Mixing for-in and if-let in Swift - swift

Question

Can I merge a for-in and if-let in one statement?
for item in array {
if let f = item as? NSDictionary {
result.addObject(newFile(f))
}
}
array is made by a JSON, so I don't know if each item is a NSDictionary or not. I have to check.
I was looking for something like this:
for item as? NSDictionary in array {
// code
}
Like Python or Ruby.

Answer 1

#nickfalk is on the right track, but we can do better. His result unfortunately returns [AnyObject], which you can't then call newFile with (I assume). But that's ok, we can get the rest of the way pretty easily.
What you want is partial map. That is to say, you want to map some (but possibly not all) of the elements of one list to another list (from AnyObject to File, if we can). So there must be some rule for choosing, and some rule for mapping. Optional let's us combine those. Let's call the function that does that f. Then its type is:
f: T->U?
So there's some magic function that will possibly convert T to U. We want to map with that. Sounds easy:
extension Array {
func partialMap<U>(f: T->U?) -> [U] {
var result = [U]()
for x in self {
if let u = f(x) {
result.append(u)
}
}
return result
}
}
So now we've hidden all the nasty mutation and var and whatnot down deep where we don't have to look at it. We have a function that takes a mapping function from "something" to "maybe something else" and returns a list of "something elses that we could map."
Now everything is nice and immutable and reusable:
let result = array.partialMap { ($0 as? NSDictionary).map(newFile) }
Whoa there. What's that map in the middle? Well, as? returns NSDictionary?. When you map an optional, then if the optional is None, it returns None, otherwise it applies the function to the value and wraps it in Some. So this whole thing takes AnyObject and returns File? just like we wanted. One partialMap later we have our answer.

Answer 2

I would probably just go for something like:
let result = array.filter() { $0 is NSDictionary }
If you need result to be an NSDictionary array, you can just cast it:
let result = array.filter() { $0 is NSDictionary } as [NSDictionary]

Answer 3

If your goal is to reduce an NSArray to an array only containing NSDictionary filter is a very powerful tool. Create the appropriate filtering function:
func filterForNSDictionary(object: AnyObject) -> Bool{
return object.isKindOfClass(NSDictionary)
}
Then simply pass in you array and function to the filter function
let result = filter(array, filterForNSDictionary)
As #RobNapier points out my solution above will end up with a result array being of the type [AnyObject] this can of course easily be remedied:
let result = filter(array, filterForNSDictionary) as [NSDictionary]
This could be considered risky, if you force the array to be of the wrong type. as [NSString] (for instance9 would most likely blow up in your face down the line...
Rob's solution being pure awesome cleverness of course and #MattGibson delivering the perfect shorthand, while exposing me as an absolute beginner in this field.