update string from a dictionary with the values from matching keys

Question

def endcode(msg,secret_d):
    for ch in msg:
        for key,value in secret_d:
             if ch == key:
                 msg[ch] = value
    return msg
encode('CAN YOU READ THIS',{'A':'4','E':'3','T':'7','I':'1','S':'5'})

This is my code. What I am trying to do here is for every characters in a string msg, the function should search in the dictionary and replace it with the mapping string if the character ch is a key in the dictionary secret_d.

If ch is not a key in secret_d than keep it unchanged.

For the example, the final result is should be 'C4N YOU R34D 7H15'

Hey OP, have you come any further with the help of the answers you got? — timgeb, Apr 01 '16 at 11:52

rb612 · Accepted Answer · 2016-04-01T05:24:33.097

Your function name is endcode but you are calling encode.

But more important, I'll give you a hint to what's going on. This isn't going to totally work, but it's going to get you back on track.

def endcode(msg,secret_d):
    newstr=""
    for ch in msg:
        for key,value in secret_d.iteritems():
            if ch == key:
                newstr=newstr+value
    print(msg)
endcode('CAN YOU READ THIS',{'A':'4','E':'3','T':'7','I':'1','S':'5'})

But if you want a complete answer, here is mine.

score 0 · Answer 2 · edited May 23 '17 at 11:50

A few issues:

As rb612 pointed out, there's a typo in your function definition ("endcode")
you are doing nothing with the return value of your function after calling it
msg[ch] is trying to assign items in a string, but that's not possible, strings are immutable. You'll have to build a new string. You cannot "update" it.
in order to iterate over (key, value) pairs of a dictionary d, you must iterate over d.items(). Iterating over d will iterate over the keys only.

That being said, here's my suggestion how to write this:

>>> def encode(msg, replacers):
...     return ''.join([replacers.get(c, c) for c in msg])
... 
>>> result = encode('CAN YOU READ THIS',{'A':'4','E':'3','T':'7','I':'1','S':'5'})
>>> result
'C4N YOU R34D 7H15'

Things to note:

dict.get can be called with a fallback value as the second argument. I'm telling it to just return the current character if it cannot be found within the dictionary.
I'm using a list comprehension as the argument for str.join instead of a generator expression for performance reasons, here's an excellent explanation.

This is definitely a good answer. Mine (now edited) will do the same but overall not nearly as clean and compact. — rb612, Apr 01 '16 at 05:26

update string from a dictionary with the values from matching keys

2 Answers2