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Showing posts with label Collections. Show all posts
Showing posts with label Collections. Show all posts

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.

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
Method 1 : 
var sortedArray = swiftArray.sorted { $0.localizedCaseInsensitiveCompare($1) == ComparisonResult.orderedAscending }

Method 2 : 
let sortedNames = names.sort { $ < $ }
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] 

Why array method containObject has id as parameter in ios?

Ans : id is a generic type. This means that the compiler will expect any object type there, and will not enforce restrictions. It can be useful if you're expecting to use more than one class of objects there. So you can add anything of type id to an NSArray.

So array method containObject has id type as parameter.

- (BOOL)containsObject:(ObjectType)anObject;

Usage :

bool bVal = [arr containObject:@5];

How to take common elements from two array in ios?


Swift higher order function is very useful. See below example.

let fruitsArray = ["apple", "mango", "blueberry", "orange"]
let vegArray = ["tomato", "potato", "mango", "blueberry"]

// only Swift 1
let output = fruitsArray.filter{ contains(vegArray, $0) }

// in Swift 2 and above
let output = fruitsArray.filter{ vegArray.contains($0) }
// or
let output = fruitsArray.filter(vegArray.contains)

Array vs Set

let array1: Array = ...
let array2: Array = ...

// Array
let commonElements = array1.filter(array2.contains)

// vs Set
let commonElements = Array(Set(array1).intersection(Set(array2)))

// or (performance wise equivalent)

let commonElements: Array = Set(array1).filter(Set(array2).contains)

Another way in Swift 4.0

   var someHash: [String: Bool] = [:]

   fruitsArray.forEach { someHash[$0] = true }

   var commonItems = [String]()

   vegArray.forEach { veg in
    if someHash[veg] ?? false {


What are higher order functinos in swift?

Ans : Higher order functions are functions that operate on other functions by taking function as argument or returning function.

 Sorted : Sorting array. It may takes closure as argument and returns array .
let numbers : [Int] =  [1,5,2,4,3]
let arrNumbers = numbers.sorted()

By default it returns in ascending order.

numbers.sorted(by: (Int,Int) -> Bool)  - > How we want to sort array then sorted(by:)
numbers.sorted((a,b) -> Bool in return a > b } // descending order
numbers.sorted(by : >)

      Map : It iterates through the array that is calling it and changes each element of the array based on the closure passes to the method.

Use map to loop over a collection and apply the same

operation to each element in the collection. It returns after

applying transform.

let arrInt = [1,8,4,6]
I want add 1 in every number in array.
Without loop, using map, $0 + 1)  // <- This is shortest code for doing map

output : [2,9,5,7]

How map works :

The map function has a single argument which is a closure (a

function) that it calls as it loops over the collection. This

closure takes the element from the collection as an argument and

returns a result. The map function returns these results in an


Another example :

let chocolateAmt = [“Dairy Milk”:20.0, “Munch”:10]

I want to increase price 5%,
so using map,{ (key, value) in
   value + value*0.05

output : [“Dairy Milk”:21.0, “Munch”:10.5]

Another Example :

If we want index in map,

let arrInt = [1, 2, 4, 5]
let indexElement = arrInt.enumerated().map { (index,element) in
return "\(index):\(element)"
print(indexElement) // [“0:1”, “1:2”, “2:4”, “3:5”]

CompactMap: Use this method to receive an array of nonoptional values when your transformation produces an optional value.

let scores = ["1", "2", "three", "four", "5"]

let mapped: [Int?] = { str in Int(str) }
// [1, 2, nil, nil, 5] - Two nil values as "three" and "four" are strings.

let compactMapped: [Int] = scores.compactMap { str in Int(str) }
// [1, 2, 5] - The nil values for "three" and "four" are filtered out. 

FlatMapFlatmap is used to flatten a collection of collections.

Flatmap is joined word of Flat + Map. So as per name, it applies map function over collection and do flatten collection.

Test 1 :

let codes = [["abc","def","ghi"],["jkl","mno","pqr"]]
let newCodes = codes.flatMap { $ {$0.uppercased()} }

Output : ["ABC","DEF","GHI","JKL","MNO","PQR"] 

Test 2 :

let codes = ["abc","def","ghi"]
let newCodes = codes.flatMap { $0.uppercased() }

Output : ["A","B","C","D","E","F","G","H","I"]

-> In Test 2, first it applies map so half output is like 

["ABC","DEF","GHI"] and after that it applies flat so now full 

output is : ["A","B","C","D","E","F","G","H","I"]

Tip : 

1. String is collection from Swift 4.

2.If you do flatmap a collection containing optional values, 

flatmap will only consider the non-nil values.

let codes = [1,2,nil,3,nil,5]

let newCodesFlatMap = codes.flatMap { return $0 }
output : [1,2,3,5]

let newCodesFlatMap = { return $0 }

output : [optional(1),optional(2),nil,optional(3),nil,optional(5)]

The output of map became a collection of optional int ([Int?]) 

only because the array had nil — value in it. Otherwise it would 

have been an Int array. It is benefit of using flatMap.

Filter : Return array with elements which fulfill filter condition.           
let numbersLessThanFive = numbers.filter { (a) -> bool in return a  <  5 }
let numbersLessThanFive = numbers.filter { $0 < 5}

          Reduce : It is used to combine all element in array to make one single value.
let sumOfNumbers = numbers.reduce(0, { $0 + $1 }) 

Difference between map and flatmap : 

Big difference is Map considers nil but flatmap  remove nil value from collection.