Let's start with a single array, like:

Copylet raceResult = ["one", "two", "four"]

If we want to combine each element with an offset counting from 0, we can use Array.enumerated(), along with map.

Copylet numberedRaceResult = raceResult
    .enumerated()
    .map { offset, element in "\(offset). \(element)" }

for numberedResult in numberedRaceResult {
    print(numberedResult)
}

// Prints:
// 0. one
// 1. two
// 2. four

You can see that I didn't call print inside the closure passed to map. You can do this, but it kind of defeats the purpose of map (which is to create an equal-sized output array from the transformed elements of the input array), because the result would be unused. In that case, it makes more sense to just use a for loop or a call to forEach, like @Sh_Khan showed.

To handle a nested array, it's much the same. We can use the same logic as for one array, but apply it to each sub-array.

Copylet 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)
}

// Prints:
// 0.0. one
// 0.1. two
// 0.2. four
// 1.0. two
// 1.1. one
// 1.2. five
// 1.3. six
// 2.0. two
// 2.1. one
// 2.2. four
// 2.3. ten
// 3.0. one
// 3.1. two
// 3.2. four

You'll notice that I used flatMap on the outter array, instead of a simple map. You can change it back and forth and compare the result. In short, flatMap gives you a single flat array of string results, rather than an array of sub-arrays of strings.