In-class notes for 04/14/2014

CS 121B (CS1), Spring 2014

• Homework assignment

• Questions on past homework assignments welcomed. I will select one per day.

• Quiz moved to Friday this week. (Objects, genomes)

• Grade reports...

Heartbleed security bug

• First message: Change your passwords ASAP on Facebook, Google/Gmail, Instagram, Pinterest, Tumblr, Yahoo, Yahoo Mail, GoDaddy, Flickr, Minecraft, Netflix, SoundCloud, YouTube, Box, DropBox, GitHub, OKCupid, Wikipedia (if you have an account), Wunderlist, maybe others.

  Note: Most of these have taken corrective action, but changing your password insures that your account will be safe from then on.

• But don't change your password yet on Skype, Twitter, Wordpress, and other sites that haven't/may not have taken action yet. (And remove anything you don't want to be seen!)

• The technology problem: OpenSSH is free security software that is used by most websites that need security (https, green padlock). A bug was discovered by security experts that enables someone to see files on sites that use OpenSSH. The fix requires a software upgrade for OpenSSH, and not all companies have done this yet.

• Security is an example of a computing ethics issue. Others include:

  • Accessibility - access for differently abled people

  • Intellectual property - who owns the ideas expressed in software and systems?

  • Privacy - controlling who may know what about you

  • Honesty and deception using computing

  • Risk and reliability of computing systems

  • Uses of power related to technology

  • Quality of life, which may be affected for better or worse using computing technology

• St. Olaf has an innovative course on Ethical Issues in Software Design (ESD, CS 263) that takes a social-science approach to learning about ethical issues. The course includes a term-long team project in which students consult with the ethical issues in real companies/entities about real software. (Spring, prerequisite SD)

Submitted questions on assignments and technology

• Solution to colorString() problem developed: turtles; solving simpler problem first; looping through string; spacing past drawn characters; then adding turtle color using dictionary

Upcoming

• Homework 24

Submitted questions on readings

Defining classes

• Example class specs: Account.html, and spec for Plant class

• Memory diagram of an Account object (see end of Account.html)

  Objects in the same class have the same state-variable structure, although each has its own separate memory locations.

• Syntax of a class definition

WMR exercises

• Exercise 1:

  Compute the average ratings for all movies

  • Sample data set. For that data set, the results should be:

            1	3.3333
            3	4
            6	4.5
            8	4.5
    

  • mapper() specs

    # IN format
    #    key is a netflix record   value is empty string
    #    NOTE: netflix record format is   movieID,reviewerID,rating,date
    # OUT format
    #    FILL THIS IN
    

  • reducer() specs

    # IN format
    #    FILL THIS IN
    # OUT format
    #    key is a movieID  value is the mean movie rating for that movie
    

  • Hints: (a) COMPLETE THE SPECS FIRST -- what intermediate key-value pairs will you need? (b) Use two accumulators in reducer, one for sum and one for count, in order to compute the mean

  Sample data set:

  1,1596531,5,2004-01-23
  3,2318004,4,2004-02-05
  6,110641,5,2003-12-15
  8,1447639,4,2005-03-27
  8,2557899,5,2005-04-16
  6,52076,4,2004-10-05
  1,13651,3,2004-06-16
  1,1374216,2,2005-09-23
  

• Exercise 2:

  Compute word frequencies per book and overall frequencies, in a single map-reduce cycle.

  Example input:

          cathat	The cat in the hat
          cathat	wore the hat
          cathat	to the cat hat party.
          OwlCat	The owl and the pussy cat went to sea
  

  Final key-value pairs emitted from reducer:

          The	OwlCat 1
          The	cathat 1
          The	2
          and	OwlCat 1
          and	1
          cat	cathat 2
          cat	OwlCat 1
          cat	3
          ...
  

  • mapper() specs

    # IN format
    #    key is a book ID  value is a line of text from that book
    # OUT format
    #    FILL THIS IN
    

  • reducer() specs

    # IN format
    #    FILL THIS IN
    # OUT formats (there are two)
    #    1. key is a word  value is a book ID, a space, and freq of key in that book
    #    2. key is a word  value is total freq of that word in all books
    

  Hints:

  (1) fill out the specs first -- what should key-value pairs be for example? (2) Use a fake "book id" 'zzzz' to count frequencies for all books, and handle that fake ID specially in reducer to get second output format.

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