# index(_:ofObjectPassingTest:) Returns the index of an object in the ordered set that passes a test in a given block for a given set of enumeration options. ``` func index(_ opts: NSEnumerationOptions = [], ofObjectPassingTest predicate: (Any, Int, UnsafeMutablePointer) -> Bool) -> Int ``` ## Parameters `opts` A bitmask that specifies the options for the enumeration (whether it should be performed concurrently and whether it should be performed in reverse order). `predicate` The block to apply to elements in the ordered set. The block takes three arguments: - obj: The element in the array. - idx: The index of the element in the ordered set. - stop: A reference to a Boolean value. The block can set the value to to stop further processing of the set. The `stop` argument is an out-only argument. You should only ever set this value to within the block. The block returns a Boolean value that indicates whether obj passed the test. ## Return Value The index whose corresponding value in the ordered set passes the test specified by `predicate` and `opts`. If no objects in the ordered set pass the test, returns `NSNotFound`. ## Discussion By default, the enumeration starts with the first object and continues serially through the ordered set to the last object. You can specify [`concurrent`](/documentation/Foundation/NSEnumerationOptions/concurrent) and/or [`reverse`](/documentation/Foundation/NSEnumerationOptions/reverse) as enumeration options to modify this behavior. --- 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)