declaring variables

var mutableDouble = 1.0
mutableDouble = 2.0

let constantDouble = 1.0
// errror constantDouble = 2.0

var optionalDouble:Double ?= nil
optionalDouble = 1.0

if let definiteDouble = optionalDouble {
definiteDouble
}

variable types

Int 1,2,500
Float 0.5
Double
Bool true, false
String "string"
Classname UIView, UIImage