Locating the Highest Index of a Substring in a Range with Python’s rindex

💡 Problem Formulation: You are given a string and need to find the last occurrence of a specified substring within it. Python’s rindex() method can be used to return the highest index at which the substring is found. For example, in the string “This is an example sentence; it serves as an example.”, with substring “an”, you would want to return the index 37, which corresponds to the start of the last occurrence of “an” within the given string.

Method 1: Using rindex() without Range

The rindex() method can find the highest index where a substring occurs in a string. It throws a ValueError if the substring is not found. The method can be called as str.rindex(sub[, start[, end]]), where sub is the substring, and start and end are optional arguments to limit the search within a specific range.

Here’s an example:

text = "Look over there: see the sea!"
try:
    index = text.rindex("sea")
    print(f"The highest index of the substring is: {index}")
except ValueError:
    print("Substring not found.")

Output:

The highest index of the substring is: 25

This code snippet attempts to find the last occurrence of the substring “sea” in the given text. It prints the highest index at which the substring is found. If the substring does not occur, a ValueError is caught, indicating the substring was not found in the string.

Method 2: Using rindex() with Range

Searching for a substring within a specified range using rindex() is helpful when you want to limit the search to a certain part of the string. This can be done by setting the start and end parameters. If the substring is not within the range, a ValueError will be thrown.

Here’s an example:

text = "Python is cool, but not as cool as Pythonista say."
try:
    index = text.rindex("cool", 10, 30)
    print(f"Last occurrence within the range is at index: {index}")
except ValueError:
    print("Substring not found within the specified range.")

Output:

Last occurrence within the range is at index: 17

In this example, the code looks for the last occurrence of “cool” within the characters indexed from 10 to 30. As the substring is found at index 17 within this range, it prints the index. If not found, a ValueError exception would be raised.

Method 3: Using rindex() in a Function

Encapsulating the search functionality in a function can provide a clean and reusable way of applying the rindex() method. The function can accept the string, substring, and optional range parameters, and return the highest index or a custom message if the substring is not found.

Here’s an example:

def find_last_occurrence(text, sub, start=None, end=None):
    try:
        index = text.rindex(sub, start, end)
        return f"Last occurrence of '{sub}' found at index: {index}"
    except ValueError:
        return f"Substring '{sub}' not found in the specified range."

result = find_last_occurrence("Finding sequences within sequences", "seq", 0, 20)
print(result)

Output:

Last occurrence of 'seq' found at index: 19

This function find_last_occurrence() takes the string to search, the substring, and optional start and end indices for the range. When called with a specific range, it returns a message with the last occurrence of the substring or a not found message.

Method 4: Handling Exceptions

Python’s rindex() method will throw a ValueError if the substring is not found in the specified range. Handling this exception allows for graceful degradation of the application or provides alternative logic paths when the substring is absent.

Here’s an example:

text = "Error handling in Python can be handled elegantly."
sub = "handled"

try:
    index = text.rindex(sub, 0, 25)
except ValueError:
    index = None

if index is not None:
    print(f"Substring '{sub}' found at index: {index}")
else:
    print(f"Substring '{sub}' not found in the given range.")

Output:

Substring 'handled' not found in the given range.

This example tries to find the substring “handled” within the first 25 characters. Since it’s not within this range, the catch block sets index to None, and the program informs the user that the substring was not found.

Bonus One-Liner Method 5: Using Lambda and Exception Handling

For those who enjoy concise code, a one-liner using lambda functions and exception handling can achieve the same substring search with rindex.

Here’s an example:

find_last = lambda text, sub, start, end: text.rindex(sub, start, end) if sub in text[start:end] else "Not found"
print(find_last("The quick brown fox jumps over the lazy dog", "fox", 0, 30))

Output:

This one-liner creates a lambda function find_last that takes the string, substring, and range parameters. It uses a conditional expression to either return the index found by rindex() or a “Not found” message.

Summary/Discussion

Method 1: Using rindex() without Range. Simple and direct. Not suited for searches within specific ranges.
Method 2: Using rindex() with Range. Precise control within a specified range. Requires knowing the range beforehand.
Method 3: Using rindex() in a Function. Reusable and clean. Slightly more overhead due to function calls.
Method 4: Handling Exceptions with rindex(). More robust code. Extra code needed for exception handling.
Method 5: Lambda One-Liner. Elegant and concise for simple use cases. Less readable for complex scenarios or exception handling.