How to replace characters in a string in python

Question

How to replace characters in a string which we know the exact indexes in python?

Ex : name = "ABCDEFGH" I need to change all odd index positions characters into '$' character.

    name = "A$C$E$G$"

(Considered indexes bigin from 0 )

csunday95 · Answer 1 · 2015-07-23T20:23:07.680

12

Also '$'.join(s[::2]) Just takes even letters, casts them to a list of chars and then interleaves $

 ''.join(['$' if i in idx else s[i] for i in range(len(s))])

works for any index array idx

edited Jul 23 '15 at 20:23

answered Jul 23 '15 at 20:01

csunday95

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3

You don't even need the `list()` call. :) – TigerhawkT3 Jul 23 '15 at 20:02
Question is based on the indices so this is not a proper way! – Mazdak Jul 23 '15 at 20:04
1

In general for this sort of manipulation, convert to a list, manipulate the indices in the list then join back to a string, but in this specific example as you showed it collapses down beautifully into a join and slice with skip. – Duncan Jul 23 '15 at 20:05
1

@Kasramvd, it's an [XY Problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). – TigerhawkT3 Jul 23 '15 at 20:05
@TigerhawkT3 Yeah a sort of! but not completely! – Mazdak Jul 23 '15 at 20:08
@Kasramvd Added a general soln as well. Pythonic but not as elegant. – csunday95 Jul 23 '15 at 20:27

Mazdak · Answer 2 · 2015-07-23T20:06:07.353

3

You can use enumerate to loop over the string and get the indices in each iteration then based your logic you can keep the proper elements :

>>> ''.join([j if i%2==0 else '$' for i,j in enumerate(name)])
'A$C$E$G$'

edited Jul 23 '15 at 20:06

answered Jul 23 '15 at 20:00

Mazdak

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You could shorten this slightly with `'$' if i%2 else j`. – TigerhawkT3 Jul 23 '15 at 20:05

score 1 · Answer 3 · answered Jul 23 '15 at 20:09

1

You can reference string elements by index and form a new string. Something like this should work:

startingstring = 'mylittlestring'
nstr = ''
for i in range(0,len(startingstring)):
    if i % 2 == 0:
            nstr += startingstring[i]
    else:
            nstr += '$'

Then do with nstr as you like.

answered Jul 23 '15 at 20:09

Andrew

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The reason why the other solutions use a list with `join` is to avoid this - you are creating a new string object each time you use `+=`. – cdarke Jul 23 '15 at 20:13
I did not know that. I figured it was, in effect, appended to an existing string. What's the detail of the operation? Does it (1) create a new string consisting of the old `nstr` concatenated with the new character, (2) delete the old `nstr` and (3) move the new string to `nstr`? – Andrew Jul 23 '15 at 20:18
You can't append to an existing string because strings are immutable in Python. – TigerhawkT3 Jul 23 '15 at 20:20
Just about, yes. Python is not the only language like this, PHP and Java are examples that are the same. Perl is the odd one out (in so many ways). – cdarke Jul 23 '15 at 20:22
Good to know. Thanks for the post. – Andrew Jul 23 '15 at 20:24
The discussion is worthwhile, although often repeated here. It would be good to keep. – cdarke Jul 23 '15 at 20:26

Thilina Chathuranga · Accepted Answer · 2015-07-23T20:42:03.617

1

name = "ABCDEFGH"
nameL = list(name)

for i in range(len(nameL)):
    if i%2==1:
        nameL[i] = '$'

name = ''.join(nameL)
print(name)

edited Jul 23 '15 at 20:42

answered Jul 23 '15 at 20:16

Thilina Chathuranga

113
6

How to replace characters in a string in python

4 Answers4