COMP2511

☕  8.2 - Iterator Pattern

In this lecture

Why?​

  • Understand the concepts of iterators and iterables
  • Understand the motivation for the Iterator Pattern
  • ​Discuss implementation of the Iterator Pattern in different languages 

 

How does a for loop actually work?

List<String> shoppingList = new ArrayList<String>(
  Arrays.asList(new String[] {
    "apple", "banana", "pineapple", "orange"
}));

for (String item : shoppingList) {
    System.out.println(item);
}

Under the hood

Iterator<String> iter = shoppingList.iterator();
while (iter.hasNext()) {
    String item = iter.next();
    System.out.println(item);
}
  • An iterator is an object that enables a programmer to traverse a container
  • Allows us to access the contents of a data structure while abstracting away its underlying representation
  • In Java, for loops are an abstraction of iterators
  • Iterators can tell us:
    • Do we have any elements left?
    • What is the next element?

Iterators

Custom Iterators

Traversing a Data Structure

  • Aggregate entities (Containers)
    • Stacks, Queues, Lists, Trees, Graphs, Cycles
  • How do we traverse an aggregate entity without exposing its underlying representation?
  • Maintain abstraction and encapsulation
  • Initial solution - a method in the interface
    • What if we want multiple ways to traverse the container?

Abstracting the Traversal

  • Seperate Containers, Iterators and Algorithms
  • Allows for many possible ways of traversal
  • Avoid bloating interfaces with different traversal methods
  • Client (Algorithm) requests an iterator from the container
  • Container needs to provide a method for creating an iterator, to show that it is iterable

Iterators vs Iterables

  • An iterable is an object that can be iterated over
  • All iterators are iterable, but not all iterables are iterators
  • For loops only need to be given something iterable

Iterator Invalidation

  • What happens when we modify something we're iterating over?
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9]

for number in numbers:
    if number == 3 or number == 4:
        numbers.remove(number)

print(numbers)

Design by Contract

  • In many languages, part of the postconditions of iterators is that modifying the container in certain ways causes the iterator to become invalidated (the behaviour of the iterator is undefined)
    • Python
    • C++

Iterator Invalidation: Java

  • What happens when we modify something we're iterating over?
List<Integer> numbers = new ArrayList<Integer>(
  Arrays.asList(new Integer[] {1, 2, 3, 4, 5, 6, 7, 8, 9}
));

for (Integer number : numbers) {
    if (number.equals(3) || number.equals(4)) {
        numbers.remove(number);
    }
}

System.out.println(numbers);

Iterator Invalidation: Java

  • What happens when we modify something we're iterating over?
Exception in thread "main" java.util.ConcurrentModificationException
        at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1043)
        at java.base/java.util.ArrayList$Itr.next(ArrayList.java:997)
        at dungeonmania.DungeonManiaController.main(IterExample.java:120)

Generators

  • A functional way of writing iterators
  • Defined via generator functions instead of classes
  • Example generator
def shopping_list():
    yield 'apple'
    yield 'orange'
    yield 'banana'
    yield 'pineapple'

for item in shopping_list():
    print(item)

Iterator Categories (C++)

  • Output (Write-only)
  • Input (Read-only)
  • Forward (most iterators, standard Java iterators)
  • Bidirectional (forward and backwards)
  • Random Access (iterators which function as arrays)