CSC 207.01 2019S, Class 14: Iterators

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Quiz
• Preparation
• Lab

Preliminaries

News / Etc.

• I spent the weekend grading a CSC 151 exam. Our lab may a bit more ad-hoc than usual.
• I will be off at SIGCSE for the rest of the week. Dr. Vostinar will be covering our classes.

Upcoming work

• Assignment 5 due Thursday night.
• Exam 1 to be distributed Friday.
• Reading for Wednesday: Osera, Chapter 4
• Lab writeup: 3
  • To csc207-01-grader@grinnell.edu
  • Subject: CSC 207.01 Writeup for Class 14 (Your names)
  • Include the code for your iterator in the body of the message.

Extra credit

Extra credit (Academic/Artistic)

Extra credit (Peer)

• Neverland Players, Friday (8:30), Saturday (2:00 and 7:30), and Sunday (2:00)

Extra credit (Wellness)

• Student Wellness Fair, Tuesday, 5-7pm, JRC 1st and 2nd floors.

Extra credit (Wellness, Regular)

• 30 Minutes of Mindfulness at SHACS every Monday 4:15-4:45
• Any organized exercise. (See previous eboards for a list.)
• 60 minutes of some solitary self-care activities that are unrelated to academics or work. Your email reflection must explain how the activity contributed to your wellness.
• 60 minutes of some shared self-care activity with friends. Your email reflection must explain how the activity contributed to your wellness.

Extra credit (Misc)

Other good things

Questions

Talk to me about the toString method in hashes.

    for (int i = 0; i <= 255; i++) {
      pen.println(String.format("%d: %x", i, i));
    } // for

Why did Alice and Bob turn into Anna and Bob?

Foolish consistency is the hobgoblin of small minds. I’ll fix it.

What’s a nonce?

Something that’s hard to explain.

A bit like a password.

It’s a number that allows us to generate a valid hash.

How does mining relate to nonces?

Mining is a process in which we repeatedly generate candidate nonces until we find one that gives us a valid hash.

What makes a hash valid?

For this assignment, the first three bytes must be zero.

What’s a byte?

An integer between 0 and 255.

How much effort to mine?

In this case, it appears that only 1/(256^3) hashes are valid, so about 256^3 nonces must be generated.

Quiz

If you finish early, take the time to sit quietly. Alternately, get a drink of water or use the restroom.

Lab Prep

What is an iterator?

An object that allows you to visit each element of a collection one by one.

How does it do that?

Through two methods, next() and hasNext().

Client repeatedly calls next() to visit the elements. Knows to stop when hasNext() returns false.

Why do we use them?

Allows the client to access the elements in the collection.

Lets us write more general code.

Provides encapsulation: Client does not need to know the innards of our collection.

What is an anonymous inner class?

A class that you can make within another class.

They don’t have a name, that’s why they’re anonymous.

Why do we use them?

Good for helper classes (just like anonymous functions were usuful as helpers in Scheme), particularly when we only want to use them once.

How do we write them?

Something like the following. Here’s a counter that increments by ten, starts at 5, and resets to 0.

  new Counter() {
    int i = 5;
    public int get() { return i; }
    public int increment() { i += 10; }
    public int reset() { i = 0; }
  }

foo(new Predicate<String>() { public boolean test(String str) return false; });

Lab

Writeup: Exercise 3

public ArrayBasedQueueIterator<T> implements Iterator<T> {
  ArrayBasedQueue<T> abq;
  int i;

  public ArrayBasedQueueIterator(ArrayBasedQueue<T> abq) {
    this.abq = abq;
  }
  // ...  this.abq.values[pos] ...

}

public ArrayBasedQueue<T> implements Iterable<T> {
  // ...
  Iterator<T> iterator() {
    return new ArrayBasedQueueIterator<T>(this);
  }
  // ...
}