What is lazy property?

Ans : 

When to use lazy initialization is when the initial value for a property is not known until after the object is initialized.

For example, if you have a Person class and a personalizedGreeting property. The personalizedGreeting property can be lazily instantiated after the object is created so it can contain the name of the person.

class Person {      
  var name: String
    
  lazy var personalizedGreeting: String = {
    return "Hello, \(self.name)!"
  }()
    
  init(name: String) {
    self.name = name
  }
}

When you initialize a person, their personal greeting hasn’t been created yet:

let person = Person(name: "John Doe") // person.personalizedGreeting is nil But when you attempt to print out the personalized greeting, it’s calculated on-the-fly:

NSLog(person.personalizedGreeting)

Bonus Tip : You do need to declare your lazy property using the var keyword, not the let keyword, because constants must always have a value before initialization completes.

Benefit of lazy property increase performance in terms of speed.

Explain defer keyword or defer statement in Swift

Ans : Defer block is executed just before return statement or exit of block. Let's have a look to following chunk of code for better understanding

      func defer()  { 
          print("Start") 
          var value: String? 
         defer { 
              if let v = value { 
                   print("Ending execution of \(v)")
              } 
        } 
      value = "defer function" 
      print("End") 
   }

So printing sequence is like following :

Start
End
Ending execution of defer function

So in short defer block will be called lastly in the current block. It will be executed in any case like before return, before the break, before throw exception. Yes, if exception comes, then also defer block will be called before program crash or going to exception.

If there is multiple defer block in same method, Then defer block will execute from down to up side.

Write a program to find missing number of array of 1 to n.

Ans : 

      import Foundation

        

        let arr = [1,2,4,5,8,6,9,7]

        let n = arr.count

        let total = ((n + 1)*(n + 2))/2

        var missingNum = total

        for item in arr {

            missingNum = missingNum - item

        }

        print(missingNum)

Write a program to distinguish lowercase and uppercase character from String in swift

Ans : 

Method 1 : 

let string = "iOSiQA is Very Helpful WebSite to Prepare for iOS Interview."
var output = ""

for chr in string {
    var str = String(chr)
    if str.lowercaseString != str {
        output += str
    }
}
print(output)


>>> OSQAVHWPOSI

Method 2 : 

let testString = "iOSiQA is Very Helpful WebSite to Prepare for iOS Interview."
let lowerCase = NSCharacterSet.lowercaseLetterCharacterSet()
let upperCase = NSCharacterSet.uppercaseLetterCharacterSet()

for currentCharacter in testString.utf16 {
  if lowerCase.characterIsMember(currentCharacter) {
    println("Character code \(currentCharacter) is lowercase.")
  } else if upperCase.characterIsMember(currentCharacter) {
    println("Character code \(currentCharacter) is UPPERCASE.")
  } else {
    println("Character code \(currentCharacter) is neither upper- nor lowercase.")
  }

}

Method 3 : 

let testString = "Åke röstet un café in Владивосток!"
let lowerCase = CharacterSet.lowercaseLetters
let upperCase = CharacterSet.uppercaseLetters

for currentCharacter in testString.unicodeScalars {
    if lowerCase.contains(currentCharacter) {
        print("Character code \(currentCharacter) is lowercase.")
    } else if upperCase.contains(currentCharacter) {
        print("Character code \(currentCharacter) is UPPERCASE.")
    } else {
        print("Character code \(currentCharacter) is neither upper- nor lowercase.")
    }
}

Method 4 :   to identify character is uppercase or lowercase

extension Character {

        func isUpperCase() -> Bool {
            return CharacterSet.uppercaseLetters.contains(self.unicodeScalars.first!)
        }

        func isLowerCase() -> Bool {
            return CharacterSet.lowercaseLetters.contains(self.unicodeScalars.first!)
        }


  }

Write a program to reverse string in swift

Ans : 

Using Self made function : 

func reverse(_ s: String) -> String {
       var str = ""
       //.String is just like array of characters

       for character in s.characters {
            str = "\(character)" + str
            print ( str)
       }
   return str
}

print (reverse("!pleH"))


Using Swift Direct method : 

Swift 4.0

let str = "abc"

String(str.reversed())

or 

String(str.characters.reversed())

Difference between mutable array and immutable array?

Ans : 

Mutable Array content can be changed but Immutable Array content can not be changed.

You can insert, update, remove item from Mutable Array, but same is not possible in Immutable Array. There is no any methods like RemoveItem or ReplaceItem for Immutable Array, which are available for Mutable Array.

So in Immutable Array, content can not be changed after initialisation. It is either be initialised by content or by nil.

Use of Immutable array 

1. Used when we want to give array to other coder who can't be change array content.
2. Used when only fixed contents are there like array of months, days which can't be changed.

CollectionViewDelegate and CollectionViewDataSource Methods

Ans :
Popular methods is designed by blue color.

CollectionViewDelegate Methods : 

Managing the Selected Cells

func collectionView(UICollectionView, shouldSelectItemAt: IndexPath) -> Bool

Asks the delegate if the specified item should be selected.

func collectionView(UICollectionView, didSelectItemAt: IndexPath)

Tells the delegate that the item at the specified index path was selected.

func collectionView(UICollectionView, shouldDeselectItemAt: IndexPath) -> Bool

Asks the delegate if the specified item should be deselected.

func collectionView(UICollectionView, didDeselectItemAt: IndexPath)

Tells the delegate that the item at the specified path was deselected.

Managing Cell Highlighting

func collectionView(UICollectionView, shouldHighlightItemAt: IndexPath) -> Bool

Asks the delegate if the item should be highlighted during tracking.

func collectionView(UICollectionView, didHighlightItemAt: IndexPath)

Tells the delegate that the item at the specified index path was highlighted.

func collectionView(UICollectionView, didUnhighlightItemAt: IndexPath)

Tells the delegate that the highlight was removed from the item at the specified index path.

Tracking the Addition and Removal of Views

func collectionView(UICollectionView, willDisplay: UICollectionViewCell, forItemAt: IndexPath)

Tells the delegate that the specified cell is about to be displayed in the collection view.

func collectionView(UICollectionView, willDisplaySupplementaryView: UICollectionReusableView, forElementKind: String, at: IndexPath)

Tells the delegate that the specified supplementary view is about to be displayed in the collection view.

func collectionView(UICollectionView, didEndDisplaying: UICollectionViewCell, forItemAt: IndexPath)

Tells the delegate that the specified cell was removed from the collection view.

func collectionView(UICollectionView, didEndDisplayingSupplementaryView: UICollectionReusableView, forElementOfKind: String, at: IndexPath)

Tells the delegate that the specified supplementary view was removed from the collection view.

Handling Layout Changes

func collectionView(UICollectionView, transitionLayoutForOldLayout: UICollectionViewLayout, newLayout: UICollectionViewLayout) -> UICollectionViewTransitionLayout

Asks for the custom transition layout to use when moving between the specified layouts.

func collectionView(UICollectionView, targetContentOffsetForProposedContentOffset: CGPoint) -> CGPoint

Gives the delegate an opportunity to customize the content offset for layout changes and animated updates.

func collectionView(UICollectionView, targetIndexPathForMoveFromItemAt: IndexPath, toProposedIndexPath: IndexPath) -> IndexPath

Asks the delegate for the index path to use when moving an item.

Managing Actions for Cells

func collectionView(UICollectionView, shouldShowMenuForItemAt: IndexPath) -> Bool

Asks the delegate if an action menu should be displayed for the specified item.

func collectionView(UICollectionView, canPerformAction: Selector, forItemAt: IndexPath, withSender: Any?) -> Bool

Asks the delegate if it can perform the specified action on an item in the collection view.

func collectionView(UICollectionView, performAction: Selector, forItemAt: IndexPath, withSender: Any?)

Tells the delegate to perform the specified action on an item in the collection view.

Managing Focus in a Collection View

func collectionView(UICollectionView, canFocusItemAt: IndexPath) -> Bool

Asks the delegate whether the item at the specified index path can be focused.

func indexPathForPreferredFocusedView(in: UICollectionView) -> IndexPath?

Asks the delegate for the index path of the cell that should be focused.

func collectionView(UICollectionView, shouldUpdateFocusIn: UICollectionViewFocusUpdateContext) -> Bool

Asks the delegate whether a change in focus should occur.

func collectionView(UICollectionView, didUpdateFocusIn: UICollectionViewFocusUpdateContext, with: UIFocusAnimationCoordinator)

Tells the delegate that a focus update occurred.

Controlling the Spring-Loading Behavior

func collectionView(UICollectionView, shouldSpringLoadItemAt: IndexPath, with: UISpringLoadedInteractionContext) -> Bool

Returns a Boolean value indicating whether you want the spring-loading interaction effect displayed for the specified item.

UICollectionViewDataSource Methods : 

Getting Item and Section Metrics

func collectionView(UICollectionView, numberOfItemsInSection: Int) -> Int  (Required)

Asks your data source object for the number of items in the specified section.

func numberOfSections(in: UICollectionView) -> Int

Asks your data source object for the number of sections in the collection view.

Getting Views for Items

func collectionView(UICollectionView, cellForItemAt: IndexPath) -> UICollectionViewCell  (Required)

Asks your data source object for the cell that corresponds to the specified item in the collection view.

func collectionView(UICollectionView, viewForSupplementaryElementOfKind: String, at: IndexPath) -> UICollectionReusableView

Asks your data source object to provide a supplementary view to display in the collection view.

Reordering Items

func collectionView(UICollectionView, canMoveItemAt: IndexPath) -> Bool

Asks your data source object whether the specified item can be moved to another location in the collection view.

func collectionView(UICollectionView, moveItemAt: IndexPath, to: IndexPath)

Tells your data source object to move the specified item to its new location.

Configuring an Index

func indexTitles(for: UICollectionView) -> [String]?

Asks the data source to return the titles for the index items to display for the collection view.

func collectionView(UICollectionView, indexPathForIndexTitle: String, at: Int) -> IndexPath

Asks the data source to return the index path of a collection view item that corresponds to one of your index entries.

Write program for ascending sorting Int Array in any language

Ans : 
(Interviewer told syntax doesn't matter, write pseudo-code type program)

1. Not using direct function.

 void main()
    {

        int i, j, a, n, number[30];
        printf("Enter the value of N \n");
        scanf("%d", &n);

        printf("Enter the numbers \n");
        for (i = 0; i < n; ++i)
            scanf("%d", &number[i]);

        for (i = 0; i < n; ++i)
        {

            for (j = i + 1; j < n; ++j)
            {

                if (number[i] > number[j])
                {

                    a =  number[i];
                    number[i] = number[j];
                    number[j] = a;

                }

            }

        }

        printf("The numbers arranged in ascending order are given below \n");
        for (i = 0; i < n; ++i)
            printf("%d\n", number[i]);

    }

2. If we write in swift using higher order function, then

var numbers = [45,6,113,56,8,56,43,78]

print(numbers.sort()) //Sorting in Ascending order

print(numbers.sort(>)) //Sorting in Descending order

Interviewer mostly asked this question to check your programming logic. So he/she don't require syntax.

Q. Sorting strings array
A.
Method 1 : 
var sortedArray = swiftArray.sorted { $0.localizedCaseInsensitiveCompare($1) == ComparisonResult.orderedAscending }

Method 2 : 
let sortedNames = names.sort { $0.name < $1.name }
let sortedNames = names.sorted(by: <)

Difference between Swift Array and Objective C Array

Ans : Swift arrays (Array and for short, []) are passed by value which means
that every object contained will be copied.

NSArray are implemented as classes (and bridged from ObjC) so they're passed as references.

In Swift, we can declare three type of array.
One is Array or [],
Second is NSArray,
Third is NSMutableArray.

NSArray and NSMutableArray are coming from Objective C by bridge. So interviewer may ask what are difference between types arrays in swift?

Extract value from Array : 

Get 3rd Value from Array

Objective C : [arr objectAtIndex:2] // 2 due to index start from 0 in array

Swift : arr[2] 

Equilibrium Problem - Left and right side and top and bottom side sum of any point are same in matrix

Problem Description :

#import <Foundation/Foundation.h>


int leftArraySum(NSMutableArray *A,int n)
{
    NSInteger sum  = 0;
    for(int i=0;i<n;i++)
    {
        sum += [[A objectAtIndex:i] integerValue];   
    }
    return sum;
}

int reightArraySum(NSMutableArray *A,int n)
{
     NSInteger sum  = 0;
    for(int i=n+1;i<[A count];i++)
    {
        sum += [[A objectAtIndex:i] integerValue];   
    }
    return sum;
}

int topArraySum(NSMutableArray *A,int n)
{
     NSInteger sum  = 0;
    for(int i=0;i<n;i++)
    {
        sum += [[A objectAtIndex:i] integerValue];   
    }
    return sum;
}

int bottomArraySum(NSMutableArray *A,int n)
{
     NSInteger sum  = 0;
    for(int i=n+1;i<[A count];i++)
    {
        sum += [[A objectAtIndex:i] integerValue];   
    }
    return sum;
}

int solution(NSMutableArray *A) {
    // write your code in Objective-C 2.0
 
    NSMutableArray *rowArr = [A objectAtIndex:0];
    int colCount = [rowArr count];
    int rowCount = [A count];
 
    NSMutableArray *arrRowSum = [[NSMutableArray alloc] initWithCapacity: rowCount];
    NSMutableArray *arrColSum = [[NSMutableArray alloc] initWithCapacity: colCount];
 
    for(int i=0;i<rowCount;i++)
    {
        NSArray *colArr = [A objectAtIndex:i];
        NSNumber * sum = [colArr valueForKeyPath:@"@sum.self"];
        [arrRowSum insertObject:sum atIndex:i];
    }
 
    for(int i=0;i<colCount;i++)
    {
     
        NSInteger sum = 0;
        for(int j=0;j<rowCount;j++)
        {
           sum +=  [[[A objectAtIndex:j] objectAtIndex:i] integerValue];
        }
        NSNumber *sumVal = [NSNumber numberWithInt:sum];
        [arrColSum insertObject:sumVal atIndex:i];
    }
 
    int result = 0;
    for (int i=1;i<rowCount;i++)
    {
        int left = leftArraySum(arrRowSum,i);
        int right = reightArraySum(A,i);
        if(left  == right)
        {
            for(int j=0;j<colCount;j++)
            {
                int top = topArraySum(A,j);
                int bottom = bottomArraySum(A,j);
                   if(top == bottom )
                {
                    result = i+j;
                    break;
                }
            }
        }
    }
 
    return result;
}