Swift - best practice to find the longest string at [String] array

Question

I'm trying to find what is the most effective way to get the longest string in a string array. For example :

let array = ["I'm Roi","I'm asking here","Game Of Thrones is just good"]

and the outcome will be - "Game Of Thrones is just good"

I've tried using the maxElement func, tho it's give the max string in a alphabetic ideas(maxElement()).

Any suggestions? Thanks!

Answer 1

Instead of sorting which is O(n log(n)) for a good sort, use max(by:) which is O(n) on Array providing it a closure to compare string lengths:

Swift 4:

For Swift 4 you can get the string length with the count property on String:

let array = ["I'm Roi","I'm asking here","Game Of Thrones is just good"]

if let max = array.max(by: {$1.count > $0.count}) {
    print(max)
}

Swift 3:

Use .characters.count on String to get the string lengths:

let array = ["I'm Roi","I'm asking here","Game Of Thrones is just good"]

if let max = array.max(by: {$1.characters.count > $0.characters.count}) {
    print(max)
}

Swift 2:

Use maxElement on Array providing it a closure to compare string lengths:

let array = ["I'm Roi","I'm asking here","Game Of Thrones is just good"]

if let max = array.maxElement({$1.characters.count > $0.characters.count}) {
    print(max)
}

Note: maxElement is O(n). A good sort is O(n log(n)), so for large arrays, this will be much faster than sorting.

Answer 2

You can use reduce to do this. It will iterate through your array, keeping track of the current longest string, and then return it when finished.

For example:

let array = ["I'm Roi","I'm asking here","Game Of Thrones is just good"]

if let longestString = array.reduce(Optional<String>.None, combine:{$0?.characters.count > $1.characters.count ? $0:$1}) {
    print(longestString) // "Game Of Thrones is just good"
}

^{(Note that Optional.None is now Optional.none in Swift 3)}

This uses an nil starting value to account for the fact that the array could be empty, as pointed out by @JHZ (it will return nil in that case). If you know your array has at least one element, you can simplify it to:

let longestString = array.reduce("") {$0.characters.count > $1.characters.count ? $0:$1}

Because it only iterates through each element once, it will quicker than using sort(). I did a quick benchmark and sort() appears around 20x slower (although no point in premature optimisation, I feel it is worth mentioning).

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Edit: I recommend you go with @vacawama's solution as it's even cleaner than reduce!