How do I fetch the i'th element from a Swift ArraySlice? - arrays

Question

Below I am trying to fetch the i'th element of the ArraySlice draggignFan. The code builds fine (no warnings) but the program dies at runtime on the line where I try to index the slice like a normal array:
var draggingFan : ArraySlice<Card>?
...
if let draggingFan = draggingFan {
for i in 1 ..< draggingFan.count {
let card = draggingFan[i] // EXECUTION ERROR HERE
...
}
}
According to the docs there is a first and last method (which I use elsewhere with no problem). So how do I index an ArraySlice in Swift? (Note: I am intentionally skipping the 0'th index in the slice -- that's needed elsewhere).

Answer 1

The indices of the ArraySlice still match those of the original array. In your case, you are accessing index 1 which is not in your slice. If you offset the index by draggingFan.startIndex it will work:
if let draggingFan = draggingFan {
for i in 1 ..< draggingFan.count {
let card = draggingFan[draggingFan.startIndex + i]
...
}
}
Alternatively:
if let draggingFan = draggingFan {
for i in draggingFan.startIndex + 1 ..< draggingFan.endIndex {
let card = draggingFan[i]
...
}
}

Answer 2

This will access the values from the second element in the slice to the last element in the slice:
let original = [1,2,3,4,5,6] // Int array to demonstrate
var draggingFan : ArraySlice<Int>?
draggingFan = original[1...4] // create the slice
if let draggingFan = draggingFan {
// so there's no errors just slice the slice and iterate over it
for i in draggingFan[(draggingFan.startIndex+1)..<draggingFan.endIndex] {
print(i, terminator: ", ")
}
}
Output:
3, 4, 5,

Answer 3

The reason you are having this problem is that the slice maintains the original index numbers of the sequence you got it from. Thus, element 1 is not in this slice.
For example, consider this code:
let arr = [1,2,3,4,5,6,7,8,9]
let slice = arr[2...5]
Now what is slice[1]? It isn't 4, even though that is the second thing in the slice. It's 2, because the slice still points into the original array. In other words, slice[1] is out of the slice's range! That is why you're getting a runtime error.
What to do? Well, the actual indexes of the slice are its indices. That is what you want to cycle thru. But... You don't want the first element pointed to by the slice. So you need to advance the startIndex of the range you're going to iterate through. Thus:
if let draggingFan = draggingFan {
var ixs = draggingFan.indices
ixs.startIndex = ixs.startIndex.advancedBy(1)
for i in ixs {
// ... now your code will work ...
}
}
However, in my view, there's no need to index the slice at all, and you shouldn't be doing so. You should cycle through the slice itself, not thru its indexes. You have this:
for i in 1 ..< draggingFan.count
But that is much like saying
for aCard in draggingFan
...except that you want to drop the first element of the slice. Then drop it! Say this:
for aCard in draggingFan.dropFirst()
To see that this will work, try this in a playground:
let arr = [1,2,3,4,5,6,7,8,9]
let slice = arr[2...5]
for anInt in slice.dropFirst() {
print(anInt) // 4, 5, 6
}
As you can see, we are cycling through exactly the desired elements, with no reference to index numbers at all.

Answer 4

To iterate over the elements in the slice:
draggingFan?.forEach({ (element)
...
})
As far as I know, the get a specific element, it needs to be converted back to an array e.g.
let draggingFanArray = Array(draggingFan!)
Here's the playground code I used to toy around with various scenarios:
import Cocoa
var a: Array<Int>?
var b: ArraySlice<Int>?
a = [1, 2, 3, 4, 5, 6, 7]
b = a![3...5]
let count = b!.count
b!.forEach({ (element) in
print("\(element)")
})
let c = Array(b!)
print(c[2])
edit ArraySlice extension though:
extension ArraySlice {
func elementAtIndex(index: Int)->AnyObject?{
return Array(self)[index] as? AnyObject
}
}

Answer 5

If I have an array:
var arr = [1, 2, 3, 4, 5, 6, 7] // [1, 2, 3, 4, 5, 6, 7]
And I take a slice of the array:
let slice = arr[3..<arr.count] // [4, 5, 6, 7]
This slice will have a startIndex of 3, which means that indexing starts at 3 and ends at 6.
Now if I want a slice containing everything but the first element, I can use the dropFirst() method:
let sliceMinusFirst = slice.dropFirst() // [5, 6, 7]
And at this point, sliceMinusFirst has a startIndex of 4, which means my indexes range from 4 to 6.
Now if I wish to iterate over these to do something with the items, I can do the following:
for item in sliceMinusFirst {
print(item)
}
Alternatively, I can do it with forEach:
sliceMinusFirst.forEach { item in
print(item)
}
By using these forms of iteration, the fact that the startIndex is nonzero doesn't even matter, because I don't use the indices directly. And it also doesn't matter that, after taking a slice, I wanted to drop the first item. I was able to do that easily. I could have even done that at the time I wanted to do the iteration:
slice.dropFirst().forEach { item in
print(item)
}
Here I dropped the first item from the original slice, without creating any intermediate variables.
Remember that if you need to actually use the index, you're probably doing something wrong. And if you genuinely do need the index, make sure you understand what's going on.
Also if you want to get back to zero-based indexing once you make a slice, you can create an array from your slice:
let sliceArray = Array(slice) // [4, 5, 6, 7]
sliceArray.startIndex // 0