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I am trying to replace every instance of a substring in a string using python. The following is the code that I have done and it is giving me some weird result.

def main():
    s='IN GOING GO'
    x='IN'
    y='aa'

    print(rep_str(s,x,y))

def rep_str(s,x,y):
    result=""
    if x in s:
        for i in range(len(s)):
            if s[i:i+len(x)] == x:
                result=result+y
            else:
                result=result+s[i+1:i+len(x)]          

    return result


main()

I am not allowed to use the replace method. In fact, my function is supposed to do what replace function does in python. The following is the output that I am getting.

aa GOIaaG GO

I would appreciate if someone could give me some input about how to change the logic to get the right out put i.e. aa GOaaG GO.

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9
  • Why aren't you allowed to use the replace method? Best practices is to use the built-in string methods. Commented Feb 15, 2014 at 21:17
  • @Will could be for learning purposes, etc. It's like practicing writing sorting functions or creating ADTs Commented Feb 15, 2014 at 21:18
  • Ah, got it. In that case I won't suggest regular expressions as an alternative. Thanks @JesseMu Commented Feb 15, 2014 at 21:19
  • import re; re.sub(x,y,s) Commented Feb 15, 2014 at 21:20
  • 1
    Looks like you forgot to skip len(x) characters after appending y to result, and continue to iterate characters one by one instead. Commented Feb 15, 2014 at 21:21

3 Answers 3

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3

As I mentioned in comments, the mistake is that you are not skipping len(x) characters after match. Also in keyword is quite high-level routine (it does not less than search part of search & replace), so here is fixed version without in:

def rep_str(string, search, replacement):
    result = ''
    i = 0
    while i < len(string):
        if string[i : i + len(search)] == search:
            result += replacement
            i += len(search)
        else:
            result += string[i]
            i += 1
    return result
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4 Comments

What is the output that you are getting? I tried your code and I am not getting any output.
It's a replacement for original rep_str function; try it in your code with main function.
Sorry, forgot to add main() at the end.
Thanks, I changed my code after seeing your input about skipping the length of the substring that was supposed to be replace and that was a really good suggestion. However, I did not think about adding 1 to i in the else statement and that is why probably I did not get the right output. Thanks for taking time to look at my code.
1

If you just want to have the result try with:

import re
re.sub(r'IN','aa','IN GOING GO')

but if you need some logic then you should compare for blocks of same length as the pattern, not char by char

@Zag asnwer is better, because can compare longer patterns and has return when it does not match nothing but if you want to get your code running you need to skip for when you have a match like this : 

def rep_str(s,x,y):
    result=""
    skip = False
    if x in s:
        for i in range(len(s)):
            if skip:
                skip = False
                continue
            if s[i:i+2] == x:
                result+=y
                skip = True
            else:
                result+=s[i:i+1] 
        return result
    else:
        return s

but your code won't work when you will call the function with rep_str("A example test","test", "function") for example.

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1 Comment

Thanks for your input too. I tried your code and it's giving me the right input too. I learned something new from your code. So, thanks for that.
1

If you are allow to use the index() function, you can try this:

def main():
    s='IN GOING GO'
    x='IN'
    y='aa'

    print(rep_str(s,x,y))


def rep_str(s,x,y):
    while x in s:
        s = s[:s.index(x)] + y + s[s.index(x) + len(x):]

    return s


main()
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