Function to replace all "&int-int" with the respective string slices in an input string

Question

I have a school project question (for Python) that goes like this:

Given a string_input such as "abcd&1-4efg", the function must remove the "&1-4" and insert the string slice from 1 to 4 where the "&1-4" was.

eg. if string_input = "abcd&1-4efg",

1. "&1-4" is removed.

2. The remaining characters are indexed as follows: a=0, b=1, c=2, d=3, e=4, f=5, g=6

3. The new string becomes: "abcdbcdeefg"

I've managed to write a long chunk of code to do this, but I'm wondering if anyone has any more efficient solutions?

Things to note:

1. The instructions can include double digits (eg. &10-15)
2. If the index isn't found, the returned string should print "?" for every missing index (eg. "abcd&5-10efgh" would return "abcdfgh???efgh")
3. Intructions can be back-to-back (eg. "&10-15abcdef&1-5&4-5pqrs")

The code I've written is:

def expand(text):
    text += "|"
    import string
    digits_dash = string.digits + "-"

    idx_ref_str = ""
    replace_list = []
    record_val = False
    output_to_list = []
    instruct = ""
    and_idx_mark = 0

    #builds replace_list & idx_ref_list
    for idx in range(len(text)):
        if text[idx] == "&" and record_val==True:
            output_to_list.append(instruct)
            output_to_list.append(and_idx_mark)
            replace_list.append(output_to_list)
            output_to_list, instruct, inst_idx, and_idx_mark = [],"",0,0

            and_idx_mark = idx
            continue
        elif text[idx] == "&":
            record_val = True
            and_idx_mark = idx
            continue

        #executes if currently in instruction part
        if record_val == True:
            #adds to instruct
            if text[idx] in digits_dash:
                instruct += text[idx]
            #take info, add to replace list
            else:
                output_to_list.append(instruct)
                output_to_list.append(and_idx_mark)
                replace_list.append(output_to_list)
                output_to_list, instruct, inst_idx, and_idx_mark, record_val = [],"",0,0,False

        #executes otherwise
        if record_val == False:
            idx_ref_str += text[idx]

    idx_ref_str = idx_ref_str[:-1]
    text = text[:-1]

    #converts str to int indexes in replace list[x][2]
    for item in replace_list:
        start_idx = ""
        end_idx = ""
        #find start idx
        for char in item[0]:
            if char in string.digits:
                start_idx += char
            elif char == "-":
                start_idx = int(start_idx)
                break
        #find end idx
        for char in item[0][::-1]:
            if char in string.digits:
                end_idx = char + end_idx
            elif char == "-":
                end_idx = int(end_idx)
                break

        start_end_list = [start_idx,end_idx]
        item+=start_end_list

    #split text into parts in list
    count = 0
    text_block = ""
    text_block_list = []

    idx_replace = 0

    for char in text:
        if char == "&":
            text_block_list.append(text_block)
            text_block = ""
            count += len(replace_list[idx_replace][0])
            idx_replace +=1
        elif count > 0:
            count -= 1
        else:
            text_block += char

    text_block_list.append(text_block)

    #creates output str
    output_str = ""

    for idx in range(len(text_block_list)-1):
        output_str += text_block_list[idx]

        #creates to_add var to add to output_str
        start_repl = replace_list[idx][1]
        end_repl = replace_list[idx][1] + len(replace_list[idx][0])

        find_start = replace_list[idx][2]
        find_end = replace_list[idx][3]

        if end_idx >= len(idx_ref_str):
            gap = end_idx + 1 - len(idx_ref_str)
            to_add = idx_ref_str[find_start:] + "?" * gap
        else:
            to_add = idx_ref_str[find_start:find_end+1]
        
        output_str += to_add

    output_str += text_block_list[-1]

    return output_str

Answer 1

Indeed there is. For such tasks you should use regex expressions.

import re

WILDCARD_REGEX = "&[0-9]+-[0-9]+"


def get_text_without_wildcards(text):
    return re.sub(WILDCARD_REGEX, "", text)


def find_wildcard(text):
    wildcard = re.search(WILDCARD_REGEX, text)

    if wildcard is not None:
        return wildcard.group(0)

    return None


def parse_wildcard(wildcard):
    first_index, last_index = re.split("-", wildcard[1:])
    return int(first_index), int(last_index)


def get_replacement(begin, end, text):
    if end + 1 >= len(text):
        return "?"

    return text[begin:end+1]


def replace(text):
    clean_text = get_text_without_wildcards(text)
    wildcard = find_wildcard(text)

    while wildcard is not None:
        first_index, last_index = parse_wildcard(wildcard)
        replacement = get_replacement(first_index, last_index, clean_text)
        text = re.sub(wildcard, replacement, text)
        wildcard = find_wildcard(text)

    return text

print(replace("&10-15abcdef&1-5&4-5pqrs"))

The main part is: replace wildcards until there are none left.

The regex expression is simple: &[0-9]+-[0-9]+ - which means match all strings that have "&<numbers>-<numbers>".

Parsing the wildcard is also simple: skip the & and split the remainder on -. The result is a list of length 2. All you need to do is parse the elements to int.

When you have the beginning and end of the replacement string, you just need to substring the string without wildcards (clean_text).

All that is left is replacing the wildcard with the replacement string.