Homework 4 (due Tuesday, Oct 29th, at 11:59pm PST)
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Watch the videos below, and do the homework.

Videos
------

Watch these videos (they're from last year, so there's a bit of a
context gap at the beginning; to catch up, just connect to the IPython
Notebook and run the contents of the 'course-00-update-notebooks'
notebook, and you'll be good).

1. `Loading data from local files <http://youtu.be/HL1Nuzog9kc>`__.

2. `Using modules in Python: introducing 'math' and 'csv' <http://youtu.be/YHS9MJZTVwY>`__.

See the Python stdlib `math docs
<http://docs.python.org/library/math.html>`__ and `csv docs
<http://docs.python.org/library/csv.html>`__.

**Also** see Doug Hellmann's excellent intermediate introductions to
these at his `Python Module of the Week (PyMOTW) site
<http://www.doughellmann.com/PyMOTW/>`__: `math -- mathematical
functions <http://www.doughellmann.com/PyMOTW/math/>`__ and `csv --
comma-separated value files
<http://www.doughellmann.com/PyMOTW/csv/>`__.  You don't *need* more
than what I've shown in the screencast to do the homework, but you
might consider skimming these to see what other goodies are available
in these modules in the stdlib.

4. `Introducing dictionaries <http://youtu.be/Pt0LlpDUcwk>`__.

Also see the Software Carpentry lecture on dictionaries: http://software-carpentry.org/4_0/setdict/dict/

5. `Plotting in ipython notebook with matplotlib <http://youtu.be/YSK3CboAOvU>`__.

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Homework 4
----------

1. After you update the notebooks repository on your EC2 instance by
   running the 'course-00-update-notebooks' notebook, you will have a
   new directory '/usr/local/notebooks/quartet', containing four files:
   q1.csv, q2.csv, q3.csv, and q4.csv.  Each of these files will be
   available to your program as ::

      '/usr/local/notebooks/quartet/q1.csv'

   etc. You can see their contents here:

      https://github.com/beacon-center/intro-computational-science/tree/master/quartet

   Write a notebook to:

   a. Load in the data in these files into two lists.  Please do this
      using a single function that takes a filename as an argument and
      returns two lists, x and y -- you might want to name them x1 and y1
      for q1.csv, x2 and y2 for q2.csv, etc.

   b. Calculate and print the average and standard deviation of y1,
      y2, y3, and y4.

   c. Plot the x and y points on four different plots.

   Hand in the homework by uploading it as a gist: log in via ssh,
   and run ::

      apt-get install -y ruby1.9.1 rubygems

   Then install 'gist'::

      gem install gist

   and when you're done with the notebook, save it, and execute
   '!gist notebook-name.ipynb'.  Then send me the gist info.

2. Write a function to 'invert' dictionaries, i.e. the function should
   take a single dictionary, and create a new dictionary that contains
   all of the values from the first dictionary as keys in the second,
   and all of the keys from the first dictionary as values associated with
   the appropriate key in the second dictionary.

   One possible approach:

     Do (b) by hand (i.e. no functions or loops), first; just write out
     the statements to create a new dictionary and assign the correct
     values to it.  Then do (c), too.

   Another approach:

     First, write a function that takes in a dictionary and returns
     a new, empty dictionary.

     Second, modify the function so that it prints out each key/value
     pair in the input dictionary.

     Third, modify the function so that it prints out each value, key
     pair.  (i.e. flip the order of things in the print statement.)

     Fourth, modify the function so that it assigns the value/key
     pair to the new dictionary rather than just printing the value/key.

   For the homework,

   a. Write the function; name it something like 'invert_d'.

   b. Take the following test dictionary (you can copy/paste into the notebook)::

        first_d = { 'first': 1, 'second': 2 }

      and make sure your function turns it into this::

        { 1: 'first', 2: 'second' }

      Write this as an assert statement::

        assert invert_d(first_d) == test_d

      Note!  If you swipe some online code it will probably not pass this
      test, so be careful :).

   c. Observe what happens when you pass in this dictionary::

        bad_d = { 'one': 1, 'uno': 1 }

      Explain what happens in your own words (English, please...).
      Is this 'expected' behavior?  Why or why not?

      Bonus pixie dust: Name one or more possible alternatives to this
      behavior.

  Also hand this notebook in as a gist.