Changing multiple characters by other characters in a string

Question

I'm trying to manipulate a string.

After extracting all the vowels from a string, I want to replace all the 'v' with 'b' and all the 'b' with 'v' from the same string (i.g. "accveioub" would become ccvb first, then ccbv).

I'm having problem to swap the characters. I end up getting ccvv and I figured I'd get that based of this code. I'm thinking of iterating through the string and using an if statement basically staying if the character at index i .equals"v" then replace it with "b" and an else statement that says "b" replace it with "v" and then append or join the characters together?

Here's my code

def Problem4():
    volString = {"a", "e", "i", "o", "u", "A", "E", "I", "O", "U"}
    s = "accveioub"
    chars = []
    index = 0

    #Removes all the vowels with the for loop
    for i in s:
        if i not in volString:
            chars.append(i)
    s2 = "".join(chars)
    print(s2)
    print(s2.replace("v", "b"))
    print(s2.replace("b", "v"))

>>> Problem4()
ccvb
ccbb
ccvv
>>>

`replace` does not modify the original string (s2) but returns a new string. — Selcuk, Mar 05 '16 at 05:41

Ian · Accepted Answer · 2016-03-05T05:59:36.853

4

You have done it half-way actually, the only thing to take note is that when you want to "swap" the string, you got to create "temporary" string instead of replacing is directly.

What you did was this:

ccvb 
ccbb #cannot distinguish between what was changed to b and the original b
ccvv #thus both are changed together

Consider using the non existent character in the string as the first replacement. Let say, I now change all b with 1 first:

s2 = s2.replace("b","1")
s2 = s2.replace("v","b")
s2 = s2.replace("1","v")

Then you will get:

ccvb #original
ccv1 #replace b with 1
ccb1 #replace v with b
ccbv #replace 1 with v

The most important point here is the use of the temporary string

edited Mar 05 '16 at 05:59

answered Mar 05 '16 at 05:40

Ian

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Hmm i see what you're saying that would also be a good solution to use. kinda like aliasing i that's what it's called and then use use that variable as an instance or place holder for that value or string. – Usman Bashiru Mar 05 '16 at 05:57
Yes it does its funny how the answer was right in front of my face haha how do i reward you with the correct answer if it is possible on this website? – Usman Bashiru Mar 05 '16 at 06:05
@UsmanBashiru Right. sometimes small things can pass by under our nose. :) simply tick (V) my answer as the accepted answer. :) that is in the V symbol top left of my answer. In fact, this is how it is done in all Stack Exchange website. Once your reputation is > 15, you could also upvote any answer/questions which you find useful in any Stack Exchange site. :) you can *upvote* as many answer as you want, but can only *accept one* answer (supposedly best fit for you). – Ian Mar 05 '16 at 06:07
@UsmanBashiru more info on [accepting an answer](http://stackoverflow.com/help/accepted-answer) on this site. – Ian Mar 05 '16 at 06:09
If this was a general purpose function, it might be difficult to find an appropriate intermediate value, e.g. the above would break with an input string of 'four11bvaa'. – AChampion Mar 05 '16 at 17:04

AChampion · Answer 2 · 2016-03-05T23:58:30.373

Python's comprehension and generator expressions are quite powerful.
You can do the vowel removal the same time as you do the join():

>>> volString = {"a", "e", "i", "o", "u", "A", "E", "I", "O", "U"}
>>> s = 'accveioub'
>>> ''.join(c for c in s if c not in volString)
'ccvb'

In fact you do the swap in the join() too but this maybe a little too verbose:

>>> ''.join('v' if c == 'b' else 'b' if c == 'v' else c for c in s if c not in volString)
'ccbv'

You could pull out the swap code into a mapping dict (dict.get(k, v) returns v if k is not in the dictionary):

>>> vbmap = {'b': 'v', 'v': 'b'}
>>> ''.join(vbmap.get(c, c) for c in s if c not in volString)
'ccbv'

The advantage being these solutions avoid all the intermediate string creations.

score 0 · Answer 3 · answered Mar 05 '16 at 06:12

You should use intermediate replacement to get proper result in python:

s2.replace('b',' ')
s2.replace('v','b')
s2.replace(' ','v')

In other words steps are following

Replace b to ' ' space
Replace 'v' to 'b'
Replace ' ' space (that was your b character) to 'v'

Good luck!

Changing multiple characters by other characters in a string

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