CS414 Lab 9: Cypher Tools

For this lab, you will prepare a several files, one for each exercise.

Before you begin, create a folder for your programs. Then download these starting files:

• crypto.py
• quotes.txt

Turning in your work

Late penalties: 1 day late: -5%, 2 days: -10%, -3 days: -20%.

To turn your work in, go to mycourses.unh.edu, find CS414, open the syllabus, and click on Lab 8. Then click Submit Assignment, click Choose file, and upload crypto.py .

Exercises:

1. String replacement : Run IDLE, and open the file crypto.py. The function replace_text() doesn't work: run the program to see for yourself.

   The error is in this line:

               text[index] = new_text
             

   The problem is that strings are immutable (you can't change any part of a string). Try using list slices instead:

               text[index] = text[ : index] + new_text + text[index + 1 :]
             

   Also, try another approach: explode the letters into a list, change a list entry, and join the letters again:

               text_letters = list(text)
               text_letters[index] = new_text
               text = ''.join(text_letters)
             

2. Scramble a string : Implement the function scrambled() .

   Proceed as follows:

   • explode text into a list
   • call random.shuffle(), which modifies the list
   • join the scrambled list together again

3. Caesar cypher : Implement the function shift_encypher() .

   Notice the string alphabet. Initialize it as follows:

               alphabet = string.lowercase
             

   (This has the same result). You will now create a cypher alphabet, with the letters shifted forward by shift spaces. For example, if shift==3, you should get 'defghijklmnopqrstuvwxyzabc'.

   To do this, you should initialize cypher_alphabet by joining together two slices of alphabet:

   • the first shift characters
   • the characters after the first shift characters.
   The remaining job is to replace each letter in text with the corresponding letter in cypher_alphabet. Proceed as follows:
   • create a result string, initially empty, ie "" .
   • make a loop that visits each letter in text:

                     for letter in text:
                     ...
                   

   • in the loop, check if letter is in the alphabet (the text may have spaces or punctuation). If not, just add the original letter to the result.
   • otherwise, find the location of letter within alphabet (use the index() function).
   • using the same location, get a letter from cypher_alphabet and add it to the result.

4. Caesar cypher 2 : Implement shift_decypher, which performs the action opposite to shift_encypher. This function is almost identical, except that you should find a location in cypher_alphabet, and get the same letter in alphabet

5. Reflowed text : Implement print_reflowed, which opens a file, reads the text, and prints its text with a maximum witdh.

   The function includes code to read a file and join all its text into one long string. The remaining task is this:

   • Split() the text into a list of words.
   • Make a for-loop that visits each word in the list.
   • Keep track of the length of the current printed line:
     • If the next word, plus a space, will exceed width, do a print first.
     • Otherwise, print the word using a trailing comma , to continue the current line.
   Don't forget to reset the line length at the right time.