CODEWITHSUNDEEP
swiftswift2
CocoaUser
CocoaUser

Reputation: 1429

Insert a new element into an array in Swift

Code:

let oldNums: [Int] = [1, 2, 3, 4, 5 ,6 , 7, 8, 9, 10]
var newArray = oldNums[1..<4]
newArray.insert(99, atIndex: 0) // <-- crash here
newArray.insert(99, atIndex: 1) // <-- work very well

I thank the newArray is a new mutable variable. So I get confuse. Why? I can't insert a new element into "newArray"

Upvotes: 1

Views: 194

Answers (1)

Martin R
Martin R

Reputation: 540105

oldNums[1..<4] is not an array, but an ArraySlice:

An Array-like type that represents a sub-sequence of any Array, ContiguousArray, or other ArraySlice.

The indices of array slices are not zero-based, but correspond to the indices of the original array. This is a change that came with Swift 2 and is documented in the Xcode 7.0 Release notes:

For consistency and better composition of generic code, ArraySlice indices are no longer always zero-based but map directly onto the indices of the collection they are slicing and maintain that mapping even after mutations.

In your case:

let oldNums: [Int] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
var newArray = oldNums[1..<4]
print(newArray.indices)
// 1..<4

So 0 is an invalid index for insert(), and that's why

newArray.insert(99, atIndex: 0)

crashes. To insert an element at the beginning of the slice, you can use

newArray.insert(99, atIndex: newArray.startIndex)

To create a "real" array instead of a slice, use the Array() constructor:

let oldNums: [Int] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
var newArray = Array(oldNums[1..<4])
print(newArray.indices)
// 0..<3

newArray.insert(99, atIndex:0)
print(newArray)
// [99, 2, 3, 4]

Upvotes: 4

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