CPSC 120 B

Lecture 33 - File Input/Output

As usual, create two directories for today's class. Create a directory called lecture33 under activities, and a directory called lab33 under labs.

File I/O

I have shown you how you can trick python into reading a file by using a file as standard input. This works as a bit of a hack, but sometimes we want to actually read a true file from the system. We can even write files using Python as well

Turingscraft won't do File IO apparently, so additional in class activities! As with last weeks Turingscraft exercises, you should work on your own for this activity.

Create a program in a file called fileio.py. The program should read a file called in.txt that consists of a list of integers, each on their own line. The program should write a file called out.txt that is the same as in.txt with all even integers removed. For example:

$ cat in.txt
1
2
3
4
5
6
$ python fileio.py
$ cat out.txt
1
3
5

Many of the programs that you use on a daily basis all have some element of persistence to them. For example, Emacs can read and write text files that the user writes, or the Messages list on your phone keeps track of all the text-messages you have received. In fact, these programs would be worthless if they couldn't store this data.

Even programs that you wouldn't typically think of require a large amount of stored data in order to accomplish their tasks. Mozilla Firefox, for example, creates an entire directory to store the files necessary to store the information that persists between browsing sessions. It is located in a hidden folder in your home directory called .mozilla/.

In fact, if you have ever get an error opening Firefox because it claims there is already a browser instance open, you can solve the problem by removing one of these persistent files:

$ cd ~/.mozilla/firefox/<some random string of characters>.default/
$ rm lock .parentlock

Since (seemingly) all useful programs rely on files, today you will write such a program. Your program will allow the user to write and store some information using your program. The choice is up to you, but here are some ideas:

• To-Do List:

  Let the user store their current list of things to do.

• CD/DVD Collection:

  Allow the user to catalog their CD/DVD collection.

• Journal:

  Let the user dictate their deepest and darkest secrets to your program. No peeking!

• Travel Blog:

  Let the user keep track of everywhere they have been.

Details

Create a program called user_database.py. This program will allow the user to enter in their information, and have it saved for later. This exercise is pretty open ended, but there are a few restrictions:

• Your program must read and store information that the user types in via the input function.

• Your program should ask the user which file they would like to use for this program at start-up.

• The main portion of your program will be a menu driven system that allows the users to add entries or read all entries. They should also be able to save their changes, and exit the program gracefully.

Example

$ python3 user_database.py
Please enter the file you wish to load from and save to: scotty_db
Welcome to the CD collection processor!
Your choices are:
(A) Add an entry
(R) Read all entries
(E) Exit the program
(S) Write collection to file

Enter your choice: A
Album Title: The Wall
Album Artist: Pink Floyd
Welcome to the CD collection processor!
Your choices are:
(A) Add an entry
(R) Read all entries
(E) Exit the program
(S) Write collection to file

Enter your choice: R
The Wall - Pink Floyd
Welcome to the CD collection processor!
Your choices are:
(A) Add an entry
(R) Read all entries
(E) Exit the program
(S) Write collection to file

Enter your choice: S
Welcome to the CD collection processor!
Your choices are:
(A) Add an entry
(R) Read all entries
(E) Exit the program
(S) Write collection to file

Enter your choice: E
$cat scotty_db
The Wall - Pink Floyd

Hint

$ cat scotty_db
The Wall - Pink Floyd
$ cat #scotty_db#
The Wall - Pink Floyd
Physical Graffiti - Led Zeppelin
Alpocalypse - Weird Al Yankovic
Incredibad - The Lonely Island
Greatest Hits - Queen
What a Wonderful World - Louis Armstrong
$ python3 user_database.py
Please enter the file you wish to load from and save to: scotty_db
There is a recovery file for this database.  Would you like to recover? Y
Welcome to the CD collection processor!
Your choices are:
(A) Add an entry
(R) Read all entries
(E) Exit the program
(S) Write collection to file

Enter your choice: R
The Wall - Pink Floyd
Physical Graffiti - Led Zeppelin
Alpocalypse - Weird Al Yankovic
Incredibad - The Lonely Island
Greatest Hits - Queen
What a Wonderful World - Louis Armstrong
...

Cryptography can be used to make the contents of a message secret, but sometimes you want to hide that a message is even being sent. Steganography is one way to hide messages that takes advantage of our inability to perceive small changes in colors. For example the image below consists of two boxes with different colors.

The left box has the RGB values (161, 187, 150). The box on the right has the RGB values (162, 188, 151). The two boxes appear to be the same color because the RGB values are similar. A message can be hidden in an image by making small changes to the RGB values of an existing image.

A common way of modifying an image is to change only the least significant bits of the RGB values. The least significant bit of a number is the right-most bit and represents the value 2⁰, or 1. Therefore, the most a color value will change if the least significant bit is change is 1, which the above image demonstrates as imperceptible. The following whale image, for example, has a message encoded in it.

Details

Create a function called decode_image(ppm_file_name) in a file called steganography.py. The program should return the secret message that is embedded in the specified image file. The message is encoded as 8-bit ASCII in the least significant bit of the image's color data. Assume that the number of color values in the file is a factor of 8. Once you have tested your program on a simple example, you can test it on the above whale image.

Some things to keep in mind:

• The first three lines of a PPM file specify header information for the image. You will have to skip these three lines.

• Each subsequent line specifies one color value of a pixel (Red, Green, and Blue repectively). You need to extract the least significant bit of each of these values.

• Once you have processed 8 color values, you will have to convert the binary sequence represented by their least significant bits into a single character. This is one character in the hidden message.

Example

>>> image_file = open("demo.ppm", "r")
>>> print(image_file.read())
P3
1 8
255
122
101
215
42
2
128
200
13
64
203
155
130
36
172
91
180
88
77
55
22
44
66
33
99
>>> print(decode_image("demo.ppm"))
abc

Hint

Submission

When you have finished, create a tar file of your lab33 directory. To create a tar file, execute the following commands:

cd ~/cs120/labs
tar czvf lab33.tgz lab33/

To submit your activity, go to cseval.roanoke.edu. You should see an available assignment called Lab Assignment 33. Only one of your pair should submit your activity. Make sure both partners are listed in the header of your files.

Do not forget to email your partner today's files!