Recursively replace characters in string using Python

Question

def replaceChar(myString, oldChar, newChar):

    # if myString has no characters left, return newString
    if myString == '':
        return newString

    elif myString[0] == oldChar:
        # then add newChar to newString

    else:
        # then add myString[0] to newString

    # Chop the first character off of myString
    myString = myString[1:]

    # recurse
    replaceChar(myString, oldChar, newChar)

Since strings are immutable, I can't add newChar or oldChar to newString. I can't make newString a list, because it will get overwritten with each recursive loop. I also can't define that list outside of the function because the rules state that everything must be inside the function. How can I add these characters to my new string?

Sure. replaceChar('Do you have some sample input', 'o', 'X') would return 'DX yXu have sXme sample input' — user5619350
– user5619350, Commented Aug 2, 2016 at 22:33

sberry · Accepted Answer · 2016-08-02 22:58:00Z

1

Obviously you would never actually use a recursive solution like this for this type of problem, but here it is anyway:

def replaceChar(inval, old, new):
    if inval == '':
        return ''
    if inval[0] == old:
        return new + replaceChar(inval[1:], old, new)
    return inval[0] + replaceChar(inval[1:], old, new)

And

print(replaceChar('Do you have some sample input', 'o', 'X'))

yields

DX yXu have sXme sample input

answered Aug 2, 2016 at 22:58

sberry

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3 Comments

user5619350 Over a year ago

Why do we return the empty string?

user5619350 Over a year ago

I know why. It's because when we returned (new or inval[0] ) + replaceChar(inval[1:] ...) we declared that we were concatenating something with new or inval[0]. We must provide a value, and specifically a string value so that concatenation is possible. That's why we can't just return, or pass, etc. We return "" in order to concatenate a string type with no value.

sberry Over a year ago

@manbearpig1 The best lesson I learned a long time ago with recursion is the first step is determining your end case. And yes, you want to always return strings.

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