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SOLVED: Checkpoint 5 Xcode warning

Forums > 100 Days of SwiftUI
@ntfontoura  

Hi everyone, I already finished the checkpoint 5 with this solution bellow without warnings in Xcode:

import Cocoa

//CHECKPOINT 5
let luckyNumbers = [7, 4, 38, 21, 16, 15, 12, 33, 31, 49]

let checkPoint5 = { (luckyNumber: [Int]) in
        luckyNumber.filter {!($0.isMultiple(of: 2))}
        .sorted()
        .map { print("\($0) is a lucky number") }
}

checkPoint5(luckyNumbers)

So I tried to put the return Void after the Closure parameter, but I received the follow warning in Xcode: Result of call to 'map' is unused

import Cocoa

//CHECKPOINT 5
let luckyNumbers = [7, 4, 38, 21, 16, 15, 12, 33, 31, 49]

let checkPoint5 = { (luckyNumber: [Int]) -> Void in
        luckyNumber.filter {!($0.isMultiple(of: 2))}
        .sorted()
        .map { print("\($0) is a lucky number") }  //the warning is here
}

checkPoint5(luckyNumbers)

I'm just curious, why this happened when I put the Void return? I cant figure out why.

3      

@roosterboy  HWS+

Because your code is returning a value but you are telling the compiler that your closure doesn't return anything.

This code:

luckyNumber.filter {!($0.isMultiple(of: 2))}
.sorted()
.map { print("\($0) is a lucky number") }

returns [Void], not Void

4      

@Greenamberred  HWS+

You have an incompatible closure declaration and usage.

When you use -> Void, you are telling the compiler that the function, or in this case the closure, does not return a result.

for example

func printMessage(_ theMessage: String) {   // no Void is used here
    print("Message: \(theMessage)")
}

let outputMessage: (String) -> Void = outputMessage
outputMessage ("A void message")

Conversely, when there is a result returned ( in this case the result of the filtering, sorting and mapping), but the closure (or function) declares -> Void, then effectively you are trying to discard the result, when maybe you shouldn't. Hence the warning that the result (of the last modifier - .map) is unused.

The compiler does not throw an error, because it can in this case ignore the result from the closure, however giving you a warning to check your code to make sure it is as you intended.

4      

@behrouz  

the map method's return value is not Void. You can instead use the method forEach.

let luckyNumbers = [7, 4, 38, 21, 16, 15, 12, 33, 31, 49]

luckyNumbers.filter { !$0.isMultiple(of: 2) }
.sorted { $0 < $1 }
.map { "\($0) is a lucky number" }
.forEach { print($0) }

3      

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