python 2.7 replacing a raw_input string with a value from a dictionary

Question

I am very new to programming (6 weeks, self taught on the net with "codecademy" and "python the hard way"). I decided it was time I started experimenting at writing code without direction and I hit a wall with my second project.

I am trying to make a "secret coder" that takes a raw_input string and replaces every letters in it with the next one in the alphabet. With my very limited knowledge, I figured that a dictionary should be the way to go. With a "little" googling help, I wrote this:

alpha = {"a" : "b", "b" : "c", "c" : "d", "d" : "e", "e" : "f",  "f" : "g","g" : "h"
         , "h" : "i", "i" : "j", "j" : "k", "k" : "l", "l" : "m","m" : "n", "n" : "o"
         , "o" : "p", "p" : "q", "q" : "r", "r" : "s","s" : "t", "t" : "u", "u" : "v"
         , "v" : "w", "w" : "x", "x" : "y", "y" : "z", "z" : "a"}

entry = raw_input("Please write a sentence you want to encode: ")

def encode(entry, letters):
    for k, v in letters.iteritems():
        if k in alpha:
            entry = entry.replace(k, v)
    return entry
print encode(entry, alpha)

The problem that I have is that only half the letters in my string are replaced by the correct values from the dictionary. "a" and "b" will both be printed as "c" when "a" should be printed as "b" and "b" should be printed as "c" and so on.

illustration

Where I get completely lost is that, when I replaced every value in my dictionary with numbers, it worked perfectly.

illustration bis

That's the gist of it, I really don't get what is wrong with my code.

Thank you in advance for your help.

PS: This was my very first post on stackoverflow, hopefully I did everything the way I should.

EDIT: Since I cannot give reputation yet, I will just thank you all here for your helpful answers. I can see a bit clearer now where my mistake is and I will take the info provided here to fix my code and work on understanding it properly. Also I can see that there are much more logical and straightforward approches to solving this kind of problem. Functions are still a bit blurry to me but I guess it is normal so early on.

Answer 1

The problem is that since you're iterating over the dictionary instead of the string you might replace original character many times. For example with given input of 'ab' the first replace will result to 'bb' and second to 'cc' in case that iteritems returns the keys in order a, b. Note that since dictionary is unordered collection the order of returned items is random.

You could fix the problem by using generator expression to iterate over the source string and join to create the result:

def encode(entry, letters):
    return ''.join(letters.get(c, c) for c in entry)

The above example is calling get instead of using index operator to handle the cases where alpha doesn't contain a letter in source string. The difference between get and index operator is that get takes second argument that is default value which will be returned in case that key doesn't exist. In the above example the default value is the character itself.

Answer 2

Xetnus is correct you need to loop through entry variable instead of the letters. maybe something like this

def encode(entry, letters):
    new_entry = ''
    for letter in entry:
        if letter in letters:
            new_entry += letters[letter]
    return new_entry
print encode(entry, alpha)

Answer 3

def encode(entry, letters):
    for k, v in letters.iteritems():
        if k in alpha:
            entry = entry.replace(k, v)
    return entry
print encode(entry, alpha)

Calling encode(entry, alpha) in your print function, you pass the encode() function the alpha dictionary to be the letters variable in the function. Then, you use the iterator to loop through the letters variable (which is the alpha dictionary). Then your if statement checks to see if that k (which you just got from the letters variable, which is the alpha dictionary) is in the alpha dictionary, which of course it is.

You'll need to do something like this instead:

for k, v in letters.iteritems():
    if k in entry:
        entry = entry.replace(k, v)

Answer 4

Dictionaries aren't ordered, so you are replacing some letters ('a' -> 'b') and others are getting skipped over.

For your edification what you are doing is called a rot (rotation). The most well-known one is rot13.

Writing things yourself is helpful when you first start out, but there is a pretty cool standard library function called string.maketrans which will do exactly this!

Answer 5

Others have pointed out how to do this correctly using a dictionary as lookup. However, you can also use the str.translate method which avoids having to do the look up and rebuilding a string.

import string

trans = string.maketrans(
    'abcdefghijklmnopqrstuvwxyz', # from...
    'bcdefghijklmnopqrstuvwxyza' # to
)

print raw_input('Please enter something to encode: ').translate(trans)