DESCRIPTION

Students:  Please keep in mind the OMSI rules.  Save your files often,
make sure OMSI fills your entire screen at all times, etc.  Remember
that clicking CopyQtoA will copy the entire question box to the answer
box.  

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*  Don't forget to FREQUENTLY save and submit your work, even work    *
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In questions involving code which will PARTIALLY be given to you in the
question specs, you need to add new lines.  There may not be information
given as to where the lines should be inserted.  Do not modify the lines
already there unless instructed to do so.

If a question includes test code, make sure to include it in your
submission.

IF YOU FINISH EARLY, please leave the exam room.  If you stay, close
your laptop and sit quietly.

QUESTION -ext .py -run 'python omsi_answer1.py'

The iterator class cq on p.121 may be somewhat inefficient, due to all
that rearranging of the list self.q.  (Note that although we used cq as
a circular queue of clients, it was generally applicable.) Here you will
modify it to a new class, cqIdxed, that simply makes use of a list
index.  The index starts at 0, increasing by 1 on each iteration, until
it goes past the end of the list, upon which it reverts to 0.  Complete
the code:

class cqIdxed:
   def __init__(self,q):
      self.q = q
      self.idx = 0
      self.lenQ = len(self.q)
   def __iter__(self): return self

myQ = cqIdxed([5,12,13])
i = 0
for qElement in myQ:
   print qElement  # prints 5,12,13,5,12
   i += 1
   if i >= 5: break 

QUESTION -ext .py -run 'python omsi_answer2.py'

Here you will write a class collDirs, "collection of directories", which
will be an iterator for processing each directory in the collection.  If
an element of the collection turns out to not be a directory, the code
will print an error message and raise a StopIteration exception.

class dirColl(list):
   def __init__(self,dirList):   
      self.dirList = dirList
      self.numDirs = len(dirList)
      self.i = 0  # indexes the collection
   def __iter__(self): return self

import os

try: 
   os.mkdir('x')
except: 
   pass
try: 
   os.mkdir('y')
except: 
   pass
try: 
   f = open('z','w')
   f.writelines('123\n')
   f.close()
except:
   pass

dc = dirColl(['x','y'])
for d in dc: print d  # prints 'x', 'y'

dc1 = dirColl(['x','z','y'])
for d in dc1: print d  # prints 'x', then 'z is not a directory'