# Booleans

## The `bool`

type

If you recall from the Variables lesson,
one of basic types listed was `bool`

. This type represents a **boolean**, which is a
single true/false value.

The Boolean Expressions
we learned about early in this Conditions lesson create a boolean value when evaluated.
So, we can make variables that are the results of such boolean expressions, and then
further use these `bool`

variables in the conditions of if statements.

C++ provides two built-in `bool`

constants - `true`

and `false`

. You can use these wherever
a `bool`

or boolean expression is expected. For example:

`bool var1 = true;`

bool var2 = false;

There are more interesting things we can do with `bool`

variables, too.
Any boolean expression is valid to assign here. Combining this with
if statements can make for shorter and more readable code in some cases.

`std::string name;`

std::cout << "Enter your first name: ";

std::cin >> name;

// User is lucky if their name has more than 5 characters

bool isLucky = name.size() > 5;

// Try changing these values yourself and see what happens!

bool isTuesday = true;

bool isCloudy = false;

std::string weather = "sunny";

if (not isTuesday and isCloudy) {

weather = "raining";

} else if (isTuesday) {

if (isCloudy) {

weather = "overcast";

} else if (not isLucky) {

weather = "downpouring";

}

}

std::cout << "It is currently " << weather << " in your location!";

The logical operators previously introduced can also be used to create new boolean variables.

`bool isPrecipitating = weather == "raining" or weather == "downpouring";`

bool isRainbowMade = isPrecipitating and not isCloudy;

bool isBoringDay = not isRainbowMade;

## Input/Output with `bool`

If you did any self-driven exploration with this new `bool`

type, you may have noticed a peculiar thing...
Printing out a `bool`

does not display a `true`

or `false`

in the terminal. Instead, it prints out a `1`

or a `0`

.
This is simply a default behavior in C++ that is related to the next section.

You can easily change this behavior by changing one of `std::cout`

's settings.
Simply "print" out the `std::boolalhpa`

built-in constant, and it will make all future boolean operations on `std::cout`

print out `true`

/`false`

instead of `1`

/`0`

.

Make sure you `#include <iomanip>`

to use `std::boolalpha`

.

`std::cout << true << " " << false << "\n";`

std::cout << std::boolalpha;

std::cout << true << " " << false << "\n";

`1 0`

true false

Conversely, `std::cin`

by default will only accept a `0`

or a `1`

when inputting into a `bool`

variable.
This behavior can also be changed by "inputting" into the `std::boolalpha`

flag.
In the example below, the user will first input `0`

, which represents `false`

.
Then, they will input `true`

once the `std::boolalpha`

setting was applied.

`bool var;`

std::cout << "Integral form: ";

std::cin >> var;

std::cin >> std::boolalpha;

std::cout << "Alphanumeric form: ";

std::cin >> var;

`Integral form: 1`

Alphanumeric form: true

As an exercise, try modifying the weather example from above to accept user input, using
the `std::boolalpha`

modifier you just learned.

The name "boolalpha" comes from a portmanteau of boolean and alphanumeric.

`int`

↔`bool`

conversions

C++ has another rather annoying behavior where booleans and integers can implicitly convert between each other.
A boolean value of `false`

is equivalent to an integer value of `0`

, and `true`

is equivalent to `1`

.
Conversely, an integral value of `0`

is equivalent to a boolean value of `false`

,
and any non-zero integer is equivalent to `true`

.

`int x = true; // x is 1`

int y = false; // y is 0

bool a = 1; // a is true

bool b = 25; // b is true

bool c = -194; // c is true

bool d = 0; // d is false

While this behavior may seem neat, it can unexpectedly crop up in many places where they are not wanted.
For this reason, if you want to intentionally trigger an `int`

↔`bool`

conversion, then you should *cast*
the value.

`int x = static_cast<int>(true); // Modern-style cast`

int y = (int) false; // C-style cast

bool a = static_cast<bool>(1);

bool b = (bool) 0;

There are two primary ways to perform a cast like this in C++ - a "static cast" and a "c-style cast".
You will learn the differences between these in the Intermediate course. For now just know that,
in the case of `int`

↔`bool`

conversions, both of the options are equivalent.
However, the `static_cast`

method is generally preferred for Modern C++.

### Unintended Conversions

These conversions can cause issues where you least expect them to. One example is in the compound boolean expressions you learned about earlier. Normally, if you want to combine multiple boolean expressions together, you use the logical operators like so:

`if (x > 10 && x < 25) { ... }`

You may be tempted to rewrite the condition as `10 < x < 25`

.
**This will result in incorrect behavior**, as this expression will always evaluate to `true`

.
This is because there is secretly an implicit `bool`

🡢`int`

conversion occurring.
This expression gets interpreted as `(10 < x) < 25`

.
Let's see how this causes this condition to always evaluate to `true`

by using
the operator precedence table:

`10 < x < 25 // Initial expression`

10 < 15 < 25 // Substitute x

(10 < 15) < 25 // Operator precedence

(true) < 25 // Evaluate subexpression

1 < 25 // bool🡢int conversion

true // Evaluate subexpression

`10 < x < 25 // Initial expression`

10 < 7 < 25 // Substitute x

(10 < 7) < 25 // Operator precedence

(false) < 25 // Evaluate subexpression

0 < 25 // bool🡢int conversion

true // Evaluate subexpression

This same issue can occur in other similar scenarios, as well.
For example, when checking if a single variable is one of many possibilities,
the correct way would be `x == 5 || x == 10 || x == 15`

.
The tempting option of `x == 5 || 10 || 15`

**will not work** for a similar reason to the above.
It will always be true. Let's see why:

`x == 5 || 10 || 15 // Initial expression`

5 == 5 || 10 || 15 // Substitute x

((5 == 5) || 10) || 15 // Operator precedence

((true) || 10) || 15 // Evaluate subexpression

(true || true) || 15 // int🡢bool conversion

true || 15 // Evaluate subexpression

true || true // int🡢bool conversion

true // Evaluate subexpression

`x == 5 || 10 || 15 // Initial expression`

7 == 5 || 10 || 15 // Substitute x

((7 == 5) || 10) || 15 // Operator precedence

((false) || 10) || 15 // Evaluate subexpression

(false || true) || 15 // int🡢bool conversion

true || 15 // Evaluate subexpression

true || true // int🡢bool conversion

true // Evaluate subexpression

## Conclusion

In summary, we learned about:

- How to break down conditional cases in our code into if statements
- How to form a logical expression out of boolean operations
- How to combine multiple conditions into a mutually exclusive group
- How to nest if statements inside of each other
- How to reduce reptition by applying the DRY Principle
- How to use boolean variables to improve our code

This was a long journey, but the skills you have picked up here in this lesson are very fundamental to the road ahead. Read over the concepts presented in this lesson multiple times, as the basics of logic and conditions are a very important skill to have.