Advanced Pattern Matching

Word Frequencies

released Fri, 15 Feb 2019
Swift Version 5.0

Word Frequencies

We have a sequence of pairs, each representing a word and its frequency in some text. Our goal is to filter out those pairs whose frequency is below or above a certain threshold, and then only return the remaining words, without their respective frequencies.

Here're our words:

let wordFreqs = [("k", 5), ("a", 7), ("b", 3)]

A simple solution would be to model this with map and filter:

let res = wordFreqs.filter({ (e) -> Bool in
    if e.1 > 3 {
        return true
    } else {
        return false
}).map { $0.0 }

However, with compactMap a map that only returns the non-nil elements, we can improve a lot upon this solution. First and foremost, we can get rid of the e.1 and instead have proper destructuring by utilizing (you guessed it) tuples. And then, we only need one call compactMap instead of filter and then map which adds unnecessary performance overhead.

let res = wordFreqs.compactMap { (e) -> String? in
    switch e {
    case let (s, t) where t > 3: return s
    default: return nil