Learn how to develop complex programs in Swift with conditionals. 

Conditionals

So far, our conditions have consisted of a single variable whose value is a Boolean, `true` or `false`. With the help of operators, we can expand on this and create conditions that utilize multiple values to achieve a Boolean result.  

In this exercise, we'll learn about one particular group of operators in Swift known as comparison operators. _Comparison operators_ determine the relationship between two operands and return a Boolean value as a result.

Swift supports the following comparison operators:

- `==` Equal to
- `!=` Not Equal to
- `>` Greater than
- `<` Less than
- `>=` Greater than or equal to
- `<=` Less than or equal to

Comparison operators are most commonly used to compare numerical values such as `Integer`s and `Double`s, though `==` and `!=` can also be used to compare `String` values.

```swift 
4 < 5        // true 
0.5 > 0.1    // true
3.5 <= 3.0   // false
12 >= 15     // false
"A" == "A"   // true 
"B" != "b"   // true
```

Notice how a capital `"B"` is not equal to a lowercase `"b"` since Swift is a case sensitive language.

Combining our knowledge of `if/else` statements and comparison operators, we can construct the following conditional to check for a student's grade on an exam:

```swift
let grade = 95

if grade > 65 {
  print("You passed!") 
} else {
  print("You failed.")
}
```

Since the student's `grade` is greater than `65`, the first code block gets executed and prints, `You passed!` 🎉. 

Comparison Operators

Until now, we’ve been working with conditionals that can handle only one condition. If that condition is met, our program follows one course of action, otherwise it follows another. 

Swift provides us with a tool called the `else if` statement which allows us to add additional conditions to a standard `if`/`else` statement. 

Here's how it works: 

```pseudo
if condition1 {
  this code runs when condition1 is true 
} else if condition2 {
  this code runs when condition2 is true 
} else if condition3 {
  this code runs when condition3 is true 
} else {
  this code runs when all previous conditions are false 
}
```

- Similarly to an `if` statement, an `else if` statement accepts a condition and a code block to execute for when that condition is `true`. 

- The `else if` statement exists only between an `if` and an `else` and cannot stand on its own like the `if` statement can.

- Any number of `else if` statements can exist between an `if` and an `else`.   

Working off the previous example, assume we'd like to update the grading scale in a school to use the [academic grading scale used in the U.S.](https://en.wikipedia.org/wiki/Academic_grading_in_the_United_States). 

We can translate numerical grades to letter grades, `"A"`, `"B"`, `"C"`, etc. with the help of multiple `else if` statements: 

```swift
let grade = 85
let letterGrade: String 

if grade >= 90 {
  letterGrade = "A"
} else if grade >= 80 {
  letterGrade = "B"
} else if grade >= 70 {
  letterGrade = "C"
} else if grade >= 60 {
  letterGrade = "D"
} else if grade < 60 {
  letterGrade = "F"
} else {
  letterGrade = "N/A"
}

print(letterGrade) 
// Prints: B
```

Since a student's numerical grade is `85`, the first `else if` statement executes, and the value of `letterGrade` becomes `"B"`. 

Else If Statements 

One super power that the `switch` statement possesses, is its ability to match values to an expression that exist within intervals. An _interval_ denotes a range used for checking whether a given value lies within that range.

In Swift, a range is indicated by three consecutive dots, `...`, also known as the closed range operator. The _closed range operator_ signifies an inclusive range where the first and last values are included in the sequence. 

Let's see these new concepts in action. In the example below, the `switch` statement determines the value of `year` and checks which century it belongs to. 

```swift 
var year = 1943

switch year {
  case 1701...1800:
    print("18th century") 
  case 1801...1900:
    print("19th century")
  case 1901...2000: 
    print("20th century")
  case 2001...2100: 
    print("21st century")
  default: 
    print("You're a time traveler!")
} 
// Prints: 20th century
```

Since the year, `1943`, falls between the interval, `1901...2000`, the code for the third case is executed and the message, `20th century` gets printed.   

Fun fact: The first electronic computer was built in 1943! 💻 

Switch Statement: Interval Matching 

On a daily basis, we’re faced with making decisions based on certain conditions; if the weather is beautiful, we'll go for a walk, or if it's rainy, we'll stay in and code! 

In Swift, the ability to determine an outcome based on a given condition also exists. It's known as a conditional. 

_Conditionals_ are powerful programming tools that introduce flexibility and complex behavior to a program by allowing it to react to change. Their power comes from being the root of all decision making within an application. 

Your computer, for example, contains authentication processes whose logic is handled by conditionals. If you are logged out, access to certain pages on your device may be restricted, or if your are logged in, access will be granted. 

In this lesson, we'll set up our own rules and conditions for programs to follow using Swift's conditional statements and operators. 

A donut factory that displays multiple conveyor belts and different types of donut flavors and toppings. The donut flavors include chocolate, strawberry, and maple. If the donut is chocolate, it goes on one conveyor belt, if it's strawberry, it goes on another, and so on. The toppings include chocolate sprinkles and rainbow sprinkles. 

Introduction to Conditionals

The `if` statement is the most basic and fundamental type of conditional in Swift. It is used to execute some code when a given condition is `true`. 

The structure of an `if` statement is:

```pseudo
if condition {
  this code will run if condition is true
}
```

- The `if` keyword is followed by a condition whose value must be `true` or `false`.
- The condition is immediately followed by a code block.
- A _code block_ is a snippet of code enclosed in a pair of curly braces, `{` `}`. 
- The code block is executed only when the condition is `true`. 
- If the condition is `false`, the code block does not run.

Suppose we're creating an app that includes an authentication process. We'd declare the variable, `isLoggedIn` to be `true` and set up the following `if` statement that prints a message to users who are logged in: 

```swift 
var isLoggedIn = true

if isLoggedIn {
  print("Welcome back!")
}
```

Since the value of the condition is `true`, the message, `Welcome back!` will get printed. If `isLoggedIn` was `false`, thus our condition `false`, the code block wouldn't execute and nothing would get printed. 

If Statement

A standard `if`/`else` statement can get pretty lengthy - at least 5 lines of code. Many developers prefer to keep their code as concise as possible and favor a shorter syntax. Swift allows us to minimize our `if`/`else` statements with a tool called the ternary conditional operator. 

The _ternary conditional operator_, denoted by a `?`, offers a shorter, more concise alternative to a standard `if/else` statement. It executes one of two expressions depending on the boolean result of the condition. 

A ternary conditional consists of three parts in the following format: 


```tex
A \ ? \ B \ : C
```

- **A** is the condition to check for<br>
- **B** is the expression to use if the condition is `true`<br>
- **C** is the expression to use if the condition is `false`<br>


Suppose we'd like to check if an order was placed successfully by a customer and print them a message. We can set up the following `if`/`else` statement:


```swift 
var orderSuccessfullyPlaced = false 

if orderSuccessfullyPlaced {
  print("Your order was received.")
} else {
  print("Something went wrong.")
}
```

Since the value of our condition is `false`, the second code block executes and `Something went wrong.` will print.

With a sprinkle of ternary magic, we can transform our code into one line, a _one liner_, and achieve the same result:

```swift 
orderSuccessfullyPlaced ? print("Your order was received.") : print("Something went wrong.")
```

Note that although the ternary conditional operator helps us develop shorter code, overusing this syntax can also result in your code being difficult to read. So, use it sparingly! 

Ternary Conditional Operator

Excellent work! In this lesson, we've learned the following concepts:

-  An `if` statement consists of a condition and code block that executes when the condition is `true`.
-  An `else` statement is immediately followed by a code block that executes when all previous conditions were `false`.
-  A _code block_ is denoted by a set of curly braces `{}`.
-  Multiple `else if` statements can be chained within an `if`/`else` to provide additional conditions.
-  Comparison operators include `<`, `>`, `<=`, `>=`, `==`, and `!=` and are used to compare the values of two operands.
-  A `switch` statement looks for the value of a case that matches the value of an expression. 
-  A `switch` statement can have cases that contain multiple items known as _compound cases_.  
-  A `switch` statement's case can include a range of values using the _closed range operator_ (`...`).

You've covered a major fundamental programming concept used in every program to control the logical flow. Feel free to utilize the empty **Review.swift** file and output terminal on the right to hone your understanding of conditionals and practice writing Swift code. 

Review

A tool most commonly used together with an `if` statement is the `else` statement. 

An `else` statement is used to execute a block of code when the condition of an `if` statement is `false`. Think of the `else` to be synonymous with the word, _otherwise_. Do this if a condition is true, otherwise, do something _else_: 

```pseudo
if condition {
   this code will run if condition is true
} else {
   this code will run if condition is false 
}
``` 

Two rules to remember: 

- An `else` statement must be used in conjunction with an `if` statement and cannot stand on its own.  
- An `else` statement does not accept a condition and is immediately followed by a code block. 

In the previous exercise, we created an `if` statement that prints a friendly message to  logged in users. Let's use an `else` statement to complete this logic and print a message to users who are not logged in: 

```swift
var isLoggedIn = false

if isLoggedIn {
  print("Welcome back!") 
} else {
  print("Access Denied.")  
}
// Prints: Access Denied.
```

Since the value of `isLoggedIn` is `false`, our condition is therefore, `false`, and the code block following the `else` will execute. 

Else Statement

Another noteworthy ability of the `switch` statement is its use of multiple values in a single case. These are known as _compound cases_. The `switch` statement will match each value within a compound case to the given expression.

The following code checks the value of `country` and determines the continent on which it is located using a `switch` statement. Since a continent may consist of multiple countries, compound cases deem useful: 

```swift 
var country = "India"

switch country {
  case "USA", "Mexico", "Canada":
    print("\(country) is in North America. 🌏")
  case "South Africa", "Nigeria", "Kenya":
    print("\(country) is in Africa. 🌍")
  case "Bangladesh", "China", "India":
    print("\(country) is in Asia. 🌏")
  default: 
    print("This country is somewhere in the world!")
} 
// Prints: India is in Asia. 🌏
```

Notice how...

- The multiple values or items in a compound case are separated by a comma.
- We used string interpolation to output the value of `country` within the `String` of the `print()` statement. 

Switch Statement: Compound Cases

Another neat feature available for the cases of a `switch` statement is the `where` clause. 

The `where` clause allows for additional pattern matching for a given expression. It can also be used with loops and other conditionals such as `if` statements. 

Assume we're creating a program that determines if a random integer between 0 and 10 is even or odd. We can write the following program:   

```swift 
var randomNumber = Int.random(in: 0...10)

switch randomNumber {
  case let x where x % 2 == 0:
    print("\(randomNumber) is even")
  case let x where x % 2 == 1:
    print("\(randomNumber) is odd")
  default:
    print("Invalid")
}
```

Let's dive into what's happening on the first line:

```swift 
var randomNumber = Int.random(in: 0...10)
```

- We're generating a random integer, `Int`, using the built-in Swift method, `.random()` which returns an arbitrary value from a range of numbers. Notice how we're using the closed range operator, `...`, to denote a numerical range.

- We then assign the randomly generated value to the variable, `randomNumber`. We'll be working more with `.random()` in the following lessons.  

Following the variable declaration is a standard `switch` statement that checks the value of `randomNumber`:

```swift 
switch randomNumber {
  case let x where x % 2 == 0:
    print("\(randomNumber) is even")
  case let x where x % 2 == 1:
    print("\(randomNumber) is odd")
  default:
    print("Invalid")
}

// Prints: 7 is odd
```

- Each `case` contains a variable declaration followed by a `where` clause and a condition. When a condition is `true`, the code for that `case` will execute. 

- The `let` keyword followed by the `x` creates a temporary binding to the `randomNumber` value. This means that the value of `x` temporarily becomes the value of `randomNumber`. If `randomNumber` is 5, then `x` is 5! 

- The `let` keyword is specifically used here instead of `var` since the value of `x` will not be reassigned at any point throughout the `switch` statement, thus its value will always be constant. If `var` is used, Swift will display a compiler warning recommending us to use `let` instead: 

```error 
Numbers.swift:6:12: warning: variable 'x' was never mutated; consider changing to 'let' constant
```

**Note**: a compiler warning is not an error. Your program should still run even with a warning.  

- Lastly, the `where` condition checks if `x` is divisible by 2 with or without a remainder and determines if the `randomNumber` is even or odd. 

If you run this code, chances are your output will be different from ours since the number generated each time is random!

Switch Statement: where Clause 

Another type of conditional statement that exists in Swift is the `switch` statement. The _switch statement_ is a popular programming tool used to check the value of a given expression against multiple cases. The `switch` statement is a lot more powerful in Swift than it is in other programming languages, thus we'll be dedicating the next few exercises to explore its features. 

Unlike the `if` statement, a `switch` statement does not check for the value of a condition and instead finds and matches a case to a given expression. 

Let's take a look at an example where a `switch` statement can be used. The code below uses multiple `else if` statements within an `if`/`else` to match a landmark to a given `city`:

```swift 
var city = "Rome"

if city == "Rapa Nui" { 
  print("Moai 🗿")
} else if city == "New York" {
  print("Statue of Liberty 🗽")
} else if city == "Rome" {
  print("Colosseum 🏛")
} else {
  print("A famous landmark is the Eiffel Tower!")
}
```

Since this code involves a series of `else if` comparisons, it's the perfect candidate for a `switch` statement rewrite:

```swift
switch city {
  case "Rapa Nui":
    print("Moai 🗿")
  case "New York":  
    print("Statue of Liberty 🗽")
  case "Rome":
    print("Colosseum 🏛")
  default: 
    print("A famous landmark is the Eiffel Tower!")
}
```

Notice how...

* Our new conditional begins with the `switch` keyword and is followed by the variable, `city` which acts as the expression. The value of the expression, originally `"Rome"`, is checked against each `case` within the `switch` block.

* The corresponding code to execute for a `case` is followed by a colon, `:`. 

* Once the value has been matched with a `case`, the code for that `case` is executed and the `switch` completes its checking. 

* Very much similar to an `else` statement, if a matching value isn't found, the `default` statement gets evaluated. 