1. Concepts of Programming Languages
Dr. Mohamed Yehia Dahab

2. Iterator
We often want to access every item in a data structure or collection in turn
We call this
traversing ,
iterating over,
stepping through or
visiting every item
Example with a data structure (array): for (int i = 0; i < arr.length(); i++) // do something to arr[i]

3. Iterator (Cont’) What if we want to traverse a collection of objects?
A list, a stack, a queue …
Its underlying implementation may not be known to us
Java provides a common scheme for stepping through all elements in any collection, called an iterator

4. What is an Iterator?
An iterator is a mechanism used to step through the elements of a collection one by one
Each element is “Visited” exactly once

5. Why use Iterators?
Traversing through the elements of a collection is very common in programming, and iterators provide a uniform way of doing so (Reusability)
Advantage? Using an iterator, we don’t need to know how the collection is implemented!
2-5

6. Iterator Interface (in Java)
The Java API has a generic interface called Iterator<T> that specifies what methods are required of an iterator
public boolean hasNext( ); returns true if there are more elements in the iteration
public <T> next( ); returns the next element in the iteration
public void remove( ); removes the last element returned by the iterator (optional operation)
public void set( <T> Obj); Set the last element returned by the iterator (optional operation)
……….
It is in the java.util package of the Java API

7. Iterator Interface (in Java) (Cont’)
For Forward Traversing
Next()
hasNext()
For Backward Traversing
Previous()
hasPrevious()
For Modifying
remove
set

8. Iterator Interface (in Java) (Cont’)
public static void main (String[] args) {
ArrayList al = new ArrayList();
al.add("Mohamed");
al.add("Ragab");
al.add("Ramadan");
Iterator itr = al.iterator();
while(itr.hasNext()) {
Object obj = itr.next();
System.out.print(obj + "-->"); }

9. Using an Iterator in an Application
Example: Suppose we had an unordered list that was created by ArrayUnorderedList<Person> myList =
new ArrayUnorderedList<Person>();
and then had items added to it…
// Use iterator to display contents of list Iterator<Person> iter = myList.iterator();  while(iter.hasNext() )
{     System.out.println(iter.next()); } 

10. Using an Iterator in an Application
// Print just the addresses now
// Note that we have to start a new iteration!
iter = myList.iterator(); // start new iteration  while(iter.hasNext() ) { System.out.println(iter.next().get ()); }

11. C++ Iterators STL iterator’s value can represent 3 kinds of states:
Standard Template Library (STL)
STL iterators generalize different uses of pointers
STL iterator’s value can represent 3 kinds of states:
Dereferenceable (points to a valid location in a range)
Past the end (points just past last valid location in a range)
A closed/open range: [start, end)
Singular (points to nothing)

12. C++ Iterators (Cont’)
Pointers and integers support iteration but not all collections are ordinal data structures.
for (int * p = A; p – A < MAX; ++p) {
cout << *p << endl; }
for (unsigned int i = 0; i < MAX; ++i) {
cout << A[i] << endl;

13. Models of C++ Iterators
Forward Iterator
Linked-list style access (slist)
hash_set<string>::iterator
Bidirectional Iterator
Bi-linked-list style access (list)
list<int>::iterator
set<string>::iterator
Random Access Iterator
Array/buffer style access (vector, deque)
vector<int>::iterator

14. C++ Example using namespace std;
vector<int> V1; // Add some elements to V1
V1.push_back(1);
V1.push_back(5);
V1.push_back(7);
for(int y=0; y<V1.size(); y++) {
cout<<V1[y]; }

15. C++ Example (Cont’) vector<int>::iterator V1Iterator;
for(V1Iterator = V1.begin(); V1Iterator != V1.end(); V1Iterator++) {
cout<<*V1Iterator; }

16. C++ Example (Cont’)
the iterators associated with vectors are random access iterators so you could use arithmetic of the form
Iterator + I
vector<int> myIntVector;
vector<int>::iterator myIntVectorIterator;
myIntVectorIterator = myIntVector.begin() + 2;

17. Bidirectional Iterator
std::list<int> mylist;
for (int i=1; i<=5; ++i)
mylist.push_back(i);
for (std::list<int>::reverse_iterator riter=mylist.rbegin(); riter!=mylist.rend(); ++riter)
std::cout << *riter;

18. https://ideone.com/YsRGV4
Output Iterators 
#include <iostream> // std::cout
#include <iterator> // std::ostream_iterator
#include <vector> // std::vector
#include <algorithm> // std::copy
int main () {
std::vector<int> myvector;
for (int i=1; i<10; ++i)
myvector.push_back(i*10);
std::ostream_iterator<int> out_it (std::cout,“ and ");
std::copy ( myvector.begin(), myvector.end(), out_it );
return 0; }

19. Output Iterators #include <iostream> #include <vector>
#include <algorithm>
#include <fstream>
using namespace std;
int main () {
int ary[] = {2,5,7,2,8,9};
ofstream ofile("TEST.DAT"); // write to STDOUT
copy(ary,ary+6,ostream_iterator<int>(cout," - "));
// write into file "TEST.DAT" copy(ary,ary+6,ostream_iterator<int>(ofile,"\n"));
ofile.close();
cout << endl;
return 0; }