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In Swift, an index can be maintained using flatMap by mapping over an array and returning a tuple containing the index and element. This tuple can then be flattened into a single-dimensional array using flatMap.

Here's an example:

let names = ["Alice", "Bob", "Charlie"]
let indexedNames = names.enumerated().flatMap { (index, name) -> [(Int, String)] in
    return [(index, name)]
}
// indexedNames is [(0, "Alice"), (1, "Bob"), (2, "Charlie")]

In this example, we first call the enumerated() method on the names array, which returns a sequence of tuples containing the index and element of each element in the array. We then use flatMap to map over the enumerated sequence and return an array of tuples, which are then flattened into a single-dimensional array. The resulting indexedNames array contains tuples that represent the index and element for each element in the original names array.