In-class notes for 03/31/2014

CS 121B (CS1), Spring 2014

• Homework assignment

• Quiz returned

Submitted questions on assignments and technology

Upcoming

• Homework

• Instead of reading more of the WMR module, I will present some additional topics in class.

• Quiz on Wednesday. See below for topics.

Exercise: Strategies for counting words

an
death
great
I
love 
say
that
the

Word counting with WMR

• Question: How count word frequencies in a very large body (corpus) of text?

• WMR mapper code for word frequency count

  # IN keys and values:
  #   key: holds lines of text from the corpus   value: is empty
  # OUT keys and values:
  #   key: one word from that text  value: a string representing the number 1
  
  def mapper(key, value):
      words=key.split()
      for word in words:
          Wmr.emit(word, '1')
  

• WMR reducer code for word frequency count

  # IN keys and values:
  #   key: one word from a corpus  value: a string representing the number 1
  #   NOTE:  the values are packed into an interator  iter
  # OUT keys and values:  
  #   key: that one word from a corpus  value: the sum of the 1's from iter
  
  def reducer(key, iter):
      sum = 0
      for s in iter:
          sum = sum + int(s)
      Wmr.emit(key, str(sum))
  

• Try it! You can use the WMR installation at http://www.cs.stolaf.edu/wmr

  • Use your St. Olaf username and password. WMR should work on any browser.

Submitted questions on readings

To study for quiz

• Recursion with list arguments, including nested (non-linear) recursion

  • Problems similar to homework

  • Include #assert comments that describe the logical state of a computation, located on the line just before the next guard. Example

  • Use recursion only, no iteration (no loops)

• Tracing recursion

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