How to handle initial nil value for reduce functions

Question

I would like to learn and use more functional programming in Swift. So, I've been trying various things in playground. I don't understand Reduce, though. The basic textbook examples work, but I can't get my head around this problem.

I have an array of strings called "toDoItems". I would like to get the longest string in this array. What is the best practice for handling the initial nil value in such cases? I think this probably happens often. I thought of writing a custom function and use it.

func optionalMax(maxSofar: Int?, newElement: Int) -> Int {
    if let definiteMaxSofar = maxSofar {
        return max(definiteMaxSofar, newElement)
    }
    return newElement
}   

// Just testing - nums is an array of Ints. Works.
var maxValueOfInts = nums.reduce(0) { optionalMax($0, $1) }   

// ERROR: cannot invoke 'reduce' with an argument list of type ‘(nil, (_,_)->_)'
var longestOfStrings = toDoItems.reduce(nil) { optionalMax(count($0), count($1)) }

Answer 1

It might just be that Swift does not automatically infer the type of your initial value. Try making it clear by explicitly declaring it:

var longestOfStrings = toDoItems.reduce(nil as Int?) { optionalMax($0, count($1)) }

By the way notice that I do not count on $0 (your accumulator) since it is not a String but an optional Int Int?

Generally to avoid confusion reading the code later, I explicitly label the accumulator as a and the element coming in from the serie as x:

var longestOfStrings = toDoItems.reduce(nil as Int?) { a, x in optionalMax(a, count(x)) }

This way should be clearer than $0 and $1 in code when the accumulator or the single element are used.

Hope this helps

Answer 2

Initialise it with an empty string "" rather than nil. Or you could even initialise it with the first element of the array, but an empty string seems better.