21+-+Iterators - Overview EECS 280 Programming and...

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11/30/2009 1 Iterators EECS 280 Programming and Introductory Data Structures 1 Overview 2 circle5 So far, we've looked at several different kinds of abstractions: 1. Functional generalization 2. Abstract data types 3. Containers circle5 Today, we'll talk about what happens when containers and functional abstraction meet. Overview 3 circle5 Several weeks ago, we talked about a collection of abstract data types: circle5 IntSet circle5 a subtype of an IntSet called a MaxIntSet circle5 MaxInstSet was just like an IntSet , with one additional method: int max(); // REQUIRES: set is non-empty // EFFECTS: returns the largest element in set. circle5 max walked down the set of integers, computing the maximum. Iterators Introduction 4 circle5 While max works, it is unsatisfactory since there are many computations that could be performed on a set of integers. circle5 As ADT designers, we have at least two choices: 1. Predict all of a client's needs, and provide a method for each of them. (Sum, min, max, ...) 2. Provide an abstraction that can access each object in the set, with exactly-once semantics. circle5 The second is preferable and is called an "iterative abstraction", or commonly an "iterator" because it allows clients to iterate over the members of some container. circle5 In principle, any container class could support an iterator. Iterators Introduction 5 circle5 An iterator is a separate abstraction from the container itself. There are two reasons why this is so: 1. An iterator's lifetime is shorter than its container's: the container must exist before you iterate over its contents, and the container can't be destroyed safely before the iterator is. 2. A container can have more than one "live" iterator at a time. circle5 So, iterators are a separate abstract data type. circle5 Usually, for each container type one has, there is also (at least) one iterator type defined. Iterators Details 6 circle5 An iterator has the following operations: 1. create : create a new iterator, bound to some particular instance of a container class. circle5 Once an iterator is created, the underlying container cannot be modified in any way, else the iterator is invalid. circle5 The intuition behind this is that the iterator depends on the representation of the container – if that changes, the iterator is likely to miss an element or return an element that no longer exists. circle5 In general, an iterator makes no guarantee about the order in which objects are returned, though it is possible to define specific iterators with such a guarantee (like when working with sorted lists).
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11/30/2009 2 Iterators Details 7 circle5 An iterator has the following operations: 2. next : return an object from the container.
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