Inserting array into array

Question

So I wanted to insert the objects of an array into another array. Swift arrays seem to missing an equivalent method to - (void)insertObjects:(NSArray *)objects atIndexes:(NSIndexSet *)indexes, but all is well because we can use the following syntax:

var array = [0,4,5]
array[1..<1] = [1,2,3] // array = [0,1,2,3,4,5]

All is well? Not quite! The following gives a compile-time error ("[Int] is not convertible to Int"):

var array = [0,4,5]
var array2 = [1,2,3]
array[1..<1] = array2

Is there a sane explanation for this?

Edit:

Ok, the following works (thanks Greg):

array[1..<1] = array2[0..<array2.count]

As well as this:

let slice = array2[0..<array2.count]
array[1..<1] = slice

But right now I'm utterly confused how this works. Gregs explanation that I was trying to insert array2 itself into array makes sense, but I fail to see the difference to just using an array literal, as well as why it works with a slice (which seems to be an undocumented implementation detail?).

Slice conforms to the ArrayLiteralConvertible protocol, that's why array[1..<1] = [1,2,3] works. But there is no automatic conversion from Array to Slice or vice versa. — Martin R
– Martin R, Commented Dec 9, 2014 at 10:42

Greg · Accepted Answer · 2014-12-09 10:17:40Z

4

This is happening because you want to insert an array2 (Array) not elements from the array, try this:

array[1..<1] = array2[0..<array2.count]

answered Dec 9, 2014 at 10:17

Greg

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rintaro · Accepted Answer · 2014-12-09 13:03:06Z

3

Range subscription on Array<T> works only with Slice<T>.

subscript (subRange: Range<Int>) -> Slice<T>

In your case, array2 is not a Slice<Int>. That's why you see the error.

Usually, you should use replaceRange() or splice(), which works with arbitrary CollectionType.

array.replaceRange(1..<1, with: array2)
// OR
array.splice(array2, atIndex: 1)

Moreover, assigning to range on Array has some bugs I think. for example:

var array = [0,1,2,3,4,5]
array[1...5] = array[1...1]

The result array should be [0, 1] but it actually remains to be [0,1,2,3,4,5]. It only happens if the startIndex of the range is the same, and the slice is constructed from the same array.

On the other hand, replaceRange() works as expeceted:

var array = [0,1,2,3,4,5]
array.replaceRange(1...5, with: array[1...1])
// -> [0,1]

edited Dec 9, 2014 at 13:03

answered Dec 9, 2014 at 12:28

rintaro

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1 Comment

mkeiser Over a year ago

Thanks for the explanation and for showing me the correct method to use.

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