or False
#Returns False

Again, a more realistic example is helpful:

sunnyDay = True

rainyDay = False

sunnyDay or rainyDay

#Returns True

Comparison Operators

While the equality and inequality operators were used earlier in this lesson to test the equality (or lack thereof) of two variables, the comparison operators are used to compare values.

There are four comparison operators in Python, and their syntax mimics their mathematical counterparts. They are presented and explain below:

>
#Greater than

<
#Less than

>=
#Greater than or equal to

<=
#Less than or equal to

Examples of how to use these comparison operators are demonstrated below:

1 < 2

#Returns True

2 < 1

#Returns False

1 > 0

#Returns True

0 > 1

#Returns False

1 <= 2

#Returns True

3 <= 2 

#Returns False

3 <= 3 

#Returns True

3 >= 3

#Returns True

3 >=  2

#Returns True

2 >=  3

#Returns False

We have spent a great deal of time in this lesson learning various logical operators. In the next section, we will learn how to use 'if statements'.

If Statements

'If statements' are used in computer programming to control whether or not a block of code is executed depending on some outside condition. In Python, if statements are controlled by boolean variables.

If statements have the following general syntax in Python:

if(statement):

	action

Let's break this down:

• Statement: this is a boolean condition that controls whether or not the code in the if statement will run
• Action: this is the code that will run if the 'statement' is True. The indent before this action is key - without the indent after the if statement's colon, the code will not compile correctly and will throw an error.

Let's consider a real example of an if statement in action:

age = 20

if(age < 20):

	print("You are less than 20 years old!")

if(age == 20):

	print("You are exactly 20 years old!")

if(age > 20):

	print("You are more than 20 years old!")

If statements can also be used to modify other variables.

For example, consider an hourly worker being paid $25/hour who gets 2x overtime for all hours above 40 hours per week. This section of if statements calculates his weekly compensation after accounting for his overtime pay:

totalHours = 37

hourlyPay = 25

if(totalHours < 40):

	weeklyPay = totalHours * hourlyPay

if(totalHours >= 40):

	weeklyPay = 40 * hourlyPay + (totalHours - 40) * 2 * hourlyPay

Elif and Else Statements

As we saw in the last section, you can pair together multiple if statements to account for different scenarios. In this section, we will discuss a better, more readable way of using different logic scenarios: if else statements.

If else statements are another way to run conditional code that is considered more readable than chained if statements. If else statements have the following appearance:

if(statement):

	action

else:

	different action

An example of this logic is below:

You can also use elif statements to test multiple conditions before executing the catch-all code held within the else block. Elif stands for 'else if', and is executed as follows:

if(statement1):

	action1

elif(statement2):

	action2

else:

	action3

There is no limit to how many elseif statements can be in one statement. For example, consider the following code:

items = 7

if(items == 0):

    print('There are no items in your shopping cart.')

elif (items == 1):

    print('There is 1 item in your shopping cart.')

elif (items == 2):

    print('There are 2 items in your shopping cart.')

elif (items == 3):

    print('There are 3 items in your shopping cart.')

elif (items == 4):

    print('There are 4 items in your shopping cart.')

elif (items == 5):

    print('There are 5 items in your shopping cart.')

elif (items == 6):

    print('There are 6 items in your shopping cart.')

elif (items == 7):

    print('There are 7 items in your shopping cart.')

elif (items == 8):

    print('There are 8 items in your shopping cart.')

elif (items == 9):

    print('There are 9 items in your shopping cart.')

elif (items == 10):

    print('There are 10 items in your shopping cart.')

else:

    print('There are too many items to count in your shopping cart!')

Important Note: The code above is for illustration's purpose only. The best way to solve the shopping cart item would be to use the f string interpolation that we learned earlier in this course, like this:

items = 20

if(items == 0):

    print('There are no items in your shopping cart.')

elif (items == 1):

    print('There is 1 item in your shopping cart.')

else:

    print(f'There are {items} items in your shopping cart!')

#Returns 'There are 20 items in your shopping cart!'

Moving On

This lesson provided a detailed explanation of logical operators and if statements in Python.

This is unquestionably the hardest topic we've covered yet in this course. To make sure you understand it fully, the next lesson of this course will work through numerous practice problems to help you solidify your knowledge of these fundamental concepts.