If statements in swift - xcode

Question

I am new to swift and trying to understand this code that I am reading out of a book. Can somebody explain the if statement that sets the private searches Dictionary? Also what does the statement pairs as [String : String] mean? I am confused about the term as.Also I don't understand how the if statement is executed if you are declaring a constant and not comparing it? Shouldn't the if statement check for something then execute the following code if that is true?
import Foundation
// delegate protocol enables Model to notify controller when data changes
protocol ModelDelegate {
func modelDataChanged()
}
// this will manage the saved searches
class Model {
// keys used for storing the app's data in app's NSUserDefaults
private let pairsKey = "TwitterSearchesKVPairs"
private let tagsKey = "TwitterSearchesKeyOrder"
private var searches: Dictionary <String, String> = [:] // stores tag-query pairs
private var tags: Array<String> = [] // stores tags in user-specified order
private let delegate: ModelDelegate // delegate is MasterViewController
// initializes the Model object aka constructor
init(delegate: ModelDelegate) {
self.delegate = delegate
// get the NSUserDefaults object for the app
let userDefaults = NSUserDefaults.standardUserDefaults()
// get Dictionary of the app's tag-query pairs
if let pairs = userDefaults.dictionaryForKey(pairsKey) {
self.searches = pairs as [String : String]
}
// get Array with the app's tag order
if let tags = userDefaults.arrayForKey(tagsKey) {
self.tags = tags as [String]
}

Answer 1

"if let" is for conditional unwrapping and used to safely unwrap an optional value. "as" is used for casting but in this case you need to use as? (conditional casting). You can combine both in just one sentence if you want:
// get Dictionary of the app's tag-query pairs
if let pairs = userDefaults.dictionaryForKey(pairsKey) {
self.searches = pairs as [String : String]
}
// get Array with the app's tag order
if let tags = userDefaults.arrayForKey(tagsKey) {
self.tags = tags as [String]
}
can also be written as :
if let searches = NSUserDefaults.standardUserDefaults().dictionaryForKey(pairsKey) as? [String : String] {
println(searches.description)
}
if let tags = NSUserDefaults.standardUserDefaults().arrayForKey(tagsKey) as? [String] {
println(tags.description)
}
// [String : String] means a Dictionary where its keys and values are String types
NSUserDefaults.standardUserDefaults().setObject(["book":"brown","car":"black"], forKey: "searches")
if let searches = NSUserDefaults.standardUserDefaults().dictionaryForKey("searches") as? [String : String] {
println(searches["book"]!) // "brown"
println(searches["car"]!) // "black"
}