NSIndexSet (like its mutable counterpart, NSMutableIndexSet) is a sorted collection of unique unsigned integers. Think of it like an NSRange that supports non-contiguous series. It has wicked fast operations for finding indexes in ranges or set intersections, and comes with all of the convenience methods you’d expect in a Foundation collection class.

You’ll find NSIndexSet used throughout the Foundation framework. Anytime a method gets multiple elements from a sorted collection, such as an array or a table view’s data source, you can be sure that an NSIndexSet parameter will be somewhere in the mix.

If you look hard enough, you may start to find aspects of your data model that could be represented with NSIndexSet. For example, AFNetworking uses an index set to represent HTTP response status codes: the user defines a set of “acceptable” codes (in the 2XX range, by default), and the response is checked by using containsIndex:.

The Swift standard library includes PermutationGenerator, an often-overlooked type that dovetails nicely with NSIndexSet. PermutationGenerator wraps a collection and a sequence of indexes (sound familiar?) to allow easy iteration:

let streetscape = ["Ashmead", "Belmont", "Clifton", "Douglas", "Euclid", "Fairmont", 
						"Girard", "Harvard", "Irving", "Kenyon", "Lamont", "Monroe", 
						"Newton", "Otis", "Perry", "Quincy"]

let selectedIndices = NSMutableIndexSet(indexesInRange: NSRange(0...2))

for street in PermutationGenerator(elements: streetscape, indices: selectedIndices) {
// Ashmead, Belmont, Clifton, Fairmont, Monroe, Newton, Otis

Here are a few more ideas to get you thinking in terms of index sets:

  • Have a list of user preferences, and want to store which ones are switched on or off? Use a single NSIndexSet in combination with an enum typedef.
  • Filtering a list of items by a set of composable conditions? Ditch the NSPredicate; instead, cache the indexes of objects that fulfill each condition, and then get the union or intersection of those indexes as conditions are added and removed.

Overall, NSIndexSet is a solid class. A fair bit nerdier than its collection class siblings, but it has its place. At the very least, it’s a prime example of the great functionality that you find by paying attention to what Foundation uses in its own APIs.


Questions? Corrections? Issues and pull requests are always welcome.

This article uses Swift version 2.0 and was last reviewed on September 8, 2015. Find status information for all articles on the status page.

Written by Mattt

Mattt (@mattt) is a writer and developer in Portland, Oregon.

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