How to replace particular characters of a string with the elements of a list in an efficient way?

Question

There is a string:

input_str = 'The substring of "python" from index @ to index @ inclusive is "tho"'

and a list of indices:

idx_list = [2, 4]

I want to replace the character @ in str_input with each element of the idx_list to have the following output:

output_str = 'The substring of "python" from index 2 to index 4 inclusive is "tho"'

So I have coded it as follows:

def replace_char(input_str, idx_list):
    output_str = ""
    idx = 0
    for i in range(0, len(input_str)):
        if input_str[i] == '@':
            output_str += str(idx_list[idx])
            idx += 1
        else:
            output_str += input_str[i]
    return output_str

I wonder if there is any shorter and faster way than the concatenation that I have used?

Asked By: plpm

||

Source

Answer 1

One concise approach uses re.sub with a callback function:

input_str = 'The substring of "python" from index @ to index @ inclusive is "tho"'
idx_list = [2, 4]
output_str = re.sub(r'bindex @', lambda m: str(idx_list.pop(0)), input_str)
print(output_str)

# The substring of "python" from 2 to 4 inclusive is "tho"

The idea here is that every time a match of index @ is found, we replace with the first entry in the list of indices. We also then pop that first index, so that it doesn’t get used again.

Answered By: Tim Biegeleisen

Answer 2

Could you use formatted strings?
It would look like this:

idx_list = [2,4]
input_str = f'The substring of "python" from index {idx_list[0]} to index {idx_list[1]} inclusive is "tho"'

Answered By: Alex S

Answer 3

You’re basically recreating str.format() using a slightly different format string. see https://docs.python.org/3/tutorial/inputoutput.html#the-string-format-method

If you just want to fix the format to exactly match how python already does the task you’re doing, just replace ‘@’ with ‘{}’

input_str = 'The substring of "python" from index @ to index @ inclusive is "tho"'
idx_list = [2, 4]
print(input_str.replace('@', '{}').format(*idx_list))

But if the original string is really big, you may not want to add the replace step. Since we want to favor iterators in that case, write a generator to go through the string and yield its characters, except replace ‘@’ with the next argument in your idx_list.

# my first draft is a bit clunky
def replace_char(input_str, idx_list):
    def replace(s, args):
        for char in s:
            if char == '@':
                yield str(next(args))
            else:
                yield char
    return ''.join(replace(input_str, iter(idx_list)))

input_str = 'The substring of "python" from index @ to index @ inclusive is "tho"'
idx_list = [2, 4]
print(replace_char(input_str, idx_list))

This is not really any different from your original code.

Answered By: Kenny Ostrom

Answer 4

You can use this code.

def replace_char(input_str: str, min_index: int, max_index: int) -> str:
    return input_str[:min_index] + '@' * (max_index - min_index) + input_str[max_index:]

input_str = 'The substring of "python" from index @ to index @ inclusive is "tho"'

output_str = replace_char(input_str, 2, 4)
print(output_str)

# Th@@substring of "python" from index @ to index @ inclusive is "tho"  # 'Th@@substring of "python" from index @ to index @ inclusive is "tho"'

Answered By: Hoomar

How to replace particular characters of a string with the elements of a list in an efficient way?

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