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,

arrInt.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

array.

Another example :

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

I want to increase price 5%,

so using map,

chocolateAmt.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?] = scores.map { 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. 

FlatMap : Flatmap 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.map {$0.uppercased()} }

print(newCodes)

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

Test 2 :

let codes = ["abc","def","ghi"]

let newCodes = codes.flatMap { $0.uppercased() }

print(newCodes)

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 = codes.map { 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.