6.189 Exercise Opt.2

cipher.py

# OPT.2 from http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-189-a-gentle-introduction-to-programming-using-python-january-iap-2011/assignments/MIT6_189IAP11_hw1.pdf
# Secret Messages

phrase = raw_input("Enter sentence to encrypt: ")
shift = input("Enter shift value: ")

encoded_phrase = ''
for c in phrase:
    ascii = ord(c)
    if 97 <= ascii < (97+26):
        encoded_ascii = ascii - 97 # shift to zero
        encoded_ascii = encoded_ascii + shift # shift up
        encoded_ascii = encoded_ascii % 26 # loop the shift if it goes past z
        encoded_ascii = encoded_ascii + 97 # shift back up to letter range
    elif 65 <= ascii < (65+26):
        encoded_ascii = ascii - 65 # shift to zero
        encoded_ascii = encoded_ascii + shift # shift up
        encoded_ascii = encoded_ascii % 26 # loop the shift if it goes past z
        encoded_ascii = encoded_ascii + 65 # shift back up to letter range
    else:
        # do nothing; if it's not a letter it passes
        encoded_ascii = ascii
        
    encoded_c = chr(encoded_ascii)
    encoded_phrase = encoded_phrase + encoded_c

print "The encoded phrase is: " + encoded_phrase