Non empty array, accessing first element: index out of range (Swift 2.0)

Question

I have an array of 4 UIImageVIew and array of images of variable size: [UIImage].

I would like to iterate through the array of views and assign 4 last images to each view.

let photos = images.suffix(4)
for (index, view) in views.enumerate() {
  if index <= photos.count - 1 {
    view.image = photos[index]
  } else {
    view.image = nil
  }
}

Code works perfectly if there are no more than 4 elements in images. Then I got a crash: fatal error: ArraySlice index out of range.

I printed out index in for-loop and it seems that crash happens when index is 0. views.count and photos.count return 4 in the same loop.

So, I get an error when accessing the first - 0 element of an non-empty array.

In Swift 1.2 this code worked flawlessly:

let photos = suffix(images, 4)
for (index, view) in enumerate(views) {
  if index <= photos.count - 1 {
    view.image = photos[index]
  } else {
    view.image = nil
  }
}

The only change after switching to 2.0 is in suffix and enumerate methods.

Does it look like a Swift 2.0 bug or am I doing something wrong? Link to source file

Solution I've finished with:

let photos = Array(images.suffix(4))

Should work exactly as suffix() function in Swift 1.2

Hi I think re-implement suffix is unnecessary. Just create a new array with the slice passed in the initializer would do the trick. As swift structs is copy-on-write. so there is no performance or memory penalty. — Fujia
– Fujia, Commented Sep 11, 2015 at 3:15

Philippe-André Lorin · Accepted Answer · 2016-12-28 16:46:56Z

5

This is due to a recent change to ArraySlice. Now indices of ArraySlice are not zero-based. For example, if images.count == 10, then images.suffix(4).startIndex == 5. See the release note here, and search for the words “For consistency and better composition of generic code”.

edited Dec 28, 2016 at 16:46

Philippe-André Lorin

1,06110 silver badges26 bronze badges

answered Sep 10, 2015 at 10:34

Fujia

1,2429 silver badges14 bronze badges

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3 Comments

Richard Topchii Over a year ago

Thank you! Any quick-fix to ArraySlice to make it again zero-based?

Fujia Over a year ago

@RichardTopchiy Probably wrap the suffix slice in a new array, or 'photos[index + photos.startIndex]', I replied this on the train, not tested.

Richard Topchii Over a year ago

Good point! I've just re-implemented suffix() function from scratch

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