# MutableCollection A collection that supports subscript assignment. ``` protocol MutableCollection : Collection where Self.SubSequence : MutableCollection ``` ## Overview Collections that conform to `MutableCollection` gain the ability to change the value of their elements. This example shows how you can modify one of the names in an array of students. ``` var students = ["Ben", "Ivy", "Jordell", "Maxime"] if let i = students.firstIndex(of: "Maxime") { students[i] = "Max" } print(students) // Prints "["Ben", "Ivy", "Jordell", "Max"]" ``` In addition to changing the value of an individual element, you can also change the values of a slice of elements in a mutable collection. For example, you can sort *part* of a mutable collection by calling the mutable `sort()` method on a subscripted subsequence. Here’s an example that sorts the first half of an array of integers: ``` var numbers = [15, 40, 10, 30, 60, 25, 5, 100] numbers[0..<4].sort() print(numbers) // Prints "[10, 15, 30, 40, 60, 25, 5, 100]" ``` The `MutableCollection` protocol allows changing the values of a collection’s elements but not the length of the collection itself. For operations that require adding or removing elements, see the `RangeReplaceableCollection` protocol instead. # Conforming to the MutableCollection Protocol To add conformance to the `MutableCollection` protocol to your own custom collection, upgrade your type’s subscript to support both read and write access. A value stored into a subscript of a `MutableCollection` instance must subsequently be accessible at that same position. That is, for a mutable collection instance `a`, index `i`, and value `x`, the two sets of assignments in the following code sample must be equivalent: ``` a[i] = x let y = a[i] // Must be equivalent to: a[i] = x let y = x ``` ## Relationships ### Conforming Types [`Array`](/documentation/Swift/Array) [`EmptyCollection`](/documentation/Swift/EmptyCollection) [`Values-swift.struct`](/documentation/Swift/Dictionary/Values-swift.struct) [`UnsafeMutableBufferPointer`](/documentation/Swift/UnsafeMutableBufferPointer) [`UnsafeMutableRawBufferPointer`](/documentation/Swift/UnsafeMutableRawBufferPointer) [`CollectionOfOne`](/documentation/Swift/CollectionOfOne) [`ArraySlice`](/documentation/Swift/ArraySlice) [`ContiguousArray`](/documentation/Swift/ContiguousArray) [`Slice`](/documentation/Swift/Slice) ### Inherits From [`Collection`](/documentation/Swift/Collection) [`Sequence`](/documentation/Swift/Sequence) --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)