5 Best Ways to Find the Highest Substring Index in Python

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πŸ’‘ Problem Formulation: This article addresses the challenge of finding the highest index at which a specific substring is found within a larger string in Python. Given an input string, the requirement is to determine the starting index of the last occurrence of a specified substring. For instance, if the string is “hello world, hello Python”, and the substring is “hello”, the desired output is 13, corresponding to the index of the second “hello”.

Method 1: Using rfind() Method

This method involves the str.rfind() function, which returns the highest index where the specified substring is found. It searches from the end of the string towards the beginning, making it the optimal built-in function for this specific requirement.

Here’s an example:

full_string = "hello world, hello Python"
substring = "hello"
index = full_string.rfind(substring)
print(index)

Output:

13

The code snippet searches for the highest index of the substring “hello” within the full_string using the rfind() method. It prints out “13”, which is the starting index of the last occurrence of “hello” in the given string.

Method 2: Using Regular Expressions

The re module in Python allows complex string search and manipulation by utilizing regular expressions. The re.finditer() function can be used to find all occurrences of a substring and the highest index can be extracted from the last match.

Here’s an example:

import re

full_string = "hello world, hello Python"
substring = "hello"
matches = list(re.finditer(substring, full_string))
if matches:
    last_match = matches[-1]
    index = last_match.start()
    print(index)

Output:

13

This code uses the re.finditer() function to find all occurrences of “hello” in full_string. It converts the iterable of match objects to a list and gets the last match. The start() method on the last match object returns the starting index of the last occurrence of the substring, which is printed out.

Method 3: Using the rindex() Method

Similar in functionality to rfind(), the rindex() method returns the highest index where a substring is found. However, unlike rfind(), rindex() raises a ValueError if the substring is not found.

Here’s an example:

full_string = "hello world, hello Python"
substring = "hello"
try:
    index = full_string.rindex(substring)
    print(index)
except ValueError:
    print("Substring not found")

Output:

13

The code attempts to locate the substring “hello” using the rindex() method and prints the highest index of its occurrence. If the substring doesn’t exist within full_string, it catches the ValueError and prints a message instead.

Method 4: Using a Custom Function

If one desires to avoid built-in methods for educational or other purposes, a custom function can be written. This function could iterate backwards over the string, checking for the substring and returning the index of its last occurrence.

Here’s an example:

def find_last_substring_index(full_string, substring):
    index = -1
    for i in range(len(full_string) - len(substring), -1, -1):
        if full_string.startswith(substring, i):
            index = i
            break
    return index

full_string = "hello world, hello Python"
substring = "hello"
print(find_last_substring_index(full_string, substring))

Output:

13

The custom function find_last_substring_index() iterates over the string from the end to the start, checking if it starts with the given substring at each index. It returns the latest index where the substring is found.

Bonus One-Liner Method 5: List Comprehension with enumerate()

This one-liner uses list comprehension and the enumerate() function to create a list of starting indices of all occurrences of the substring, then returns the last index.

Here’s an example:

full_string = "hello world, hello Python"
substring = "hello"
indices = [i for i, _ in enumerate(full_string) if full_string.startswith(substring, i)]
index = indices[-1] if indices else -1
print(index)

Output:

13

The one-liner iterates over the full_string with index and doesn’t keep the character itself (indicated by “_”). It checks if the string starts with the substring at that index and assembles a list of valid indices. The highest index, which is the last element of the list, is printed out.

Summary/Discussion

  • Method 1: rfind(): Straightforward and built-in. The optimal choice for most use cases. It returns -1 if the substring is not found, which can be an advantage or a disadvantage depending on the application.
  • Method 2: Regular Expressions: Powerful and flexible for complex patterns. Slightly less straightforward for simple substring searches, and performance can be an issue with large texts.
  • Method 3: rindex(): Similar to rfind() but raises an exception if the substring isn’t found, which can be useful for error checking, but requires exception handling.
  • Method 4: Custom Function: Offers complete control and can be tailored to specific needs. However, it’s generally less efficient and more verbose than built-in methods.
  • Method 5: List Comprehension with enumerate(): Concise and elegant, but less readable for newcomers to Python. Also, could have performance drawbacks due to creating a list of all indices.