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

[Kay Nettle]

 >========================= 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?

There is no sys.argv[1], so you get an error.

import sys
if len(sys.argv) != 2:
        print "%s arg" % sys.argv[0]
        sys.exit(1)


 >
 >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.)

(I use mh as my mail and I've forgotten how to start a line with #
so it's indented one space)

 #!/usr/bin/python

import sys, os
def usage():
	print "%s: file" % (sys.argv[0])

if len(sys.argv) != 2:
	usage()
	sys.exit(1)

if not os.path.isfile(sys.argv[1]):
	print "No such file"
	sys.exit(1)

lc = 0
file=open(sys.argv[1])
for line in file:
	lc += 1


file.close()
print "The number of lines in %s is %d" % (sys.argv[1], lc)

 >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

 #!/usr/bin/python

import sys, os

def usage():
        print "%s: file [file]" % (sys.argv[0])

if len(sys.argv) < 2:
        usage()
        sys.exit(1)

for arg in sys.argv[1:]:
        if not os.path.isfile(arg):
                print "No such file %s" % (arg)
                continue

        lc = 0
        file=open(arg)
        for line in file:
                lc += 1

        file.close()
        print "The number of lines in %s is %d" % (arg, lc)



 >
 >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.

 #!/usr/bin/python

import sys, os

def usage():
        print "%s: file [file]" % (sys.argv[0])

if len(sys.argv) < 2:
        usage()
        sys.exit(1)

for arg in sys.argv[1:]:
        if not os.path.isfile(arg):
                print "No such file %s" % (arg)
                continue

        wc = 0
        file=open(arg)
        for line in file:
                wc += len(line.split())

        file.close()
        print "The number of words in %s is %d" % (arg, wc)

 >
 >   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?

Yes.

 >
 >   c. Here's the debugging part: why aren't they the same?
 >      (You don't have to fix it: just figure out the problem.)

Mine are the same since I didn't do line.split(' ').  line.split()
removes all the extra white spaces.