decryption_q.py

########## BEGIN DESCRIPTION ############################################
##
##  Caeser Cipher Decryption.
##  Write a function named decrytpion which accepts an encrypted English
##  phrase - ciphertext - (parameter 1) and a shift value (parameter 2)
##  and decrypts the phrase into plaintext.
##  
##  *See tests for examples of input and expected output*
##
########## END DESCRIPTION ##############################################



########## BEGIN PAIR DETAILS #####################################
##
##  Student Name:  *** WRITE YOUR NAME HERE***
## 
##  Student Number:  *** WRITE YOUR STUDENT NUMBER HERE ***
##
##  Student Name:  *** WRITE YOUR NAME HERE***
## 
##  Student Number:  *** WRITE YOUR STUDENT NUMBER HERE ***
##
########## END PAIR DETAILS #######################################



########## BEGIN MARKER'S FEEDBACK ###################################
##
##
########## END MARKER'S FEEDBACK #####################################



########## BEGIN ACCEPTANCE TESTS ####################################
##
##  This section contains the minimum tests that your program must
##  pass.  Please note, 'hard-coded' solutions are not acceptable.
##
'''
## Tests for Decryption

###### Test 1 - simple phrase, standard shift
>>> decrypt('wkh txlfn eurzq ira mxpsv ryhu wkh odcb grj', 3)
'the quick brown fox jumps over the lazy dog'

###### Test 2 - simple phrase - no shift
>>> decrypt('the quick brown fox jumps over the lazy dog', 0)
'the quick brown fox jumps over the lazy dog'

###### Test 3 - simple phrase - full 26 character shift
>>> decrypt('the quick brown fox jumps over the lazy dog', 26)
'the quick brown fox jumps over the lazy dog'

###### Test 4 - simple phrase - two full shifts
>>> decrypt('the quick brown fox jumps over the lazy dog', 52)
'the quick brown fox jumps over the lazy dog'

###### Test 5 - empty string, standard shift
>>> decrypt('', 3)
''

###### Test 6 - one space, standard shift
>>> decrypt(' ', 3)
' '

###### Test 7 - alphabet, shifted once
>>> decrypt('bcdefghijklmnopqrstuvwxyza', 1)
'abcdefghijklmnopqrstuvwxyz'

###### Test 8 - alphabet, shifted 25
>>> decrypt('zabcdefghijklmnopqrstuvwxy', 25)
'abcdefghijklmnopqrstuvwxyz'

###### Test 9 - alphabet with spaces, standard shift 
>>> decrypt('d e f g h i j k l m n o p q r s t u v w x y z a b c', 3)
'a b c d e f g h i j k l m n o p q r s t u v w x y z'

###### Test 10 - simple phrase, shifted 13
>>> decrypt('gur enva va fcnva fgnlf znvayl va gur cynva', 13)
'the rain in spain stays mainly in the plain'

'''

##
########## END ACCEPTANCE TESTS ######################################





########## BEGIN Decryption Code ###########################
##
##
##  Algorithm:
##
##  *** PUT YOUR ALGORITHM HERE ***

##  Program code:
##
##  *** PUT YOUR PROGRAM CODE BELOW ***


# Decrypt the given cyphertext using the Caesar cipher.  Shift by the given
# shift value.
def decrypt(ciphertext,shift):
	out_str = ''
	for	c in ciphertext:
		if(ord(c)==32):
			out_str+=chr(32)
		else:
			out_str+=chr((((ord(c)-ord('a'))-shift)%26)+ord('a'))
	return out_str

##
########## END Decryption Code #############################



########## BEGIN AUTOMATIC TESTING ###################################
##
##  The following code will automatically run the acceptance tests
##  when the program is "run".  Do not change anything in this
##  section.  If you want to prevent tests from running, comment
##  out the code below, but ensure that the code is uncommented when
##  you submit your program
##

# Run the tests if this is the main program:
if __name__ == '__main__':
    import doctest
    doctest.testmod(verbose=True)

##
########## END AUTOMATIC TESTING #####################################

ok
1 items had no tests:
main.decrypt
1 items passed all tests:
10 tests in main
10 tests in 2 items.
10 passed and 0 failed.
Test passed.