问题
In my lesson I was tasked with creating a Caesar Cipher decoder that takes a string of input and finds the best possible string using a letter frequencies. If not sure how much sense that made but let post the question:
Write a program which does the following. First, it should read one line of input, which is the encoded message, and will consist of capital letters and spaces. Your program must try decoding the message with all 26 possible values of the shift S; out of these 26 possible original messages, print the one which has the highest goodness. For your convenience, we will pre-define the variable letterGoodness for you, a list of length 26 which equals the values in the frequency table above
I have this code so far:
x = input()
NUM_LETTERS = 26 #Can't import modules I'm using a web based grader/compiler
def SpyCoder(S, N):
y = ""
for i in S:
x = ord(i)
x += N
if x > ord('Z'):
x -= NUM_LETTERS
elif x < ord('A'):
x += NUM_LETTERS
y += chr(x)
return y
def GoodnessFinder(S):
y = 0
for i in S:
if x != 32:
x = ord(i)
x -= ord('A')
y += letterGoodness[x]
return y
def GoodnessComparer(S):
goodnesstocompare = GoodnessFinder(S)
goodness = 0
v = ''
for i in range(0, 26):
v = SpyCoder(S, i)
goodness = GoodnessFinder(v)
if goodness > goodnesstocompare:
goodnesstocompare = goodness
return v
y = x.split()
z = ''
for i in range(0, len(y)):
if i == len(y) - 1:
z += GoodnessComparer(y[i])
print(z)
EDIT: Made changes suggested by Cristian Ciupitu Please ignore indentation errors, they probably arose when I copied my code over.
The program works like this:
- Take the input and split it into a list
- For every list value I feed it to a goodness finder.
- It takes the goodness of the string and compares everything else against and when there's a higher goodness it makes the higher one the goodness to compare.
- It then shifts that string of text by i amount to see if the goodness is higher or lower
I'm not quite sure where the problem is, the first test: LQKP OG CV GKIJV DA VJG BQQ
Prints the correct message: JOIN ME AT AT BY THE ZOO
However the next test: UIJT JT B TBNQMF MJOF PG UFYU GPS EFDSZQUJOH
Gives the a junk string of: SGHR HR Z RZLOKD KHMD NE SDWS ENQ CDBQXOSHMF
When it's supposed to be: THIS IS A SAMPLE LINE OF TEXT FOR DECRYPTING
I know I have to:
Try every shift value
Get the 'goodness' of the word
Return the string with the highest goodness.
I hope my explanation made sense as I am quite confused at the moment.
回答1:
Here is my implementation which works fine.
You should print the goodness of each possible message and see why your program output it.
letterGoodness = dict(zip(string.ascii_uppercase,
[.0817,.0149,.0278,.0425,.1270,.0223,.0202,
.0609,.0697,.0015,.0077,.0402,.0241,.0675,
.0751,.0193,.0009,.0599,.0633,.0906,.0276,
.0098,.0236,.0015,.0197,.0007]))
trans_tables = [ str.maketrans(string.ascii_uppercase,
string.ascii_uppercase[i:]+string.ascii_uppercase[:i])
for i in range(26)]
def goodness(msg):
return sum(letterGoodness.get(char, 0) for char in msg)
def all_shifts(msg):
msg = msg.upper()
for trans_table in trans_tables:
txt = msg.translate(trans_table)
yield goodness(txt), txt
print(max(all_shifts(input())))
回答2:
My final solution that works, thanks to the wonderful Cristian Ciupitu.
x = input()
NUM_LETTERS = 26 #Can't import modules I'm using a web based grader/compiler
def SpyCoder(S, N):
y = ""
for i in S:
if(i.isupper()):
x = ord(i)
x += N
if x > ord('Z'):
x -= NUM_LETTERS
elif x < ord('A'):
x += NUM_LETTERS
y += chr(x)
else:
y += " "
return y
def GoodnessFinder(S):
y = 0
for i in S:
if i.isupper():
x = ord(i)
x -= ord('A')
y += letterGoodness[x]
else:
y += 1
return y
def GoodnessComparer(S):
goodnesstocompare = GoodnessFinder(S)
goodness = 0
v = ''
best_v = S
for i in range(0, 26):
v = SpyCoder(S, i)
goodness = GoodnessFinder(v)
if goodness > goodnesstocompare:
best_v = v
goodnesstocompare = goodness
return best_v
print(GoodnessComparer(x))
Thank you for all of your help!
回答3:
I'm working on the same tutorial and used a slightly different method. This avoided creating and calling functions:
inp = input() #to hold code text
code = list(inp) #store code as a list
soln = [] #store the 'Goodness' for each of 26 possible answers
y=0 #variable to hold total goodness during calculations
clear = [] #will hold decoded text
pos=0 #position marker for a list
#for every possible value of shift
#note range as 0 to 25 are valid shifts and shift 26 = shift 0
for shift in range(0,26):
for i in code: #loop through each letter in code
if i == " ": #spaces have no score so omit them
continue
else: #if it's a letter
x = ord(i)-shift #apply the test shift
if x < 65: #prevent shifting outside A-Z range
x = x + 26
x = x - 64 #turn ord into character position in A-Z with A=1
x = letterGoodness[x-1] #turn this into the Goodness score
y = y + x #add this to a running total
soln.insert(shift-1,y) #AFTER decoding all letters in code, add total(y) to list of scores
y = 0 #reset y before next test value
bestSoln=max(soln) #find highest possible score
for i in range(0,26): #check the list of solutions for this score
if soln[i]==bestSoln: #the position in this list is the shift we need
bestShift = i+1 #+1 as the first solution is 0
for i in code: #now decode the original text using our best solution
if i == " ": #spaces are not encoded so just add these to the string
clear.insert(pos," ") #pos used to track next position for final string
pos = pos + 1
continue
else:
x = ord(i)-bestShift #same operation as before
if x < 65:
x = x + 26
z = chr(x)
clear.insert(pos,z) #add the decoded letter to the clear text
pos = pos + 1
print("".join(clear)) #join the list of clear text into one string and print it
Note that many parts of this code could (and should) be compressed, for example
x = x - 64
x = letterGoodness[x-1]
y = y + x
They are left expanded to 'show my working' for a tutorial exercise.
来源：https://stackoverflow.com/questions/10792052/python-caesar-cipher-decoder