Iterator Design (Scratch Simulation

A helpful trick for designing objects is to do an scratch simulation (scratch sim), scratched with pencil onto paper.

To implement an iterator, it takes very little coding. However, they can be tricky to design. Perfect candidates for scratch sim.

Simple Algorithm

Example #1 - Printing Objects

//Iterating over any object type
while iterator.hasNext()
	print(iterator.next());

Example #2 - Summing

//Iterating over numbers
Sum = 0
while iterator.hasNext()
	sum = sum + iterator.next();

Just Two Methods

Our entire communication with iterator is just using "hasNext" and "next".

While iterator.hasNext()
	doAnything(iterator.next());

Simulation

For an algorithm that looks so simple, it can be a head scratcher determining how to start. Who are the players (ivars)? How are parts are moving?

Time for scratch sim.

A Simulation Run

We start off simulating an iterator walking a short list.
Given this input: [10, 20, 30, 40]
The expected output is shown

Scratch Simulation

To produce the output, we need to send "get" multiple times.

We scratch them on our sketch.

10	get(0)
20	get(1)
30	get(2)
40	get(3)

Ivar Harvest 1

We see we need to send "get" to a DynamicArray. We'll call it "array".

Our first ivar of the design: "array"

Scratch Simulation (Revisited)

Now we see we have an index that is increasing: 0, 1, 2, 3

10	get(0)
20	get(1)
30	get(2)
40	get(3)

Ivar Harvest 2

Bingo, our next ivar: "nextIndex"

Design Pieces

Key Pieces

Name	Type	Description
DynArrayIterator	Class	The class to code
array	DynamicArray	ivar that we send "get" to
nextIndex	int	ivar that we use for "get" and that we increment

Key Actions

Constructor with parameter "array" (default "nextIndex" to 0)
Increment "nextIndex" after each "get"
Stop iteration when "nextIndex" is past last array position

Simple Test Code

It's super helpful to have even one simple test ready to go before we start coding. Below is an example.

Simple Test

Here we walk the iterator from start to end and make sure "next" answers 10, 20, 30, 40.

list = [10, 20, 30, 40]
iter = new Iterator(list)
index = 0
while (iter.hasNext())
	Test that iter.next() equals list[index]
	index++

Ready to Code

Now we're ready to code. Rather than just sitting down at the keyboard and trying to figure out this interesting object called an "iterator", we now have a plan.

Code up the "Simple Test Code"
Code up the iterator using the discovered "Design Pieces"

Coding Fundamentals (Chapter 201 - Ideas-Mode (Code Evaporation))

Chapter 201 - Ideas-Mode (Code Evaporation) Iterator Design (Scratch Simulation Design Trick)

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