Question

I am writing a program like unix tr, which can replace string of input.
My strategy is to find all indexes of source strings at first, and then replace them by target strings. I don’t know how to replace string by index, so I just use the slice.
However, if the length of target string doesn’t equal to the source string, this program will be wrong. I want to know what’s the way to replace string by index.

def tr(srcstr,dststr,string):
    indexes = list(find_all(string,srcstr)) #find all src indexes as list
    for index in indexes:
        string = string[:index]+dststr+string[index+len(srcstr):]
    print string

tr(‘aa’,’mm’,’aabbccaa’)
the result of this will be right: mmbbccmm
but if tr(‘aa’,’ooo’,’aabbccaa’), the output will be wrong:

ooobbcoooa

Answer 1

Python strings are immutable (as far as I can remember), so you can’t just go in and insert stuff.

Luckily Python already has a replace() function.

>>> s = "hello, world!"
>>> s1 = s.replace("l", "k")
>>> print s1
hekko, workd!