# prefix(through:)

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

Available because

`Self`

conforms to`Collection`

.

`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 collectionâ€™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]"
```