Subscript

subscript(_:)

Accesses a contiguous subrange of the collection’s elements.

Required.

Declaration

subscript(bounds: Range<Self.Index>) -> Self.SubSequence { get }

Parameters

bounds: A range of the collection’s indices. The bounds of the range must be valid indices of the collection.

Discussion

For example, using a PartialRangeFrom range expression with an array accesses the subrange from the start of the range expression until the end of the array.

let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let streetsSlice = streets[2..<5]
print(streetsSlice)
// ["Channing", "Douglas", "Evarts"]

The accessed slice uses the same indices for the same elements as the original collection. This example searches streetsSlice for one of the strings in the slice, and then uses that index in the original array.

let index = streetsSlice.firstIndex(of: "Evarts")!    // 4
print(streets[index])
// "Evarts"

Always use the slice’s startIndex property instead of assuming that its indices start at a particular value. Attempting to access an element by using an index outside the bounds of the slice may result in a runtime error, even if that index is valid for the original collection.

print(streetsSlice.startIndex)
// 2
print(streetsSlice[2])
// "Channing"

print(streetsSlice[0])
// error: Index out of bounds

Complexity: O(1)