"ambiguous subscript" when trying to return an array

Question

I'm looking to extract sub-arrays in Swift 4 and running into unexpected difficulties.

The following trivial cases work as expected:

let array1 = ["a", "b", "c","x","y","z"]
array1[2...3]  // returns ["c","x"], as it should

as does the following variation:

let m=2
let n=3
array1[m...n]  // returns ["c","x"], as it should

But if I try to create a function around this, I get an error message ambiguous subscript with base type 'Array<Any>' and index type 'CountableClosedRange<Int>':

func arrayTake(arrayIn: Array<Any>) -> Array<Any> {
    return arrayIn[2...3]
}     // no good

And of course, the following form, which is closer to what I actually want to do, fails with the same error message:

func arrayTake(m: Int, n: Int, arrayIn: Array<Any>) -> Array<Any> {
    return arrayIn[m...n]
}     // no good

I have seen other questions here about ambiguous subscripts but have not been able to translate the answers there into a solution to my problem. Any guidance would be appreciated.

Answer 1

You are getting such error because you are trying to return an ArraySlice instead of the expected Array<Any>:

Slices Are Views onto Arrays

You may want to convert such slice using a dedicated Array constructor:

func arrayTake(m: Int, n: Int, arrayIn: Array<Any>) -> Array<Any> {
    return Array<Any>(arrayIn[m...n])
}

Moreover, please note that long-term storage of ArraySlice is discouraged, a slice holds a reference to the entire storage of a larger array, not just to the portion it presents, even after the original array’s lifetime ends. Long-term storage of a slice may therefore prolong the lifetime of elements that are no longer otherwise accessible, which can appear to be memory and object leakage.

Answer 2

Problem is Datatype mismatch.

array1[2...3] This statement does not return Array it returns ArraySlice a special DataType (struct) for this kind of array operation.

So you can solve this by two way.

Change return type to ArraySlice

func arrayTake(m: Int, n: Int, arrayIn: Array<Any>) -> ArraySlice<Any> 
{   
    return arrayIn[m...n] 
}

OR

Convert ArraySlice to Array

func arrayTake(m: Int, n: Int, arrayIn: Array<Any>) -> Array<Any> {  
    return Array(arrayIn[m...n])    
}

Both will print same output but keep in mind that they are not same:

let array1 = ["a", "b", "c","x","y","z"]
let result = arrayTake(m: 2, n: 3, arrayIn: array1)

print(result) //["c","x"], as it should

Check this for more detail: https://medium.com/@marcosantadev/arrayslice-in-swift-4e18522ab4e4