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variables, Constants, and types

Variables and types

  • A Variable is a temporary container for a specific type of value.
var count: Int
var shouldReminad: Bool
var text: String
var list: [ChecklistItem]

  • Data type, or just type, of a variable determines what kind of values it can contain. Some variables hold simple values such as Int or Bool, others hold more complex objects such as String or Array.

  • Double : Similar to a Float but with more precision.

  • Character: Holds a single character. A String is a collection of Characters.

  • UInt: A variation of Int that you may encounter occasionally. The U stands for "unsigned", meaning the data type can hold positive values only. It's called unsigned because it cannot have a nagative sign (-) in front of the number. UInt can store numbers between 0 and 18 quintillion, but no negative numbers.

  • Int8, UInt8, Int16, UInt16, Int32, UInt32, Int64, UInt64.: These are all variations on Int. The difference is in how many bytes they have available to store their values. The more bytes, the bigger the values they can store.

  • CGFloat.: This isn't really a Swfit type but a type defined by the iOS SDK. It's a decimal point number like Float and Double. For historical reasons, this is used throughout UIKit for floating-point values.( The "CG" prefix stands for the __C__ore __G__raphics framework.)

Swift won't automatically convert between differnet number types. You always need to convert the values explicitly.

var i = 10
var f: Float
f = i				// error
f = Float(i) 		// OK

You don't always need to specify the type when you creat a new variable. If you give the variable an initial value, Swift uses type inference to determine the type:

var i = 10				// Int
var d = 3.14				// Double
var b = true				// Bool
var s = "Hello, world"	// String

"Hello, World" are named literal constants or just literals.

Type annotation

var f: Flaot = 3.14
var i: Double = 10
var i = 10.0

Instantiate

var item: ChecklistItem

item = ChecklistItem()

-----------------

var item = ChecklistItem()

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var item = CHecklistItem(text: "Charge my iPhone", checked: false)