Returns an array containing the non-
nil results of calling the given transformation with each element of this sequence.
- Xcode 8.0+
A closure that accepts an element of this sequence as its argument and returns an optional value.
An array of the non-
nil results of calling
transform with each element of the sequence.
Use this method to receive an array of nonoptional values when your transformation produces an optional value.
In this example, note the difference in the result of using
flat with a transformation that returns an optional
Complexity: O(m + n), where m is the length of this sequence and n is the length of the result.