Question

I have a long string and a list of [end-index, string] like the following:

long_sentence = "This is a long long long long sentence"
indices = [[6, "is"], [8, "is a"], [18, "long"], [23, "long"]]

An element 6, "is" indicates that 6 is the end index of the word "is" in the string. I want to get the following string in the end:

>> print long_sentence
This .... long ......... long sentence"

I tried an approach like this:

temp = long_sentence
for i in indices:
    temp = temp[:int(i[0]) - len(i[1])] + '.'*(len(i[1])+1) + temp[i[0]+1:]

While this seems to be working, it is taking exceptionally long time (more than 6 hours on 5000 strings inside a 300 MB file). Is there a way to speed this up?

Answer 1

Each time you do that temp = temp... assignment, Python has to create a new string (because Python strings are immutable).

What you might want to do instead is turn the string into a list of characters, then operate on the list of characters, and then join that list back together into a string again.

long_list = list(long_sentence)
for end, repstr in indices:
    long_list[end-len(repstr):end] = ['.'] * len(repstr)
new_sentence = ''.join(long_list)