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Learn how to build control flow into your Python code by including if, else, and elif statements as well as try and except statements. Expect to learn all you need to know about boolean variables and logical operators.

Control Flow

In order to build control flow into our program, we want to be able to check if something is true or not. A boolean expression is a statement that can either be `True` or `False`.

Let's go back to the 'waking up' example. The first question, "Is today a weekday?" can be written as a boolean expression:

```
Today is a weekday.
```

This expression can be `True` if today is Tuesday, or it can be `False` if today is Saturday. There are no other options. 

Consider the phrase: 
```
Friday is the best day of the week.
```
Is this a boolean expression? 

No, this statement is an opinion and is not objectively `True` or `False`. Someone else might say that "Wednesday is the best weekday," and their statement would be no less `True` or `False` than the one above since there is no objective answer to what the best day of the week is. 

How about the phrase: 
```
Sunday starts with the letter 'C'.
``` 
Is this a boolean expression? 

Yes! This expression can only be `True` or `False`, which makes it a boolean expression. Even though the statement itself is false (Sunday starts with the letter 'C'), it is still a boolean expression.

Boolean Expressions

Now that we've added conditional statements to our toolkit for building control flow, let's explore more ways to create boolean expressions. So far we know two relational [operators](https://www.codecademy.com/resources/docs/python/operators?page_req=catalog), equals and not equals, but there are a ton (well, four) more:

- `>` greater than
- `>=` greater than or equal to
- `<` less than
- `<=` less than or equal to

Let's say we're running a movie streaming platform and we want to write a program that checks if our users are over 13 when showing them a PG-13 movie. We could write something like:

```py
if age <= 13:
  print("Sorry, parental control required")
```
 
This function will take the user's age and compare it to the number 13. If `age` is less than or equal to 13, it will print out a message.

Let's try some more!


Relational Operators II

"Okay okay okay, boolean variables, boolean expressions, blah blah blah, I thought I was learning how to build control flow into my code!" 

You are, I promise you!

Understanding boolean variables and expressions is essential because they are the building blocks of [_conditional statements_](https://www.codecademy.com/resources/docs/python/conditionals?page_ref=catalog). 

Recall the waking-up example from the beginning of this lesson. The decision-making process of "Is it raining? If so, bring an umbrella" is a conditional statement. 

Here it is phrased in a different way:

```pseudo
If it is raining, then bring an umbrella.
```

Can you pick out the boolean expression here?

Right, `"it is raining"` is the boolean expression, and this conditional statement is checking to see if it is True. 

If `"it is raining" == True` then the rest of the conditional statement will be executed and you will bring an umbrella.

This is the form of a conditional statement:

```pseudo
If [it is raining], then [bring an umbrella]
```

In Python, it looks very similar:


```py
if is_raining:
  print("bring an umbrella")
```

You'll notice that instead of "then" we have a colon, `:`. That tells the computer that what's coming next is what should be executed if the condition is met. 

Let's take a look at another conditional statement:

```py
if 2 == 4 - 2: 
  print("apple")
```

Will this code print `apple` to the terminal? 

Yes, because the condition of the `if` statement, `2 == 4 - 2` is `True`.

Let's work through a couple more together.

If Statement

Imagine waking up in the morning. 

You wake up and think, "Ugh. Is it a weekday?"

If so, you have to get up and get dressed and get ready for work or school. If not, you can sleep in a bit longer and catch a couple extra Z's. But alas, it is a weekday, so you are up and dressed and you go to look outside, "What's the weather like? Do I need an umbrella?"

These questions and decisions control the flow of your morning, each step and result is a product of the conditions of the day and your surroundings.  Your computer, just like you, goes through a similar flow every time it executes code. A program will run (wake up) and start moving through its checklists, is this condition met, is that condition met, okay let's execute this code and return that value. 

This is the _control flow_ of your program. In Python, your script will execute from the top down, until there is nothing left to run. It is your job to include gateways, known as [conditional statements](https://www.codecademy.com/resources/docs/python/conditionals?page_ref=catalog), to tell the computer when it should execute certain blocks of code. If these conditions are met, then run this function.

Over the course of this lesson, you will learn how to build conditional statements using boolean expressions,  and manage the control flow in your code.

Introduction to Control Flow

As you can tell from your work with _Calvin Coolidge's Cool College_, once you start including lots of `if` statements in a function the code becomes a little cluttered and clunky. Luckily, there are other tools we can use to build control flow.

`else` statements allow us to elegantly describe what we want our code to do when certain conditions are **not** met.

`else` statements always appear in conjunction with `if` statements. Consider our waking-up example to see how this works:

``` py
if weekday:
  print("wake up at 6:30")
else:
  print("sleep in")
```

In this way, we can build if statements that execute different code if conditions are or are not met. This prevents us from needing to write `if` statements for each possible condition, we can instead write a blanket `else` statement for all the times the condition is not met.

Let's return to our `if` statement for our movie streaming platform. Previously, all it did was check if the user's age was over `13` and if so, print out a message. We can use an `else` statement to return a message in the event the user is too young to watch the movie.

```py
if age >= 13:
  print("Access granted.")
else:
  print("Sorry, you must be 13 or older to watch this movie.")
```

Else Statements

Often, the conditions you want to check in your conditional statement will require more than one boolean expression to cover. In these cases, you can build larger boolean expressions using _boolean operators_. These operators (also known as _logical operators_) combine smaller boolean expressions into larger boolean expressions.

There are three boolean operators that we will cover:

- `and`
- `or`
- `not`

Let's start with `and`.

`and` combines two boolean expressions and evaluates as `True` if both its components are `True`, but `False` otherwise.

Consider the example:

```
Oranges are a fruit and carrots are a vegetable.
```

This boolean expression is comprised of two smaller expressions, `oranges are a fruit` and `carrots are a vegetable`, both of which are `True` and connected by the boolean operator `and`, so the entire expression is `True`.

Let's look at an example of some AND statements in Python:

```py
(1 + 1 == 2) and (2 + 2 == 4)   # True

(1 > 9) and (5 != 6)            # False

(1 + 1 == 2) and (2 < 1)        # False

(0 == 10) and (1 + 1 == 1)      # False
```

Notice that in the second and third examples, even though part of the expression is `True`, the entire expression as a whole is `False` because the other statement is False. The fourth statement is also `False` because both components are `False`.

Boolean Operators: and

We have `if` statements, we have `else` statements, we can also have `elif` statements.

Now you may be asking yourself, what the heck is an `elif` statement? It's exactly what it sounds like, "else if". An `elif` statement checks another condition after the previous `if` statements conditions aren't met.

We can use `elif` statements to control the order we want our program to check each of our conditional statements. First, the `if` statement is checked, then each `elif` statement is checked from top to bottom, then finally the `else` code is executed if none of the previous conditions have been met.

Let's take a look at this in practice. The following `if` statement will display a "thank you" message after someone donates to a charity; there will be a curated message based on how much was donated.

```py
print("Thank you for the donation!")

if donation >= 1000:
  print("You've achieved platinum status")
elif donation >= 500:
  print("You've achieved gold donor status")
elif donation >= 100:
  print("You've achieved silver donor status")
else:
  print("You've achieved bronze donor status")
```

Take a second to think about this function. What would happen if all of the `elif` statements were simply `if` statements? If you donated $1100.00, then the first three messages would all print because each `if` condition had been met.

But because we used `elif` statements, it checks each condition sequentially and only prints one message. If I donate $600.00, the code first checks if that is over 1000, which it is not, then it checks if it's over 500, which it is, so it prints that message, then because all of the other statements are `elif` and `else`, none of them get checked and no more messages get printed.

Try your hand at some other `elif` statements.

Else If Statements

Now that we understand what boolean expressions are, let's learn to create them in Python.  We can create a boolean expression by using _relational operators_. 

Relational operators compare two items and return either `True` or `False`. For this reason, you will sometimes hear them called [_comparators_](https://www.codecademy.com/resources/docs/python/operators?page_req=catalog).

The two relational operators we'll cover first are:

- Equals: `==`
- Not equals: `!=`

These operators compare two items and return `True` or `False` if they are equal or not.

We can create boolean expressions by comparing two values using these operators:

```py
1 == 1     # True

2 != 4     # True

3 == 5     # False

'7' == 7   # False
``` 

Each of these is an example of a boolean expression.

Why is the last statement false? The `''` marks in `'7'` make it a string, which is different from the integer value `7`, so they are not equal. When using relational operators it is important to always be mindful of type.

Relational Operators: Equals and Not Equals

Great job! We covered a ton of material in this lesson and we've increased the number of tools in our Python toolkit by several-fold. Let's review what we've learned this lesson:

- Boolean expressions are statements that can be either `True` or `False`
- A boolean variable is a variable that is set to either `True` or `False`.
- We can create boolean expressions using relational operators: 
  - `==`: Equals
  - `!=`: Not equals
  - `>`: Greater than 
  - `>=`: Greater than or equal to
  - `<`: Less than
  - `<=`: Less than or equal to
- `if` statements can be used to create control flow in your code.
- `else` statements can be used to execute code when the conditions of an `if` statement are not met.
- `elif` statements can be used to build additional checks into your `if` statements

Let's put these skills to the test!

Review

Before we go any further, let's talk a little bit about `True` and `False`. You may notice that when you type them in the code editor (with uppercase T and F), they appear in a different color than variables or strings. This is because `True` and `False` are their own special type: [`bool`](https://www.codecademy.com/resources/docs/python/data-types?page_ref=catalog).

`True` and `False` are the only `bool` types, and any variable that is assigned one of these values is called a _boolean variable_. 

Boolean variables can be created in several ways. The easiest way is to simply assign `True` or `False` to a variable:

```py
set_to_true = True
set_to_false = False
```

You can also set a variable equal to a boolean expression.

```py
bool_one = 5 != 7 
bool_two = 1 + 1 != 2
bool_three = 3 * 3 == 9
```

These variables now contain boolean values, so when you reference them they will only return the `True` or `False` values of the expression they were assigned.

```py
print(bool_one)    # True

print(bool_two)    # False

print(bool_three)  # True
```


Boolean Variables

The final boolean operator we will cover is `not`. This operator is straightforward: when applied to any boolean expression it reverses the boolean value. So if we have a `True` statement and apply a `not` operator we get a `False` statement.

```py
not True == False
not False == True
```

Consider the following statement:

```
Oranges are not a fruit.
```

Here, we took the `True` statement `oranges are a fruit` and added a `not` operator to make the `False` statement `oranges are not a fruit`. 

This example in English is slightly different from the way it would appear in Python because in Python we add the `not` operator to the very beginning of the statement. Let's take a look at some of those:

```py
not 1 + 1 == 2  # False
not 7 < 0       # True
```

Boolean Operators: not

The boolean operator `or` combines two expressions into a larger expression that is `True` if either component is `True`.

Consider the statement

```
Oranges are a fruit or apples are a vegetable.
```

This statement is composed of two expressions: `oranges are a fruit` which is `True` and `apples are a vegetable` which is `False`. Because the two expressions are connected by the `or` operator, the entire statement is `True`. Only one component needs to be `True` for an `or` statement to be `True`.

In English, `or` implies that if one component is `True`, then the other component must be `False`. This is not true in Python. If an `or` statement has two `True` components, it is also `True`.

Let's take a look at a couple of examples in Python:

```py
True or (3 + 4 == 7)    # True
(1 - 1 == 0) or False   # True
(2 < 0) or True         # True
(3 == 8) or (3 > 4)     # False
```

Notice that each `or` statement that has at least one `True` component is `True`, but the final statement has two `False` components, so it is `False`.
