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

[Erin McLaughlin]

> ========================= Homework ============================
>
> 1. With the little example I gave earlier, the one that used
>     num = int(sys.argv[1]):
>     if you run it and don't give an argument, you'll get an error.
>     Why? Can you think of a way to check whether the user forgot to
>     supply an argument, and print an error message if so?
Because you're trying to get something that does not exist.

import sys

args = sys.argv
if len(args) != 2 :
     print "Error!"
else :
     print "Good!"

> 2. Write a program that takes a filename and prints the number of
> lines in the file. (You can check its results with wc -l filename.)
import sys

args = sys.argv
if len(args) != 2 :
     print "Usage: hw4 FILE"
else :
     f = open(args[1])
     count = 0
     for line in f :
         count += 1
     print "The file", args[1], "has", count, "lines."
> 3. How would you extend this so that you can count lines in multiple
> files, not just one? So you could say
> $ mywordcounter file1 file2 file3
import sys

args = sys.argv
if len(args) < 2 :
     print "Usage: hw4 FILE1 [FILE2]..."
else :
     for fn in args[1:] :
         f = open(fn)
         count = 0
         for line in f :
             count += 1
         print "The file", fn, "has", count, "lines."
> 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.
import sys

args = sys.argv
if len(args) != 2 :
     print "Usage: hw4 FILE"
else :
     f = open(args[1])
     count = 0
     for line in f :
         words = line.split(" ")
         count += len(words)
     print "The file", args[1], "has", count, "words."
>     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?
One off on the particular file I'm using.
>     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
Since I'm counting the words in a python file, it's counting each colon 
that comes after a for statement as a word, since there's a space before 
it.  Except this apparently is incorrect?  Because there are more than 
one colons in the file I'm using.  So the problem is blank lines/lines 
with just spaces in them.
>     d. OPTIONAL, harder: fix the problems and make your word count
>        program give the same answer as wc -w.
import sys

args = sys.argv
if len(args) != 2 :
     print "Usage: hw4 FILE"
else :
     f = open(args[1])
     count = 0
     for line in f :
         line = line.strip()
         if len(line) > 0 :
             words = line.split(" ")
             count += len(words)
             for w in words :
                 print w
         else :
             continue
     print "The file", args[1], "has", count, "words."

Victory!!