[Courses] [python] Lesson 4: Modules and command-line arguments

Leslie leslie.brothers at verizon.net
Sat Jul 9 21:30:18 UTC 2011

Help! Regarding exercise 4, my "problem" is that the results for wc -w
and for the program I wrote to count words in a file, come out the same.
I wanted to try the debugging part of the exercise, so I used different
kinds of files (some of the python programs I wrote for previous
questions; some text files with spaces, tabs, multiple lines and so on),
to try to make the results come out different and still the results come
out the same.
Is there a file that is "guaranteed" to give me a different result using
our python techniques from this lesson, and wc -w, and if so can someone
send it to me?
It would be very weird if I accidentally hit on the one way to get a
bullet-proof result with my program -- I'm not that good at programming!
But here's my program:

import sys
file = open(sys.argv[1])
y =0
for line in file :
	x = len(line.split())
	y = y + x
print y

This will probably be the only time in my life when I complain about NOT
having something to debug.
I must have missed something, somewhere.

> 4. Here's a harder problem, an exercise in debugging (which is a big
>    part of programming, sadly):
>    a. Write a program that counts words in a file (or multiple files,
>       if you prefer). Use the same split() and len() you used in
>       lesson 2.
>    b. Compare the number of words from your program to what wc -w gives.
>       (If you're on a platform that doesn't have wc, run it on a small
>       file and count by hand.) Are the answers the same?
>    c. Here's the debugging part: why aren't they the same?
>       (You don't have to fix it: just figure out the problem.)
>       Hint: if you're splitting each line into a list, try printing
>       the list to see what's in it. In python, if you have a list
>       called words, you can just say print words -- you don't have to
>       do anything fancy like you would in some languages.
>    d. OPTIONAL, harder: fix the problems and make your word count
>       program give the same answer as wc -w.
>       Hint 1: one Python function that will come in handy is strip():
>       it strips off any leading and trailing spaces. So if you have
>       a string s = "     hello, world     ", then s.strip() would
>       give you "hello, world".
>       By the way, I haven't mentioned Python's documentation, but
>       most Python modules have excellent online docs. Here's strip():
>       http://docs.python.org/library/string.html#string.strip
>       Hint 2: If you're inside a loop, say, looping over lines, and
>       you decide you don't care about this line, you can skip to the
>       next one by saying:
>           continue
>       For instance, in a loop where you don't care about negative numbers:
>       for i in list_of_numbers :
>           if i < 0 :
>               continue
>           do_stuff_for_positive_numbers(i)
>       You can break out of a loop completely with: break
>       Don't drive yourself too crazy trying to get an exact match
>       with wc. There are some special cases where splitting at spaces
>       might not give the same answer as wc -w, and there are some
>       other Python modules (specifically re, regular expressions)
>       that can do a better job. The purpose of this exercise is to
>       give you a taste of debugging, fixing problems as you find them
>       and thinking about what special cases might arise.
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