knerr cs21 notes...

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WEEK07: File I/O, top-down design
 W: start top-down design, more file I/O

 - Lab 6 due Thursday night


  Top Down Design is a problem solving technique where you:

   1. Start with General Description of Problem
   2. Break it into several high-level steps
   3. Iteratively break the steps into smaller steps until you have 
      steps that are easy to solve 

 - just like writing a paper...start with an outline, then fill
   in the second-level details, and so on until you can start
   writing each section/function

 - for example, writing a wheel-of-fortune game might start
   with something like this:

        main:     read phrases from file
                  pick one phrase for the game
                  play the game, with the chosen phrase
                  output the results
   Each one of those could be a single function, or may need to
   be broken down into multiple functions. If you think one of
   the above should be a single function, sketch it out (what
   arguments would it need, what would it do and how, and what
   should it return). If you think one of the above (ex: play the
   game) will be more than one function, do another level of 
   top-down design on that.


 - Let's try using top-down design on a "wordcount" program.
   There's a unix program called wc that tries to count the 
   number of lines, words, and characters in a file. Here's
   an example of it:

$ wc /usr/local/doc/GettysburgAddress 
  25  266 1441 /usr/local/doc/GettysburgAddress

   The wc program says there are 25 lines in that file, 266 "words",
   and 1441 characters.

   Can we write our own wordcount program???

   Here's one way to structure the main function:

         read in a file
         count the number of lines
         count the number of "words"
         output the results

   Now let's refine each of the above steps:

         read in a file
           takes one parameter: the file name to open 
           what it should do: read each line from the 
             file and store it in a list
           returns: the list of lines

   Note: there are many other ways you could do this! Let's
         try this and see if it works. Sometimes I write 
         half a program before I realize it's not going to
         work the way I've designed it...

   that should be enough to write the first step:

def readFile(fname):
  infile = open(fname, 'r')
  lines = []
  for line in infile: 
  return lines

 T E S T  AS YOU GO!!!!

 at this point, type in a simple main function and
 make sure readFile works!

def main():
  linelist = readFile("/usr/local/doc/GettysburgAddress")
  print linelist

  The next step in our top-down design is "count the number of lines".
  We could have a function do this, but it's just a one-liner (using
  len), so maybe we should just write it in main:

  nlines = len(linelist)

  The next step is count the number of "words". I keep putting
  words in quotes because we are not really going to count the
  number of english words -- we're just going to count the number
  of "things" separated by white space. For example, both of 
  these lines have 5 "words" in them:

this is really quite fun

1 2 3 4 pizza

  (really counting the number of valid English words can be done,
   it's just more complicated)

  Here's an example of using the str split method:

>>> s = "i love computer science"
>>> s.split()
['i', 'love', 'computer', 'science']

  Can we use that and len to get the number of "words"?

  Let's outline a countWords functions:

  parameters: a list of lines
  returns: number of "words" in the whole list of lines
  outline: an accumulator that sums up the number of
    words in each line

def countWords(llist):
  count = 0
  for line in llist:
    n = len(line.split())
    count = count + n
  return count

 Now we're almost done...just put it all together and make a
 nice "output results" part:

def main():
  filename = raw_input("file: ")
  linelist = readFile(filename)
  nlines = len(linelist)
  nwords = countWords(linelist)
  print "number of words = %d" % (nwords)
  print "number of lines = %d" % (nlines)


  - add a function to count the number of characters