List Iterators


You will not be asked to implement a list iterator, but it is an interesting concept to study. Most languages that have a foreach loop construct use iterators behind the scenes to keep track of the current position in the list. This is also why you should not modify the structure of the list while iterating through it using a foreach loop.

An iterator is a set of operations a data structure provides to allow users to access the items in the data structure sequentially, without requiring them to know its underlying representation. There are many reasons users might want to access the data in a list. For instance, users may want to make a copy of their list or count the number of times a piece of data was stored in the list. Or, the user might want to delete all data from a list that matches a certain specification. All of these can be handled by the user using an iterator.

At a minimum, iterators have two operations: reset and getNext. Both of these operations use the list class’s current attribute to keep track of the iterator’s current node.


The reset operation initializes or reinitializes the current pointer. It is typically used to ensure that the iterator starts at the beginning of the list. All that is required is for the current attribute be set to null.

function reset()
  current = null
end function

Get Next

The main operation of the iterator is the getNext operation. Basically, the getNext operation returns the next available node in the list if one is available. It returns null if the list is empty or if current is pointing at the last node in the list.

Lines 2 and 3 in the getNext operation check to see if we have an empty list, which results in returning the null value. If we have something in the list but current is null, this indicates that the reset operation has just been executed and we should return the data in the first node. Therefore, we set current = head in line 6. Otherwise, we set the current pointer to point to the next node in the list in line 8.

 1function getNext() returns data
 2  if isEmpty()
 3    return null
 4  end if 
 5  if current == null
 6    current = head
 7  else
 8    current =
 9  end if
10  if current == null
11    return null
12  else
13    return
14  end if
15end function

Next we return the appropriate data. If current == null we are at the end of the list and so we return the null value in line 11. If current is not equal to null, then it is pointing at a node in the list, so we simply return in line 13.

Remove current

While not technically part of the basic iterator interface, the removeCurrent operation is an example of operations that can be provided to work hand-in-hand with a list iterator. The removeCurrent operation allows the user to utilize the iterator operations to find a specific piece of data in the list and then remove that data from the list. Other operations that might be provided include being able to replace the data in the current node, or even insert a new piece of data before or after the current node.

The removeCurrent operation starts by checking to make sure that current is really pointing at a node. If it is not, then the condition is caught in line 2 and an exception is raised in line 3. Next, we set the next pointer in the previous node to point to the current node’s next pointer in lines 5 - 9, taking into account whether the current node is the first node in the list. After that, we set the next node’s previous pointer to point back at the previous node in line 10 - 14, considering whether the node is the last node in the list. Finally, we decrement size in line 15.

function removeCurrent()
  if current == null
    raise exception
  end if
  if current.previous != null =
    head =
  if != null = current.previous
    tail = current.previous
  end if
    current = current.previous
  size – size – 1
end function

Using an Iterator

There are several applications for list iterators. For our example, we will use the case where we need a function to delete all instances of data in the list that match a given piece of data. Our deleteAll function resides outside the list class, so we will have to pass in the list we want to delete the data from. We start the operation by initializing the list iterator in line 2, followed by getting the first piece of data in the list in line 3. Next, we’ll enter a while loop and stay in that loop as long as our copy of the current node in the list, listData, is not null. When it becomes null, we are at the end of the list and we can exit the loop and the function.

function deleteAll(list, data)
  listData = list.getNext()
  while listData != null
    if listData == data
    end if	
    listData = list.getNext()
  end while
end function

Once in the list, it’s a matter of checking if our listData is equal to the data we are trying to match. If it is, we call the list’s removeCurrent operation. Then, at the bottom of the list, we get the next piece of data from the list using the list iterator’s getNext operation.

By using a list iterator and a limited set of operations on the current data in the list, we can allow list users to manipulate the list while ensuring that the integrity of the list remains intact. Since we do not know ahead of time how a list will be used in a given application, an iterator with associated operations can allow the user to use the list in application-specific ways without having to add new operations to the list data structure.