Notes for October 2nd session

carla_notes_oct2.md

Programming

• Computer/Phone runs the program line by line
• 'Comments' are used to tell the computer to ignore lines of code (useful for keeping notes in the code)
• in Swift comments are created with // for single line comments or /* stuff here */ for multi-line comments

Swift

• The language we have been working with. Created by Apple and primarily used for making iPhone/Mac apps.
• 'Statically typed'
  • this means each variable has a type that cannot be changed once set
  • the type is set when the variable is first created (with let or var) using 2 ways:
  • 'implicit': let stringVar = "Hello" The computer knows stringVar is of type String
  • 'explicit': you define the type. let stringVar: String = "Hello". In this case you don't need to define it, but it can be useful elsewhere.
  • some minor exceptions to a variable staying 1 type: an Int (Integer number) can be assigned to a variable of type Double. The computer knows how to turn (cast) an Int into a Double (just add a .0; 4 is the same as 4.0)

Variables

In Swift declare a variable 2 ways: - let: this variable cannot be changed (mutated) - var: this variable can be mutated

There are 2 types of variables

1. Primitive types (the smallest, most basic types. In general these are in all languages.):

   • String: a piece of text. Must have "" around it. e.g. "Hello, my name is Josh!"
   • Int: a whole number (Integer). e.g. 4
   • Double: A decimal number e.g. 4.0, 4.1, 43242.23748374782
   • Bool: A 'boolean' value. A boolean value is a value that is either true or false

2. Objects (Not in all languages, but in almost all modern languages) a programming language that uses 'Objects' is called an "Object-oriented" language.

   • custom defined objects
   • can be written by you or be already built into the language you are using.
   • to write a custom type or 'Object' you use a class as a blueprint

Code examples:

// Comments:

// Single line comment
/*
Multi
Line 
Comment

Everything between the two symbols is ignored when running the program

*/

/* Variables: */
let immutableVariable = "You can't change me because I was created with a let."
immutableVariable = "Hello world" // This line will throw an error because `let` variables can't be changed

var mutableVariable = "You can change me"
mutableVariable = "New value" // Will work fine because variable was declared as a `var`

/* Statically typed: */
var stringVariable = "Hello world"
var doubleVariable = 12.0
var boolVariable: Bool = false // The `: Bool` is unnecessary, but can sometimes be useful for readability

print(stringVariable) // will print "Hello world"
print(doubleVariable) // will print 12.0

stringVariable = 12 // will throw an error because stringVariable has type `String` and 12 is an `Int`
doubleVariable = 10 // will NOT throw an error because while 10 is an `Int` the computer is able to 'cast' it to a Double

print(doubleVariable) // will print 10.0 since the `Int` 10 was cast to a Double

/* Defining an Object */

// Blueprint for a TodoItem object that we might use for our todo list
class TodoItem: NSObject { // Most Swift objects need to have : NSObject after their definition. Will explain someday later
    
    // Define variables of the Object here. Each todo item has a name and an isCompleted (that keeps track of if it is 
    // completed)
    //
    // A variable inside an Object is also called a 'property' of that object (here `name` is a property of `TodoItem`)
    var name: String
    var isCompleted: Bool
    
    // This is an 'initializer' and is how you create an object. Remember, a `class` is a blueprint for an object. To create
    // an object to use you need to call the initializer. This creation process is called 'instantiation' and the created 
    // object is an 'instance' of it's type.
    //
    // The format for calling an initializer (and thus creating an object) is ObjectTypeName(). In this case it would be
    // TodoItem(). This would return a TodoItem with `name` and `isCompleted` properties for you to use.
    init() {
        name = "A default name"
        isCompleted = false
    }
}

/* Creating an Object */
let myTodoItem = TodoItem() // Initializes a TodoItem using the function `init` from above
print(myTodoItem.name) // will print "A default name" since that's what we set it to in the initializer

myTodoItem.name = "New name"

print(myTodoItem.name) // will print "New name". Properties of Objects can be modified just like any variable. Objects are 
                       // essentially a variable that contains multiple other variables, as defined in the class you create.