Swift Map , Filter , Reduce

-

Swift Map , Filter , Reduce

Higher order function as defined in computer science , does at-least one of the following:

 1. returns a function as a result
2. takes one or more functions as arguments.

 Objective-C does not have direct support for higher order functions, but Swift has, which is good.

 There are several examples of higher order functions, some of them are map, filter , reduce .

 Let's check them out with Swift.

Map

Definition:

"Returns an array containing the results of mapping the given closure over the sequence’s elements."

 So we know now , map always returns an Array.

Example:
Apply an offset of 10 to userIndexes.
let userIndexes = [11,12,13,14]

let offsetUserIndexes = userIndexes.map { (userIndex) -> Int in
    return 10 + userIndex
}


let offsetUserIndexes = userIndexes.map { (userIndex) -> Int in
    return 10 + userIndex
}


Output:

[21, 22, 23, 24]

Here, we pass a function to get the result, the function adds an offset to the userIndex.  Perhaps we now know why they are called higher order functions, it helps achieve (requirement no. 2) as mentioned at the top of this page.

 There are many other ways to get the same output , basically trying to shorthand the statement.

Option 1:

Remove -> , which separates the method parameter from the function return type. Also, we don't need the return type as it's inferred from return statement.

userIndexes.map { (userIndex)  in
    return 10 + userIndex
}


Option 2 :

Remove the return from the above statement, as the Swift closures understands the return type from the operation.

userIndexes.map { (userIndex) in
    10 + userIndex
}



Option 3 :

Use of shorthand arguments. Closure with one parameter , can be replaced with $0 . It would mean we are accessing userIndex. In which case in can be eliminated too.

Remove (userIndex) and userIndex from Option 2. Also remove
"in" . Replace the userIndex with $0, which means the first parameter in the closure.

userIndexes.map { 10 + $0 }

Map on Dictionary.

Map can also be applied to dictionary. In any case output will be still be an Array. Array of String ,  Double or it can be Any Object.

 Example:
From the grocery list , get me the items which were purchased.
let groceries = ["Orange" : 4.0 , "Milk" : 3.0 , "Bread" : 2.0 ]


let groceryItemList = groceries.map { (key: String, value: Double) -> String in
    return key
}

Output:

["Milk", "Bread", "Orange"]

Filter 


 "Returns an array containing, in order, the elements of the sequence that satisfy the given predicate."

 Filter also returns an Array like Map, but the only difference here is ,it returns the item in the collection which meets the requirement.

There is no transformation applied to collection unlike Map

Example:

Give me the userIndexes which are greater than or equal to 13.

let userIndexes = [11,12,13,14]

let userIndexesGreaterThan13 = userIndexes.filter { (index) -> Bool in
    return index >= 13
}


Output:
[13, 14]

Comments

Post a Comment

Popular posts from this blog

hitTest on iOS to identify the view being hit

-
Image
HitTest on iOS Use hitTest, to identify the view being tapped. The below method allows us to identify the view being selected. We can use it, to identify the view from which we would like to hide the touch events or identify the view we are interested into. Example: In the below case when we tap on Black View we can show an alert like "Black View tapped!" , if we tap on Red View , we can show an alert  "You are not allowed to tap on Red View" .  override func hitTest ( _ point: CGPoint , with event: UIEvent ?) -> UIView ? { } Here is a simple implementation of the method, to identify the view being hit. View hierarchy being used is listed below:- Blue(parent) - Red(child) - Green(child) - Yellow (child) - Orange (child) - Black (child) - White (child) So lets' start with override of hitTest method.      override func hitTest ( _ point: CGPoint , with event: UIEvent ?) -> UIView ? {         //1         if let viewAvailable = findPoin
Read more

CMTimeMakeWithSeconds explained

-
Image
CMTime  is structure which has two main components,  timeScale  and  value. CMTIME = VALUE  / TIMESCALE where, TimeScale =  fraction of a second each unit in numerator occupies. CMTIME = Value / TimeScale 1)  public   func  CMTimeMake( _  value:  Int64 ,  _  timescale:  Int32 ) ->  CMTime CMTimeMake ( 6000 ,  10 ) Means, there are 6000 units , each unit occupies 1/10 of a second 2)  public   func  CMTimeMakeWithSeconds( _  seconds:  Float64 ,  _  preferredTimescale:  Int32 ) ->  CMTime CMTimeMakeWithSeconds(10.0,500) 10.0 = VALUE /500 Therefore VALUE = 5000 So, there are 5000 units, each occupy 1/500 of a second
Read more

How to set Custom Section Header in UITableView for ios sdk version greater than 6

-
Image
Dear Friends, As we know, that to set a custom Section Header in a TableView we need to call the following. Here we pass our own custom View. But, remember the view which we returned was never reusable, hence not memory efficient. -( UIView *) tableView:( UITableView *)tableView viewForHeaderInSection:( NSInteger )section Later version of iOS , starting from iOS 6, helps to make our code Memory efficient with few lines of code. Still here the delegate method remains the same. With the help of this tutorial we will be able achieve a Custom Header implementation using iOS 6 or greater sdk. We will be creating a app. which would look like as shown below. Please use the following steps to achieve this: Step 1: Create a .xib file for the custom header 1. Press command + N 2. Select User Interface in Left Pane. 3. Then Select a View and name it as TableHeader. Step 2: Create a Custom class for the custom header in Step 1 1. Press command
Read more