carla_notes_oct20.md

Control Structures

Control structures are things you can put in your code to perform various logic operations. (i.e. perform a line of code multiple times)

There are a few but there are 2 important ones to know:

• if statments
• for loops

if Statements

An if statment is simple. It takes a Boolean (Bool in Swift) and performs a block of code if that boolean is true. (remeber booleans are true or false).

let sampleValue = true

if sampleValue {
  print("Hello, Carla!") // This will always run since sampleValue is true
}

if statements can also have else if and else clauses. These are also pretty straightforward; an else if is just like an if and takes a boolean. However, if will run its block of code only if the boolean is true AND an if or else if clause before it hasn't run. You can have as many else ifs in an if statement as you like.

else is a catch all at the end of the statement. There can only be one else clause and it will run if no other clauses have.

Remember, for an if statement, only one clause will run.

Aside: Boolean operators. There are some operators to make more complex boolean expressions. One of them is '==' (double equals). This takes a left and a right object and evaluates to true if the left and right objects are equal to eachother.

Aside 2: Parentheses. Parentheses work the exact same way as they do in math. Expressions inside parenthese will be evaluated before other parts of the expression

(Also when I say 'expression' I just mean a statement/piece-of-code/wahtever you want to call it).

var firstCondition = false
var secondCondition = "dog" == "cat" // This will be false since the `String`s `"dog"` and `"cat"` are not equal.
var thirdCondition = (2 + 3) == 5 // This will be true since (2 + 3) equals 5

if firstCondition {
    print("This will never happen since firstCondition is false")
} else if secondCondition {
    // This also won't get called (note that if statements don't need anything inside of them, although that would be kinda 
    // dumb)
} else if thirdCondition {
    print("yay I love math! Not. Thats why I switched from math to programming.") // This will be executed
} else {
    print("everything was false! I'm so sad :(") // This won't be executed since everything else was
}

Arrays

Just like Ints, Strings, and Bools (among others), Arrays are another basic type of object in programming. Arrays are just lists of other objects.

Arrays are declared with a type and [TypeGoesHere] (brackets). The type represents the type of the object that the Array can hold. Remember, Swift is smart and can usually infer the type of an Array so you don't need to write the type unless you want to (just like other objects).

An array can only hold objects of its type, or types that 'inherit' from its type. (inherit means that a type is a subtype of another type. Cat 'inherits' from Animal)

The Array type also has the function .append(...) which lets you append an element to the array. (Remember how we defined our own functions for objects? Swift objects have lots of these).

var birdNameArray = ["chicken", "duck", "sparrow"] // birdNameArray is now an array of type `String`
birdNameArray.append("hawk") // now `birdNameArray` has the contents ["chicken", "duck", "sparrow", "hawk"]
birdNameArray.append(12) // This won't work, `4` is an `Int` not a `String`

// Assuming `Cat: Animal` and `Dog: Animal`

var animals: [Animal] = [] // Creates an empty array that holds `Animal`s. Note that the type needs to be explicitly defined 
                           // since the program 
                           // doesn't know whats going to be in the array without it.

animals.append(Cat(...)) // Remember that Cat(...) creates a new `Cat` instance. This will be fine since `Cat` has `Animal` as 
                         // a supertype
animals.append(Dog(...)) // This will also be fine as `Dog` inherits from `Animal`
animals.append("Hello I'm a String") // Not fine. Wrong type. Crashy crashy.

You can also access specific items in the array by using the syntax arrayVariable[indexToAccess] where index is an Int. The first item in the array has index 0, the second has index 1, etc.

Warning: if you try and access an index higher than the number of items in the array (commonly referred to as 'the index out of bounds') the program will crash. RIP 🙏🏼

var array = [true, false, true, true, true] // An array of booleans

var firstItem = array[0] // firstItem will be `true`
var thirdItem = array[2] // `true`

var tenthItem = array[9] // program will crash since 9 is out of bounds (the array only has 5 items)

Aside: Literals. Some of the more basic objects in swift (Array, Int, String, Double) are able to be represented in 'literal' form. This is just typing out what you want the value to be: "Hello World", 34, 23.555, ["duck"] to initialize the object. Most other objects and all objects you create need to be initialized with initializers (Cat(name: "fifi", etc.)).

for Loops

for loops are useful for performing the same operation multiple times or with multiple elements. The format is like so: for variable in array { ... } where variable is a name you give to each element as you loop through and array is an object of the array type.

var numberArray = [1, 2, 3, 4, 5, 6, 7, 8, 123586]
for item in numberArray {
    print("item is: \(item)") // Will print out every item in the list one at a time
}

Ranges

You can also use a special Array-like object called a Range to loop a certain amount of times.

for i in 0...100 { // this is the syntax for a `Range`, both 0 and 100 will be included. This prints all numbers 0-100
    print(i)
}

BONUS Control Structure: switch

The switch is very similar to an if statement. A switch statement takes a variable, and then gives a bunch of case statements that will run if that case's value equals the original variable. In swift, all switch statements must have a default case, which is like an else at the end of an if; it will run its block of code if no other cases matched.

var someString = "Taco Bell"

switch someString {
case "dog":                  // This doesn't match the given variable and wont run. Notice there aren't `{}` in each case.
    print("its a dog!")      
case "cat":
    print("its a cat!")
case "chair":
    print("its a chair!")
case "car":
    print("its a car!")
case "something":
    print("its a something!")
case "duck":
    print("its a duck!")
default:
    print("we don't know what it is :(")    // This will run. No other case statements matched the given pattern.
}

You may ask, "Why use a switch statement when you can just do if someString == "dog", else if ..."? Well good question. Technically you never need to use, but sometimes switch statements are much easier to read and understand if you are comparing against many values. In general you want code you write to be as easy to follow and understand as possible, so that future you or other people using your code will have an easy time.