Worksheet 23: Introduction to Iterator NAME:
An Active Learning approach to Data Structures using C
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/* conceptual interface */
Boolean hasNext ( );
EleType next ( );
void remove ( );
Worksheet 23: Introduction to the Iterator
In Preparation: Read Chapter 8 to learn more about the idea of encapsulation and the
iterator. If you have not done it already, complete Lessons 14 and 16 to learn more about
adding and removing values from a dynamic array.
One of the primary design principles for collection classes is encapsulation. The internal
details concerning how an implementation works are hidden behind a simple and easy to
remember interface. To use a Bag, for example, all you need know is the basic
operations are add, collect and remove. The inner workings of the implementation for the
bag are effectively hidden.
When using collections a common requirement is the
need to loop over all the elements in the collection, for
example to print them to a window. Once again it is
important that this process be performed without any
knowledge of how the collection is represented in
memory. For this person the conventional solution is
to use a mechanism termed an Iterator.
Each collection provides an associated iterator
structure. Functions are used to initialize and
manipulate this iterator. These functions are used
in combination to write a simple loop that will
cycle over the values in the collection. For
example, the code at left illustrates the use of our
dynamic array iterator.
Notice that the iterator loop exposes nothing of the
structure of the container class. The method
remove is used to delete from the collection the value most recently returned by next.
Calls on the functions hasNext and next must always be interleaved, as shown.
Note that an iterator is an object that is separate from the collection itself. The iterator is a
facilitator object, one that provides access to the container values.
An iterator for the dynamic array will work by maintaining an index into the array
representing the current location. This value is initially zero, and must be incremented by
either the function hasNext or the function next? Think about this for a moment. Which
one makes more sense? Then think carefully about the third function, remove. You must
ensure that when the loop continues after a remove the next value following the removed
element is not skipped. You can assume you have access to the dyArrayRemoveAt
function you wrote in Lesson 21.
void dyArrayRemoveAt (struct dyArray * dy, int index);
dyArrayIterator itr;
EleType current;
dyArray data;
…
dyArrayIteratorInit (&data,&itr);
while (dyArrayHasNext(&itr)) {
current = dyArrayNext(&itr);
… /* do something with current */
}
