CSC 207.02 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 (and writing some readings).
  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.
  • No office hours this week.

Upcoming work

• Assignment 5 due Thursday night.
• Exam 1 to be distributed Friday (or before).
• Reading for Wednesday: Osera, Chapter 4
• Lab writeup: Exercise 4
  • To csc207-01-grader@grinnell.edu
  • Subject: CSC 207.02 Writeup for Class 14 (Your names)

Extra credit

Extra credit (Academic/Artistic)

• Bucksbaum 152 4:00 p.m. today, talk on Bollywood and Neonationalism.
• Faulconer talk to be determined.

Extra credit (Peer)

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

• Neverland players: Friday @ 8:30, Saturday @ 2, 7:30, Sunday @ 2. Watch Grinnell student act out stories by elementary/middle students.

Questions

What’s a nonce?

Short for non-sense?

An integer that you use in a block to compute a hash.

You also use the hash of the previous block, an int that represents the data (for our assignment, the data are the money transfered between Alice/Anna and Bob) and an int that represents the block number.

The nonce helps us know whether or not a block is valid.

The nonce is a value that allows the hash to match a desired hash pattern.

This makes it harder to mess with the data.

How does mining relate to nonces?

Only certain nonces can make valid hashes for a particular set of data (prev block, money transferred, valid block).

We mine for nonces.

We mine for nonces by trying lots and lots of values until we find one that works. (a) random; (b) sequentially (Works = hash is valid)

What makes a hash valid? (At least for this HW?)

One that starts with three 0’s (hex digits)

What’s a byte?

A small integer, between 0 and 255.

Something that happens after you’ve been bit?

How much effort to mine, assuming three bytes?

1/256 for the first 0, 1/256 for the second 0, 1/256 for the third 0

About 256^3 prospective nonces must be generated

Or 16^3

Traditionally, we make it expensive to mine so that people can’t easily change the chain.

Why do we put nonces in a byte buffer?

Because we’re using MessageDigest objects to hash, and those reqire arrays of bytes. byte buffers provide the bridge.

What’s a hex value?

Four bits, 0 through 15.

How do we convert our byte to a a hex string?

String.format("%x", (int) b)

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 provides next and hasNext.

Used to step through the values in a collection.

Through repeated calls to next

Why do we use them?

Encapuslation: Client can see all values without knowing structure.

Generality: Client can write code to work with any collection.

What is an anonymous inner class?

A helper class that is unnamed.

Why do we use them?

Similar to the reasons we have anonymous helper functions: Use them in only one place, so there’s no need to name them.

Give access to the fields of the enclosing (outer) class.

How do we write them?

new Interface() { BODY; }

E.g.,

new Predicate<String>() { 
  boolean holds(String str) { 
    return string.length() < 5; 
  } 
}

Lab

Writeup: Exercise 4