1. Notes about Homework #4
Professor Hugh C. Lauer CS-1004 — Introduction to Programming for Non-Majors
(Slides include materials from Python Programming: An Introduction to Computer Science, 2nd edition, by John Zelle and copyright notes by Prof. George Heineman of Worcester Polytechnic Institute)
CS-1004, A-Term 2014
Homework #4

2. Homework #4 Maximum function length = 50 lines
However, good practice  < 25 lines!
Make your modules into logical units — e.g.,
Reading files and picking apart input
Managing the list of words
Formatting and writing output file
Control (and testing)
Within each module, make functions into logical units
E.g., reading file and scanning lines are separate logical units!
CS-1004, A-Term 2014
Homework #4

3. Yeah! — I understand all that, but I just cannot get my head around it.
CS-1004, A-Term 2014
Homework #4

4. Let’s look at the pieces
Start near the beginning …
… but not at the beginning
One module, one function
Open one input file
Read it and turn it into a list of words
Ignore punctuation for now!
Close file!!
Test it!
Write a separate module called Control, Wrapper, Test, or something like that
Prompt user (i.e., YOU) for file name, call your function for that file, get returned list, print out list
Manually check, convince yourself that it is correct
Questions so far?
How?
CS-1004, A-Term 2014
Homework #4

5. Some students put this part in file-read module!
Next Step
In a new module, write one or more functions …
… go thru list of words
i.e., with a for-loop
Use string method strip() on each word
Remove leading and trailing quotes, commas, periods, semicolons, et.
Don’t worry if you miss some punctuation
You can come back later to fix
Also make lower case
Create new list of “cleaned up” words
Test again
Modify wrapper to take list from one module, pass it to another
Inspect result manually
Some students put this part in file-read module!
CS-1004, A-Term 2014
Homework #4

6. Step #3 Still in same module — new function
Sort the list of cleaned up words
Using list method sort()
Test and inspect manually
Are the words really in alphabetical order?
Need a for-loop
Go thru alphabetical list of words
Keep two variables outside the loop
Latest word
Count
If next word in loop is same as Latest Word
Increment Count
Otherwise
Append Latest Word and its Count to yet another list
Set Latest Word to next word, Count = 1
CS-1004, A-Term 2014
Homework #4

7. Testing Step #3
Beef up your wrapper (i.e., control module or test module, or whatever you call it)
Prompt for one file name
Invoke function to read file and build up list
Invoke functions to clean up words, strip punctuation
Invoke function to sort and count list
Print out and inspect result manually
CS-1004, A-Term 2014
Homework #4

8. Step #4 – output In yet another module …
… learn how to format output lines as specified
Learn and use string method format()
§5.8.2 (pp )
Open output file
Write output lines
Close output file
Test from Wrapper
Prompt user for file name
CS-1004, A-Term 2014
Homework #4

9. Step #5 Modify wrapper to get more than one input file name
Open and get list of words from each one
Combine lists from all input files
Generate combined output
CS-1004, A-Term 2014
Homework #4

10. Step #6 Modify wrapper to get input from command line
See next slide
Option:– if command line is empty, prompt user
Put it all together and test
CS-1004, A-Term 2014
Homework #4

11. Command Lines
Windows, Macintosh, and Linux all have “command prompt” windows
Command line format:–
verb arg1 arg2 arg3 ...
verb is name of a program that carries out command action
Each arg is a string
Delimited by spaces
arg0 is verb!
Meaning:– Apply verb to the list of arguments
Don’t return till finished!
CS-1004, A-Term 2014
Homework #4

12. Operating System’s Responsibility
Pick apart command line
Create a list of strings called “argv”
Number of items is list is “argc”
Load the program named verb (i.e., arg0) into a clean memory space.
Call the function with the name main(), passing argc and argv as arguments
Wait till it returns, continue with next command line
CS-1004, A-Term 2014
Homework #4

13. Starting programs in a GUI
User “opens” a file or document
OS or Window manager consults list of file types
Finds program that opens the type of this file or document
(Essentially) constructs a command line!
As if it had been typed
Name of verb (i.e., program) as arg0
Name of file to be opened as arg1
Other arguments as needed
Calls main() function of the program!
CS-1004, A-Term 2014
Homework #4

14. python HW4.py outFile InFile1 InFile2 …
What about Python?
Command must be python or python3
Command line must be
python HW4.py outFile InFile1 InFile2 …
Getting the arguments into Python
sys.argv is a list containing the strings:–
['HW4.py', 'outFile', 'InFile1', 'InFile2', …]
Windows
Macintosh Linux
CS-1004, A-Term 2014
Homework #4

15. Questions?
CS-1004, A-Term 2014
Homework #4