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How can I replace a character in a string from a certain index? For example, I want to get the middle character from a string, like abc, and if the character is not equal to the character the user specifies, then I want to replace it.

Something like this maybe?

middle = ? # (I don't know how to get the middle of a string)

if str[middle] != char:
    str[middle].replace('')
codeforester
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Jordan Baron
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    strings are immutable, you'd need to create a new string. It would be of the form `slice_before_index + char + slice_after_index`. – Paul Rooney Jan 19 '17 at 22:34
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    strings in python are immutable... this means you must construct a new string – Joran Beasley Jan 19 '17 at 22:35
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    Does this answer your question? [Changing one character in a string in Python](https://stackoverflow.com/questions/1228299/changing-one-character-in-a-string-in-python) – Georgy Apr 18 '20 at 15:05
  • For time comparison, check out this answer: https://stackoverflow.com/questions/1228299/changing-one-character-in-a-string/22149018#22149018 – MERose Dec 11 '21 at 10:56

5 Answers5

138

As strings are immutable in Python, just create a new string which includes the value at the desired index.

Assuming you have a string s, perhaps s = "mystring"

You can quickly (and obviously) replace a portion at a desired index by placing it between "slices" of the original.

s = s[:index] + newstring + s[index + 1:]

You can find the middle by dividing your string length by 2 len(s)/2

If you're getting mystery inputs, you should take care to handle indices outside the expected range

def replacer(s, newstring, index, nofail=False):
    # raise an error if index is outside of the string
    if not nofail and index not in range(len(s)):
        raise ValueError("index outside given string")

    # if not erroring, but the index is still not in the correct range..
    if index < 0:  # add it to the beginning
        return newstring + s
    if index > len(s):  # add it to the end
        return s + newstring

    # insert the new string between "slices" of the original
    return s[:index] + newstring + s[index + 1:]

This will work as

replacer("mystring", "12", 4)
'myst12ing'
ti7
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    when trying your function above the *xrange* function threw an error. Is there a library we need to import? – yeOldeDataSmythe Oct 21 '19 at 14:23
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    Oh, I'll make an update `xrange` is Python 2.7's version of Python 3.x's `range` – ti7 Oct 21 '19 at 21:00
  • @jeffhale the action is meant to be obvious to one _reading the code here_, not necessarily obvious in general / as a possible implementation! – ti7 Sep 08 '21 at 20:58
53

You can't replace a letter in a string. Convert the string to a list, replace the letter, and convert it back to a string.

>>> s = list("Hello world")
>>> s
['H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd']
>>> s[int(len(s) / 2)] = '-'
>>> s
['H', 'e', 'l', 'l', 'o', '-', 'W', 'o', 'r', 'l', 'd']
>>> "".join(s)
'Hello-World'
Jacob Malachowski
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Strings in Python are immutable meaning you cannot replace parts of them.

You can however create a new string that is modified. Mind that this is not semantically equivalent since other references to the old string will not be updated.

You could for instance write a function:

def replace_str_index(text,index=0,replacement=''):
    return '%s%s%s'%(text[:index],replacement,text[index+1:])

or since the introduction of f-strings:

def replace_str_index(text,index=0,replacement=''):
    return f'{text[:index]}{replacement}{text[index+1:]}'

And then for instance call it with:

new_string = replace_str_index(old_string,middle)

If you do not feed a replacement, the new string will not contain the character you want to remove, you can feed it a string of arbitrary length.

For instance:

replace_str_index('hello?bye',5)

will return 'hellobye'; and:

replace_str_index('hello?bye',5,'good')

will return 'hellogoodbye'.

Willem Van Onsem
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8
# Use slicing to extract those parts of the original string to be kept
s = s[:position] + replacement + s[position+length_of_replaced:]

# Example: replace 'sat' with 'slept'
text = "The cat sat on the mat"
text = text[:8] + "slept" + text[11:]

I/P : The cat sat on the mat

O/P : The cat slept on the mat

Shivam Bharadwaj
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-2

You can also Use below method if you have to replace string between specific index

def Replace_Substring_Between_Index(singleLine,stringToReplace='',startPos,endPos):
    
       singleLine = singleLine[:startPos]+stringToReplace+singleLine[endPos:]
    
return singleLine
Milan Paliwal
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