Instance Method


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


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



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

Return Value

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


The resulting subsequence includes the element at the position end. 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) {
// Prints "[10, 20, 30, 40]"

Complexity: O(1)