Python：用不一致的结果替换字符列表中的字母

Question

I am writing a ROT13 script in python. It works by separating the string into a list of single characters and using a for-loop, goes through each letter and checks if it needs to be translated by a dictionary.

The problem is it doesn't always replace the letter in the list. I can't figure out why, but only some strings work.

Here is the code:

import string
def rot13(m):
    Data for ROT13 conversion                                                        
    alphabet = list("abcdefghijklmnopqrstuvwxyz")
    mapping = {}
    for letter in alphabet:
        mapping[letter] = alphabet[(alphabet.index(letter) + 13)%26]
    for letter in alphabet:
        mapping[letter.upper()] = alphabet[(alphabet.index(letter) +13)%26].upper()
    # Create a list of the characters in order                                         
    characters = list(m)
    # Go through each character in the list...                                         
    for character in characters:
        # Check if that character is one that needs to be changed                      
        if character in mapping:
            # Test to chcek if it is finding characters correctly (it is)              
            print "%s to %s" % (character, mapping[character])
            # replace the character with a new one (works inconsistently)              
            characters[characters.index(character)] = mapping[character]
        #Bring it all together                                                         
        result = string.join(characters, "");
    return result

print rot13("ABCDEF") # Returns NOPQRS                                                 
print rot13("ABCDEFGHIJKLMNOPQRSTUVWXYZ") # Returns original string 

The first test, with part of the uppercase alphabet, comes out just as expected. However, the full alphabet just returns the original string when put through my ROT13 function.

I am convinced the problem is with line 20, characters[characters.index(character)] = mapping[character] .

This line is supposed to replace the letter in the list with the corresponding one from the rot13 dictionary constructed in the beginning, but it doesn't always do that.

I have a line right before that prints the character it is testing and what it should be changed to based off the dictionary, and that always works. But if that does, then why not this other line?

Answer 1

characters.index(character) finds the index of the first element equal to that character.

After 13 iterations of your for character in characters loop characters will look like list("NOPQRSTUVWXYZNOPQRSTUVWXYZ") and character will be "N" so characters.index(character) will be 0 so you end up with characters[0] = mapping["N"] when you expect characters[13] = mapping["N"] . Of course mapping["N"] == "A" .

Try:

for index, character in enumerate(characters):
    # Check if that character is one that needs to be changed                      
    if character in mapping:
        # Test to chcek if it is finding characters correctly (it is)              
        print "%s to %s" % (character, mapping[character])
        # replace the character with a new one (works inconsistently)              
        characters[index] = mapping[character]
    #Bring it all together                                                         
    result = string.join(characters, "");

Answer 2

Index finds the first matching character in the list. The first 13 iterations turn the alphabet into nz repeated twice. Then the second 13 iterations undo it - the 14th iteration gets an n as its character, calculates that it needs an a instead, then calls index ('n') and gets 0 - the new index of the first 'n' in characters. It replaces that first 'n' with an 'a' and moves on.

There are a lot of better ways you could write this, but just for starters you should read about the enumerate built in function. Keeping track of the list position yourself rather than searching each time will at least get you past this problem.

Answer 3

Try this version on for size. Should be a bit easier to debug.

def translate_char(c):
    """Translate a single character using a dictionary"""
    translation_table = {
        'a':'n',
        'b':'o',
        'c':'p',
        'd':'q',
        'e':'r',
        'f':'s',
        'g':'t',
        'h':'u',
        'i':'v',
        'j':'w',
        'k':'x',
        'l':'y',
        'm':'z',
        'n':'a',
        'o':'b',
        'p':'c',
        'q':'d',
        'r':'e',
        's':'f',
        't':'g',
        'u':'h',
        'v':'i',
        'w':'j',
        'x':'k',
        'y':'l',
        'z':'m',
        'A':'N',
        'B':'O',
        'C':'P',
        'D':'Q',
        'E':'R',
        'F':'S',
        'G':'T',
        'H':'U',
        'I':'V',
        'J':'W',
        'K':'X',
        'L':'Y',
        'M':'Z',
        'N':'A',
        'O':'B',
        'P':'C',
        'Q':'D',
        'R':'E',
        'S':'F',
        'T':'G',
        'U':'H',
        'V':'I',
        'W':'J',
        'X':'K',
        'Y':'L',
        'Z':'M'}
    if c in translation_table.keys():
        return translation_table[c]
    else:
        return c    

def rot13(plaintext):
    """Translate a complete string"""
    ciphertext = ""
    for c in plaintext: 
        ciphertext = ciphertext + translate_char(c)
    return ciphertext

if __name__ == "__main__":

    plaintext = "The quick brown fox jumped over the lazy black dog."
    print rot13(plaintext)

Answer 4

I implemented as follows using maketrans form string module.

from string import maketrans
def rot_it(text):
      intab = "abcdefghijklmnopqrstuvwxyz"
      in_rot = intab + intab.upper()
      outtab = "nopqrstuvwxyzabcdefghijklm"
      out_rot = outtab + outtab.upper()
      trans = maketrans(in_rot, out_rot)
      rot13 = text.translate(trans)
      return rot13