CS414 Assignment 7

Before You Begin:

• Create a folder on your computer where your files will be kept. Create a separate python file for each question!
• Download these input files into your folder:
  • face.txt
  • flag.txt

Run-length encoding

One part of the standard used to send black-and-white faxes involves run-length encoding. This is a way to use fewer values to represent a stream of data which has lots of duplicate values.

For example, the sequence of values 5, 5, 5, 5, 2, 2, 1, 2, 2, 2, 0, 0, 0, 0 has 14 values, but could be written as 'four 5s, then two 2s, then one 1, then three 2s, then four 0s', or 4, 5, 2, 5, 1, 1, 3, 2, 4, 0 (just 10 values). Thus we have compressed the data, using 4 fewer values.

Write a program rle.py which does run-length encoding.

The program should work as follows:

• query the user for the name of an input file.
• query the user for the name of an output file.
• open the input file for reading, and read all its lines.
• close the input file.
• open the output file for writing.
• go through all the input lines that you have read:
  • split each line into fields.
  • convert each field into an int value.
  • compare the value against the previous value.
  • if they're the same, increase the run length.
  • if they're different:
    • write two numbers (length and value) to the output file.
    • since write is less powerful than print, you'll have to include a blank between output numbers.
    • Don't let the output lines get too long. If you have written 10 (length value) pairs, write a '\n' newline.
• don't forget about the final (length value) pair!
• close the output file.

Save your program in a file called rle_compress.py

Decoding

Implement another program, called rle_expand.py. It should do the opposite of the previous program.

In other words, it should ask the user for input and output file names. Then it should read from the input file, and write to the output file.

Unlike the rle_compress.py, the new program rle_expand.py should process two numbers at a time (a run length, and a value), and should write one or more values to the output file (separated by spaces, of course).

Check that both programs have been implemented correctly, by first compressing a file, then expanding it, and see if you get the original values back.

Turning in your work