5 Best Ways to Find the Lowest Index of a Substring Within a Range in Python

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πŸ’‘ Problem Formulation: Consider the task where you need to find the lowest index of a substring within a specific range in a larger string. For example, given the string “abracadabra” and the substring “abra”, you might want to know the index of “abra” within the range of index 1 to index 7. The desired output would be 3, as that’s where the substring starts within the specified range.

Method 1: Using str.find() with slicing

The str.find() method returns the lowest index of the substring if found in the given string. By slicing the string based on the specified range before calling find(), we can locate the lowest index where the substring is found within that range.

Here’s an example:

my_string = "abracadabra"
substring = "abra"
start_index = 1
end_index = 7

found_index = my_string[start_index:end_index+1].find(substring)
if found_index != -1:
    found_index += start_index  # Adjust the index according to the range

print(found_index)

Output: 3

This code snippet slices the string from the start index to the end index, searching for the substring “abra”. The offset of the starting index is then added to the result from find() to reflect the correct position in the original string.

Method 2: Using regular expressions with re.search()

Regular expressions provide a powerful toolkit for string searching. The re.search() function can be used to find a substring within a range by specifying the start and end positions as arguments.

Here’s an example:

import re

my_string = "abracadabra"
substring = "abra"
start_index = 1
end_index = 7

match = re.search(substring, my_string[start_index:end_index+1])
if match:
    found_index = start_index + match.start()

print(found_index)

Output: 3

After isolating the range using string slicing, the re.search() method searches for the substring. If found, the starting index of the match within the sliced string is added to the start index of the range, yielding the location within the original string.

Method 3: Looping through the range

This approach involves looping through the specified range of indices and checking if the substring matches the portion of the string starting from each index.

Here’s an example:

my_string = "abracadabra"
substring = "abra"
start_index = 1
end_index = 7

found_index = -1
for i in range(start_index, end_index - len(substring) + 2):
    if my_string[i:i+len(substring)] == substring:
        found_index = i
        break

print(found_index)

Output: 3

This code iterates over the specified range and checks if the substring is present starting at each position. This ensures the lowest index within the range is found. The loop stops as soon as the substring is found, effectively finding the lowest index.

Method 4: Using list comprehension and min()

In Python, list comprehension can be combined with the min() function to compactly search for the lowest index of a substring within a range. An intermediate list of indices is generated where the substring matches, and min() finds the smallest one.

Here’s an example:

my_string = "abracadabra"
substring = "abra"
start_index = 1
end_index = 7

indices = [i for i in range(start_index, end_index - len(substring) + 2) 
           if my_string.startswith(substring, i)]
found_index = min(indices) if indices else -1

print(found_index)

Output: 3

This code employs list comprehension to create a list of all indices where the substring is found within the range. The min() function then finds the smallest index, which is returned as the result.

Bonus One-Liner Method 5: Using next() and generator expression

This one-liner uses a generator expression inside the next() function to efficiently find the lowest index where the substring appears within the range, without constructing an intermediate list.

Here’s an example:

my_string = "abracadabra"
substring = "abra"
start_index = 1
end_index = 7

found_index = next((i for i in range(start_index, end_index - len(substring) + 2)
                    if my_string.startswith(substring, i)), -1)

print(found_index)

Output: 3

With the use of next(), the code instantaneously returns the lowest index from the generator expression, which lazily evaluates each index in the range until the substring is found. If not found, it defaults to -1.

Summary/Discussion

  • Method 1: Using str.find() with slicing. Strengths: Simple and intuitive. Weaknesses: Requires manual index adjustment and extra slicing.
  • Method 2: Using regular expressions with re.search(). Strengths: Powerful pattern matching capabilities. Weaknesses: Overhead of using regex for a simple task, potentially less readable.
  • Method 3: Looping through the range. Strengths: Granular control over the search process. Weaknesses: Potentially slower due to explicit looping.
  • Method 4: Using list comprehension and min(). Strengths: Elegant one-liner, very Pythonic. Weaknesses: Generates an intermediate list which could be memory intensive.
  • Method 5: Using next() and generator expression. Strengths: Efficient and concise, does not create an intermediate list. Weaknesses: Slight learning curve for those unfamiliar with generators.