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Swift Functional Programming Tutorial

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1

Learn how to program in Swift using functional programming techniques, such as map and reduce, in this Swift functional programming tutorial.

This is a companion discussion topic for the original entry at https://www.raywenderlich.com/2273-swift-functional-programming-tutorial
2

Hi Colin,

First off, thanks for the tutorial. This is a very nice explanation of the concept and great examples.

I want to know the solution of the 3141 string to int though. I can’t figure out how to return an int from the function. Here is what I have:

let stringToInt = Array([“3”, “1”, “4”, “1”])
.reduce(“”) { (first, next) in first + next }

print(Int(stringToInt))

Thanks very much.
Kevin

3

Here’s buildIndex in Swift 2.0

func buildIndex(words: [String]) -> [Entry] {
    var result = [Entry]()
    
    var letters = [Character]()
    for word in words {
        let firstLetter = Character(word.substringToIndex(word.startIndex.advancedBy(1)).uppercaseString)
        
        if !letters.contains(firstLetter) {
            letters.append(firstLetter)
        }
    }
    
    for letter in letters {
        var wordsForLetter = [String]()
        for word in words {
            let firstLetter = Character(word.substringToIndex(word.startIndex.advancedBy(1)).uppercaseString)
            
            if firstLetter == letter {
                wordsForLetter.append(word)
            }
        }
        result.append((letter, wordsForLetter))
    }
    return result
}
4

Colin, Thank you for the tutorial. I am a big fan of functional programming. Ocaml was my language of choice in the past. It’s been over 10 years since I have even thought about it. Your tutorial reintroduced it to me for Swift.
It’s a fun and thought provoking style of programming.

Here’s the build using the Swiftz library found on GitHub.

import Swiftz
extension String {
    func firstCharacterUpcase () -> Character {
        return Character(self.substringToIndex(self.startIndex.advancedBy(1)).uppercaseString)
    }
}

func buildIndexSwiftz(words:[String]) -> [Entry] {
    return words.reverse()
        .reduce([:] , combine: { $0.insertWith($1.firstCharacterUpcase(), v: [$1], combiner: (+)) } )
        .reduce([]  , combine: { $0.cons( ($1.0,$1.1) ) } )
        .sort({$0.0 <= $1.0})
}

Some notes

  1. The extension to the string is simply for readability in the final function
  2. process: reverse the array, process the words into a Dictionary of type [Character:[String], convert the dictionary into [Entry], then sort it based on the first in the tuple.
  3. I reverse the array so that the results match the order of the original imperative style. This is covering the difference between appending to an array and cons’ing an array.
5

let digits = [“3”,“1”,“4”,“1”]
let digitNum = digits.reduce(0, combine: { (total, digit) in total*10 + Int(digit)! } )
print(digitNum)

6

Great article.

For the last challenge this is what I did:

I added this code before the return:

let sortedWords = words.sorted{
$0.localizedCaseInsensitiveCompare($1) == ComparisonResult.orderedAscending
}

And off course changed the first line of the return to this:

return distinct(source: sortedWords.map(firstLetter))

Is that right?

7

Worked for me. Good job!

(P.S. I worked through this whole exercise while converting it to Swift 3 … a bit of a challenge :smiley: )

8

Great Article!

3141 example
let digits = [“3”,“1”,“4”,“1”]
let tot = digits.reduce(0) { (total, digit) in total * 10 + Int(digit)!}
tot

worked like a charm!

9

Thanks a lot for this great article. I was learning this with Swift 4.
The functions as modified for Swift 4.0 (function distinct generic changed to Comparable from Equatable and return collection sorted for the last challenge):

func distinct<T: Comparable>(_ source: [T]) -> [T] {
	var unique = [T]()
	for item in source {
		if !unique.contains(item) {
			unique.append(item)
		}
	}
	return unique.sorted(by: <)
}

func buildIndex(words: [String]) -> [Entry] {
	func firstLetter(_ str: String) -> Character {
		return Character((str as NSString).substring(to: 1).uppercased())
	}
		
	return distinct(words.map(firstLetter)).map { (letter) -> Entry in
		return (letter, words.filter { (word) -> Bool in
			firstLetter(word) == letter
		})
	}
}
10

This tutorial is more than six months old, so questions are no longer supported at the moment for it. We will update it as soon as possible. Thank you! :]