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Lab 29: Files

As usual, create a directory to hold today's files. All programs that you write today should be stored in this directory. 

$ cd ~/cs120/labs
$ mkdir lab29 
$ cd lab29

Practice 1

Write a function called average_file(opened_file), which takes as a parameter an opened file. The opened file is a file that contains only numbers, each on their own line. Your function should return a floating point number, which is the average of all of the numbers from the file.

from io import StringIO # DO NOT REMOVE. NEEDED FOR TEST CASES def average_file(opened_file): 








  
Practice 2




  Write a function called non_blank_lines(opened_file), which
  takes an opened file as a parameter.  Your function should return a list of
  lines from the file, which contains only lines that are not blank.




  

from io import StringIO # DO NOT REMOVE.  NEEDED FOR TEST CASES

def non_blank_lines(opened_file):
    
  

  

    
  

  









Steganography




  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, and a common technique
  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:





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




  
  

    
    
      
  • 
        
    
	  You are going to use the accumulator pattern here for
          strings, accumulating the least significant bit of the color
          values into a string.  A number that is odd has a 1 as the least
          significant bit, a number that is even has a 0 as the least
          significant bit.
        
    
      
    • 
      
  • 
        
    
          You can convert a binary string to a base-10 integer by
          using the built-in
	  int
          function.
        
    
      
    • 
      
  • 
        
    
          You can convert an integer to a character using the
          built-in chr
          function.
        
    
      
    • 
    

  





 




Challenge

  

    Write a function that can embed messages into images.  The
    function should take three parameters: a message, an input image
    file name, and an output image file name.  It should read in the
    input image, modify the least significant bits of the color values
    to embed the message, and write the new image to the specified
    output file.