Issues with swift map chaining

September 5, 2022

One qq about maps and chaining.
Below code works perfectly fine

    var raceResults = [["one","two","four"],["two","one","five","six"],["two","one","four","ten"],["one","two","four"]]

let numberedRaceResults = raceResults
    .enumerated()
    .flatMap { outterOffset, raceResult in
        raceResult
            .enumerated()
            .map { innerOffset, element in "\(outterOffset).\(innerOffset). \(element)" }
    }

for numberedResult in numberedRaceResults {
    print(numberedResult)
}

But when I add two print statements in it , it stops working.
Can someone-one please help me to understand

1.what’s the deal with those print statements
2. If I replace flatmap with map, code below does not work. My understanding about flatmap is it just eliminates the nil values . Is there something on top of it that flatmap does comapred to map which is why I am not getting expected results when I replace flatmap with map ?

var raceResults = [["one","two","four"],["two","one","five","six"],["two","one","four","ten"],["one","two","four"]]

let numberedRaceResults = raceResults
    .enumerated()
    .flatMap { outterOffset, raceResult in
        print(outterOffset)
        print(raceResult)
        raceResult
            .enumerated()
            .map { innerOffset, element in "\(outterOffset).\(innerOffset). \(element)" }
    }

for numberedResult in numberedRaceResults {
    print(numberedResult)
}

>Solution :

When you write many lines inside the {} of flatMap function you need to tell it what should be the result and since print returns () / void ,hence you get nothing , so you need to add a return under the two prints

    let numberedRaceResults = raceResults
        .enumerated()
        .flatMap { outterOffset, raceResult in
            print(outterOffset)
            print(raceResult)
            return raceResult // here  
                .enumerated()
                .map { innerOffset, element in "\(outterOffset).\(innerOffset). \(element)" }
        }

    for numberedResult in numberedRaceResults {
        print(numberedResult)
    }