The NSIndexSet class represents an immutable collection of unique unsigned integers, known as indexes because of the way they are used. This collection is referred to as an index set. You use index sets in your code to store indexes into some other data structure. For example, given an NSArray object, you could use an index set to identify a subset of objects in that array.
NSIndexSet is a Foundation Framework collection class that is similar to NSRange, with the notable exception of being able to support non-contiguous series. An NSIndexSet can be created from a range using the indexSetWithIndexesInRange: class constructor:
NSIndexSet expresses a collection of unique whole numbers; its purpose is to express element numbers of an ordered collection, such as an NSArray. It is available in iOS 2.0 and later, available in OS X v10.3 and later . An NSIndexSet is immutable; its mutable subclass is NSMutableIndexSet. You can form a simple NSIndexSet consisting of just one contiguous range directly, by passing an NSRange to indexSetWithIndexesInRange: but to form a more complex index set you’ll need to use NSMutableIndexSet so that you can append additional ranges.
Example :
NSRange range = NSMakeRange(0, 10);
NSIndexSet *indexSet = [NSIndexSet indexSetWithIndexesInRange:range];
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