An offset is an integer value that gives relative position of a member in the enumeration sequence of a collection. Offsets start the count of the first member at zero, just like C array indexing.
Offsets provide an alternate means to access the members of a collection, without creating a separate index object. Some collection subtypes (such as Array) support fast, direct access by integer member offset, while others support member offsets only as a shorthand for sequential access through every preceding member. Access by offsets is supported on all collections regardless of whether its speed on a particular collection type.
atOffset: and atOffset:put: raise the error OffsetOutOfRange if the offset is greater than or equal to the count of members in the collection.