Swift vs Python Performance

Question

I have some code in Python that builds a dictionary of about 250K strings (words) as keys with each value having an array of strings. The Python version runs in about 0.5 seconds. I needed to port this to Swift but my Swift port runs in 10.1 seconds, 20 times slower.

Here is the Python code:

wordsDictionary = defaultdict(list)
for word in words:
    wordsDictionary[sort_string(word)].append(word)

And here is the Swift code:

var wordsDictionary : Dictionary<String, [String]> = Dictionary()
for word in words {
    let sortedWord : String = String(word.characters.sort())
    if wordsDictionary[sortedWord] == nil {
        wordsDictionary[sortedWord] = []
    }
    wordsDictionary[sortedWord]?.append(word)
}

Is there any way to speed up the Swift version or are Swift dictionaries still that much slower than Python?

Answer 1

After changing the string sort and array allocation for the dictionary, here is my final code that executes 1.4s.

var wordsDictionary : Dictionary<String, [String]> = Dictionary()
for word in words {
    let sortedWordUTF8 = word.utf8.sort()
    let sortedWord : String = NSString(bytes: sortedWordUTF8, length: sortedWordUTF8.count, encoding: NSUTF8StringEncoding) as! String
    if wordsDictionary[sortedWord] == nil {
        wordsDictionary[sortedWord] = [word]
    }
    else {
        wordsDictionary[sortedWord]!.append(word)
    }
}

Answer 2

The key differences between your Python code and your Swift code seem to be:

As already mentioned in the comments to your question, Swift's internal string representation is quite different from that of Python and string comparison works on grapheme level rather than encoding level. As you've already eliminated this I won't go into more detail.

Next, in Swift you ask the dictionary to look up your key-string twice per iteration. Even for a hashed lookup this will likely take some time as I doubt the second access will be optimised away. You can avoid this by asking for an DictionaryIndex using the indexForKey method and reuse this. Accessing the dictionary with this index should be faster.

Lastly, you check an optional that does not need checking. The else branch in the Swift code certainly finds a non-nil dictionary entry.