Instance MethodSwift

prefix(through:)

Returns a subsequence from the start of the collection through the specified position.

func prefix(through position: Self.Index) -> Self.SubSequence

Parameters

position

The index of the last element to include in the resulting subsequence. position must be a valid index of the collection that is not equal to the endIndex property.

Returns

A subsequence up to, and including, the given position.

Overview

The resulting subsequence includes the element at the position specified by the through parameter. The following example searches for the index of the number 40 in an array of integers, and then prints the prefix of the array up to, and including, that index:

let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.firstIndex(of: 40) {
    print(numbers.prefix(through: i))
}
// Prints "[10, 20, 30, 40]"

Using the prefix(through:) method is equivalent to using a partial closed range as the collection’s subscript. The subscript notation is preferred over prefix(through:).

if let i = numbers.firstIndex(of: 40) {
    print(numbers[...i])
}
// Prints "[10, 20, 30, 40]"