Explain collections in Swift

Following are different types of collections in Swift:

Array

Set

Dictionary

Let's understand one by one : 

1. Array

• Most common and Widely used data type. 
• Ordered collection of similar types of data. 

Declaration: 

var normalArray = [Int]() // declaring an empty                        Int array like this

• The array can be any type in swift. 
• We can also declare with some initial values. 
• Swift is a strongly typed language so we don’t have to tell the type when we declare an array but we can also specify the type

var normalArray = [2,3,4,5]
var normalArray: [Int] = [2,3,4,5]

You also can declare an array with some repeated values or using one or more arrays

var normalArray = Array(repeating: 1, count: 3) //initialize normalArray with three ones var normalArray = arrayOne + arrayTwo //initialize normalArray with addign arrayOne & arrayTwo

• Insert elements into an Array: We can insert an element at the end of the array or in a valid position.

normalArray.append(12) //append element at the end of array
normalArray += [100.0] //append at the endnormalArray.insert(12, at: 0) //insert element at 0 position. This position value should be n to n-1 if array have n number of elements.

• Access Elements of Array: We can access the element of an array using a subscript syntax, passing the index value into it. Also swift provided a list of methods to get values and check different parameters.

normalArray[2]  //return second element from array
normalArray.count  // return the number of elements in array
normalArray.isEmpty  // return false if array is empty
normalArray.capacity  // return the number of elements in arraynormalArray.first  //get the first element from array
normalArray.last  //get the last elements from array
normalArray.max()  //get the max element from array
normalArray.min()  //get the min element from arraynormalArray.sorted()  // sort the array
normalArray.reversed()  //reverse an array
normalArray.shuffled()  //shuffle the elements of an array

• Replace elements of Array: Array value can set or replace in a specific index or for a range of the index.

normalArray[2] = 1 // replace the value in second index of arraynormalArray[1...3] = [12, 23, 34] // set first to third position value as 12, 23 &34

• Remove elements from Array: Array elements can remove from different ways & different positions.

normalArray.removeAll() // remove allelement from array
normalArray.removeFirst() // remove first element from array
normalArray.removeLast() // remove last element from arraynormalArray.remove(at: 2) //remove second element from arraynormalArray.removeLast(1) // remove the second element from the end index of arraynormalArray.removeFirst(2) // remove the thirdelement from the start index of the array

• Iterate over the array: Every element of an array can be traverse using loop

for i in 0..<normalArray.count{
    print(normalArray[i])
}or
for item in normalArray{
    print(item)
}

2. Dictionary

• Can hold multiple unordered data in key-value pairs. 
• Each value is associated with a unique key and any value from the dictionary can access with the associated key with it.

• Declaration: 

var normalDictionary = [Int: String]() 
// an empty "normalDictionary" with Int type key & String type value

• The key-value pair of a dictionary can be any type in swift. 
• We can also declare with some initial values. 
• Swift is a strongly typed language so we don’t have to tell the type of key-value pair when we declare a dictionary but we can also specify the type

var normalDictionary = [1: "John", 2: "Doe"]
var normalDictionary : [Int: String] = [1: "John", 2: "Doe"]

• Insert elements into a Dictionary: We can insert a value into the dictionary using a key. If the key already exists it will replace the previous value for this key elsewhere it will add a new value.

normalDictionary[3] = "Mihalos"  // Add a value "Mihalos" for key "3"

• Access Elements of Dictionary: We can access the elements of a dictionary by using the key. Also swift provided a list of methods to get values and check different parameters.

normalDictionary[2] //return the value of second key-value pair in dictionary normalDictionary.count // return the number of elements in dictionarynormalDictionary.isEmpty  // return false if dictionary is emptynormalDictionary.capacity  // return the number of elements in dictionarynormalDictionary.first  //get the first element from dictionarynormalDictionary.max { a, b in a.value < b.value}  // return key-value pair for maximum valuenormalDictionary.min()  { a, b in a.value < b.value}  // return key-value pair for minimumvaluenormalDictionary.sorted( by: { $0.0 < $1.0 }) // reurn an sorted array of dictionarynormalDictionary.reversed()  //return key-value pair in reverse ordernormalDictionary.shuffled()  //shuffle the key-value pair of dictionary

• Replace elements from a Dictionary: A value for a key is simply replaced by putting a new value for that key. We also can use “updateValue” method to replace the value for any key. But Remember “updateValue” method returns the previous value as output.

normalDictionary[3] = "Mike"
normalDictionary.updateValue("Duke", forKey: 3)

• Remove elements from Dictionary: Dictionary elements can be removed in a different way.

normalDictionary[3] = nil  // remove key-value pair for key "3"
normalDictionary.removeAll() // remove all element from dictionary arraysimpleDictionary.removeAll(keepingCapacity: true) remove all elements with keeping the capacity or not

• Iterate over the Dictionary: Every key-value pairs of a dictionary can be traverse using loop

for (key, value) in normalDictionary {
    print(key, value)
}

Instead of key-value pair we also can access only key or value from dictionary like this

for key in normalDictionary.keys{
    print(key)
}for value in normalDictionary.values{
    print(value)
}

We also can get the keys and values of a dictionary as an array.

let keys = [Int](normalDictionary.keys)  // return all the keys as an array
let values = [String](normalDictionary.values)  //return all the values as an array

3. Set

• Set is an unordered collection of the same type of data. 
• The main characteristic of the set is, set can not have any duplicate elements.

• Declaration: You can declare an empty Int type set like this

var normalSet = Set<Int>() // an empty Int type set

• The set can be any type in swift. 
• We can also declare with some initial values. 
• Swift is a strongly typed language so we don’t have to tell the type when we declare a set but we can also specify the type

var normalSet = Set([10, 20, 30, 40, 20])
varnormalSet: Set<String> = ["John", "Doe", "Hi", "John"]

You also can declare a set with some repeated values but you will get a single value for those repeated value cause set didn’t store any duplicate element.

var normalSet = Set(repeatElement(1, count: 3)) // Initialize set with three ones but set will have only one into it.

• Insert elements into a set: We can insert an element at the end of the array or in a valid position.

normalSet.insert(12) // //append element at the end of set

• Access Elements of a set: Like array you cannot access set elements using subscript syntax but you can check an element is in the set or not. Also swift provided a list of methods to get values and check different parameters.

normalSet.contains(2)  // check an element is in the set or not
normalSet.count  // return the number of elements in set
normalSet.isEmpty  // return false if set is                       empty normalSet.capacity                     // return the number of elements in setnormalSet.first  //get the first element from set
normalSet.max()  //get the max element from set
normalSet.min()  //get the min element from setnormalSet.sorted()  // sort the set
normalSet.reversed()  // reverse set
normalSet.shuffled() // shuffle set elements

• Remove elements: Set elements can remove from different way & from different positions also.

normalSet.removeFirst() // remove first element from set
normalSet.remove(1)  // remove from a specific indexnormalSet.removeAll()  // remove all elements from set
normalSet.removeAll(keepingCapacity: true)  // remove all elements with keeping the capacity or not

• Iterate over the set: Every element of a set can be traverse using loop

for data in normalSet {
    print(data)
}

The set type didn’t have any order so if you want to traverse in any order you can do it by swift provided methods

for data in normalSet.sorted() {
    print(data)
}for data in normalSet.reversed() {
    print(data)
}

• Different set operations: You also can perform different set operations within two sets and each of those operations will create a new set.

normalSet2.union(normalSet3).sorted()  // perform union opertion within normalSet2 & normalSet3
normalSet2.intersection(normalSet3) // perform intersection within two set
normalSet2.subtracting(normalSet3)  // perform subtracting
normalSet2.symmetricDifference(normalSet3)  // perform symmetricDifferencenormalSet2.isSubset(of: normalSet3)  // check one set is subset of snother or not 
normalSet2.isSuperset(of: normalSet3)  // check superset or not 
normalSet2.isDisjoint(with: normalSet3)  // check disjoint or not

There are some more interview questions answers for collections in this site. Visit here

Difference between Value type and Reference type

Value Types

A value type instance is an independent instance and holds its data in its own memory allocation. There are a few different value types: Struct, Enum, Tupple.

Struct

Let’s experiment with struct and prove that they’re value types:

Add the following code to your playground:

// 1
struct Car {
    let brand: String
    var model: String
}

// 2
var golf = Car(brand: "Volkswagen", model: "Golf")
// 3
let polo = golf

// 4
golf.model = "Golf 2019"

// 5
print(golf)
print(polo)

In the code above, you will:

• Create a Car struct with brand and model properties.
• Create a new instance of Car named golf.
• Create a copy of the golf instance, named polo.
• Change the golf.model variable to Golf 2019
• Print the 2 different instances. The first print statement prints Car(brand: "Volkswagen", model: "Golf 2019") in the Console. The second one prints Car(brand: "Volkswagen", model: "Golf"). Even if polo is a copy of golf, the instances remain independent with their own unique data copies.

With this simple playground, we’ve confirmed that structs are indeed value types.

Enum

To check that enums are value types, add this code to the playground:

// 1
enum Language {
    case italian
    case english
}

// 2
var italian = Language.italian
// 3
let english = italian

// 4
italian = .english

// 5
print(italian)
print(english)

In the code above, you will:

• Create a Language enum with italian and english cases.
• Create a new instance of Language for the italian language.
• Create a copy of the italian instance, named english.
• Change the italian instance to english.
• Print the two different instances. The first print statement prints english, and the second one prints italian. Even if english is a copy of italian, the instances remain independent.

Tuple

The last value type that we'll explore is tuple. A tuple type is a comma-separated list of zero or more types, enclosed in parentheses. You can access its values using the dot (.) notation followed by the index of the value.

You can also name the elements in a tuple and use the names to access the different values.

Add the following code to the playground:

// 1
var ironMan = ("Tony", "Stark")
// 2
let parent = ironMan

// 3
ironMan.0 = "Alfred"

// 4
print(ironMan)
print(parent)

In the code above, you will:

• Create an ironMan tuple with the strings Tony and Stark.
• Create a copy of the ironMan instance, named parent.
• Change the ironMan.0 index to Alfred.
• Print the 2 different instances. The first print, prints ("Alfred", "Stark") and the second one, prints ("Tony", "Stark"). Again, the instances remain independent.

You can now be certain that structs, enums, and tuples are value types! 

When to Use Value Types

Use value types when comparing instance data with == makes sense.
== checks if every property of the two instances is the same.
With value types you always get a unique, copied instance, and you can be sure that no other part of your app is changing the data under the hood. This is especially helpful in multi-threaded environments where a different thread could alter your data.

Use a value type when you want copies to have an independent state, and the data will be used in code across multiple threads.

In Swift, Array, String, and Dictionary are all value types.

Reference Types

In Swift, reference type instances share a single copy of their data, so that every new instance will point to the same address in memory. A typical example is a class, function, or closure.

To explore these, add a new function to your playground:

func address<T: AnyObject>(of object: T) -> Int {
    return unsafeBitCast(object, to: Int.self)
}

This function prints the address of an object, which will help you check whether you're referencing the same instance or not.

Class

The first reference type that you'll look at is a class.

Add the following code to your playground:

// 1
class Dog: CustomStringConvertible {
    var age: Int
    var weight: Int

    // 2
    var description: String {
        return "Age \(age) - Weight \(weight)"
    }

    // 3
    init(age: Int, weight: Int) {
        self.age = age
        self.weight = weight
    }
}

// 4
let doberman = Dog(age: 1, weight: 70)
// 5
let chihuahua = doberman

// 6
doberman.age = 2
// 7
chihuahua.weight = 10

// 8
print(doberman)
print(chihuahua)

// 9
print(address(of: doberman))
print(address(of: chihuahua))

In the code above, you will:

• Create a new class named Dog, that conforms to CustomStringConvertible to print the custom descriptions of the object.
• Define the custom description of the object.
• Create a new init function. This is needed because, unlike a struct, a class doesn't automatically create an initialization function based on the variables of the object.
• Create a doberman instance of Dog.
• Create a copy of doberman, named chihuahua.
• Change the doberman.age to 2.
• Change the chihuahua.weight to 10.
• Print the description of the two different instances. The first print, prints Age 2 - Weight 10, and the second one prints the same; Age 2 - Weight 10. This is because you're actually referencing the same object.
• Print the address of the two different instances. With these prints, you'll be sure that you're referencing the same address. You'll see that both print statements print the same value.

You can rest assured that a class is a reference type.

Functions and Closures

A closure is used to refer to a function along with the variables from its scope that it encapsulates. Functions are essentially closures that store references to variables in their context.

Take a look at the code below:

let closure = { print("Test") }
func function() -> (){ print("Test") }

closure()
function()

They both do the same thing.

You can find more info about closures in Swift's docs.

When to Use Reference Types

Use a reference type when comparing instance identity with ===makes sense. === checks if two objects share the same memory address.

They’re also useful when you want to create a shared, mutable state.

As a general rule, start by creating your instance as an enum, then move to a struct if you need more customization, and finally move to class when needed.