Guard in Swift

Guard statement is similar to IF statement except the fact that body of code executes when the condition in guard statement is evaluated to false.

var citizenAge:Int  //Declare a variable of type Integer

citizenAge = 65

guard citizenAge < 60  else {

// Here citizen age is 65, the guard condition is evaluated to false and the below statement is executed.

print("Senior Citizen")

return
}

print("Not a Senior Citizen") // this is executed when guard condition is true.

Guard Statement can also be used for optional binding

var playerScore:Int?    //Declare a variable a of type Optional Integer, which might or might not have value (for e.g if player did not bat, he might not have any score)

func updatePlayerScore (playerScore:Int?){

    guard let score =  playerScore else {

        print(“Player did not bat“)

        return 

    }

    print("Player score is \(score)")

}

• Unlike if statement values assigned in the Guard statement are still available after the Guard statement through out the remaining scope that encloses the Guard Statement
• Guard Statement are typically written at the top of the enclosing scope

Swift Data Types

There are two fundamental data types in Swift

Value Types 

• Int, Floating-Points, Booleans, Characters, Strings , Arrays, Dictionaries and Tuples (which are implemented as Structures)
• Optionals (which are implemented as Enumerations)

Reference Types

• Classes
• Functions and Closures 

Optionals in Swift

var a : Int            // create a swift variable of type int

var b = a + 10      //compiler will thrown an error because a is not initialized

Alternative to above

When your using ‘?”/optional , that means your explicitly telling the compiler that a may or may not have the value.

var a : Int?

var b = a! + 5     // the compiler suggest to unwrap the optional, this is called force unwrapping and program will crash if in case a does not have value so force unwrapping of optionals is always dangerous and should not do it unless your sure about var holding a value.

then what is the safe way to deal with optionals? answer is optional binding 

if let nonOptionalValue = a {

var b = nonOptionalValue + 5  // if a is nil, this statement is not executed and force unwrapping is not required in this case because we knew that a is not nil and has a value for sure.

}

Pointers

In Objective C all objects are referenced using pointers:

A pointer is a variable which stores memory address instead of value itself.

how big is int, 4 bytes

how big is char, 1 byte

how big is String, image or video?  we often don’t know and it might even change during our program so we need more flexible way to manage objects and pointers are one way of allowing us to do that.

Using pointers allows us to pass around the objects (just by passing around the address of object) without having need to copy them which is time consuming and inefficient  

E.g.   NSString* welcomeString;

         NSDate* todayDate;

welcomeString = @“Welcome to Objective C”;

Here the string 'Welcome to Objective C’ string is not directly stored in ‘welcomeString’ instead the address/memory location of 'Welcome to Objective C' is stored in ‘welcomeString which is contrary to 'int highScore = 100’ where ‘highScore’ variable directly stores value. ( this is because objects are more complex than primitive types)

if you want to make and use of simple object, we need pointers to reference them.

Note: Explicit use of pointers in Swift is not required unlike Objective C. e.g let someInstance = SomeClass ( ) whereas in objective C it is SomeClass *someInstance = [SomeClass alloc] init];

Structures in Objective C and Swift

Structure is a complex data type which contains individual elements which defer in type. 

There are many situations in programming where discrete types have to be identified together as one unit. In this situations we make use of Struct.

Structs are used to represent a record, Suppose you want to keep track of your books in a Library. you might want to track the following attributes about each book.

Struct Book {

Title

Author 

Subject

Book ID

};

Struck Person {

Name

Age

Height

Weight

Gender

Color

};

Individual elements of Struct are referred to as members

Unlike Objective C, classes in Swift, structures can have methods, properties, initializers, and conform to protocols. The main difference between classes and structures is that classes are passed by reference, while structs are passed by value.

Set vs Array

The main difference is that NSArray is an ordered collection and NSSet is an unordered list of unique elements.

Set

• Primarily access items by comparison   

E.g    let ingredients = [“Tea Powder”, “Sugar”, “Hot Water”, “Milk”, "Cardamom"]

          if ingredients.contains(“Cardamom”) {

print(“It is Cardamom Tea”)
}

• unordered
• does not allow duplicates

Array

• Can access items by Index
• Ordered
• Allow duplicates

Dynamic Typing & Dynamic Binding:

Dynamic Typing in Objective C means that the class of an object type id is unknown at the compile time and is discovered at the run time based on the message sent to the Object.

Dynamic typing allows us to declare a variable that is capable of storing any type of object regardless of its class origin, this is achieved using the Objective C id type, the id type is a special, general purpose data type that can be assigned an object of any type.

Dynamic binding takes this one step further by allowing methods on object type id to be called without prior knowledge of the type of an object currently assigned to the variable.