Write a function which replaces the n-th occurence of a string sub by a string replacement.
The function shall return the changed string or the unchanged, or the original string, if replacement didn't occur in the string.
def findnth(s, sub, n):
num = 0
start = -1
while num < n:
start = s.find(sub, start+1)
if start == -1: return -1
num += 1
return start
def replacenth(source, search, replacement, n):
pos = findnth(source, search, n)
if pos == -1: return source
return source[:pos] + replacement + source[pos+len(search):]
s = "abcxyzabcjkjkjkabclkjkjlkjabcjlj"
print(findnth(s,"abc", 4))
print(replacenth(s,"abc","---", 4))