1. Spring 2010
EECS 110: Homework III

2. Homework III
Evolving Lists & Lights On!
Caesar Cipher
Looks Good!

3. Problem 2: Caesar Cipher Overview
encipher(S, n)
Rotates the string S by n characters
Example:
encipher('AbC', 2) returns 'CdE'
encipher('xYz', 4) returns 'bCd'
decipher(S)
Retrieve the original string from an enciphered version (with unknown rotation)

4. Problem 2: Caesar Cipher encipher – Overview
Recursion
Rotate character individually
Rules:
Lower-case → lower-case
Upper-case → upper-case
Otherwise → unchanged
Character checking:
'a' <= c <= 'z'
'A' <= c <= 'Z'
Character rotation: using ASCII values

5. Problem 2: Caesar Cipher encipher – Character Rotation
ASCII values
Useful functions:
ord('a') returns 97
chr(66) returns 'B'
Example: rotate 'b' by 22 characters: chr(ord('b') + 22) returns 'x'
Note: need to take care of wrapping around A B C … X Y Z 65 66 67 88 89 90 a b c x y z 97 98 99
120
121
122

6. Problem 2: Caesar Cipher decipher – Strategy
Unknown number of rotation → Test 26 cases and pick out the best
Techniques:
English letter appearance probability (provided!)
Scrabble scores (roughly a reverse of the above)
Any other heuristics (i.e. rules of thumb)
Should work well on large strings

7. Problem 2: Caesar Cipher decipher – Example Flow
Create a list of all possibilities (there are 26) #['Dmc', 'End', 'Foe', ...]
For each possibility, calculate the sum of the probability of the letters #[0.0766, , , ...]
Return the one with the largest sum (using recursion? using for loop?)

8. Problem 3: Looks Good Overview
See class web-site for set-up instruction
Goals:
Write basic image editing tools
At least 3, one from each of the following:
Group 1: negative, gray scale
Group 2: vertical flip, horizontal flip, vertical mirror, horizontal mirror
Group 3: scale, blur, random grid

9. Problem 3: Looks Good Image
An image is a 2 dimensional list (i.e. list of lists) of pixels
Example: pixels = [[pixel, pixel], [pixel, pixel], [pixel, pixel]]
Note:
An image is a list of rows A row is a list of pixels
len(pixels) returns height
len(pixels[0]) returns width

10. Problem 3: Looks Good Pixel Color
A pixel is a tuple of 3 colors red, green, blue
Color value is from 0 to 255
Example:
(255,0,0) is red
(100,100,100) is gray
(112, 48, 160) is purple
List uses [ ], tuple uses ( )
No difference (for now)

11. Problem 3: Looks Good Put them together
pixels = [[(255,0,0), (0,255,0)], [(0,0,255), (100,100,100)], [ (0,0,0), (112,48,160)]]

12. Problem 3: Looks Good Example Code: Brighten
The beginning from modifyBase import * label = "Brighten" # change this ordinal = 1
Similarity in roles to problem 1:
modify(pic) is similar to evolve(L)
setNewPixel is similar to setNewElement
In general, modifying setNewPixel is enough, but feel free to do anything else.

13. Problem 3: Looks Good Functions to implement
Group 1 (individual pixel):
Negative: new color value = 255 – original value
Grayscale (values of red, green & blue are equal)
Group 2 (change pixel position):
Flip/Mirror horizontally/vertically
Group 3 (using multiple pixels):
Scale
Blur: take an average of surrounding pixels
Random Grid: Use random.shuffle(L)

14. Homework III
Have fun + Good luck