5 Best Ways to Check if All Occurrences of a Character Appear Together in Python

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πŸ’‘ Problem Formulation: When working with strings in Python, it’s common to encounter scenarios where you need to check if all occurrences of a specific character are contiguous within the string. For example, given the string “hello” and the character ‘l’, we want our function to return True since the ‘l’ characters are adjacent. Conversely, with the string “loop” and the character ‘o’, the function should return False.

Method 1: Using Iteration

Iteration is a straightforward approach where you examine each character in sequence to determine the character groupings. A flag is used to denote the start and stop of a sequence for the specific character, ensuring all occurrences are contiguous.

Here’s an example:

def check_occurrences(string, character):
    found = False
    for char in string:
        if char == character:
            if found:
                continue
            else:
                found = True
        else:
            if found:
                return False
    return True

print(check_occurrences("hello", "l"))  # Expected output: True
print(check_occurrences("loop", "o"))   # Expected output: False

This code snippet defines a function check_occurrences that iterates over the string character by character. If the target character is found, it uses a boolean flag to indicate its presence and ensures that subsequent characters match while adjacent. If they aren’t, it returns False.

Method 2: Using regex

Regular expressions offer a powerful way to find patterns within strings. With a well-crafted pattern, you can check for the presence of contiguous character occurrences using the regex library.

Here’s an example:

import re

def check_occurrences_regex(string, character):
    return re.fullmatch(rf'[^{character}]*{character}+[^{character}]*', string) is not None

print(check_occurrences_regex("hello", "l"))  # Expected output: True
print(check_occurrences_regex("loop", "o"))   # Expected output: False

This code snippet uses the regex pattern to define a string that does not contain the target character, followed by one or more occurrences of the character, and then any characters not matching the target again. If the string matches this pattern, all occurrences of the character are together.

Method 3: Using str.find and str.rfind

The str.find() and str.rfind() methods return the lowest and highest indexes of the substring if found. Comparing the indexes for the first and last occurrence can determine grouping.

Here’s an example:

def check_occurrences_find(string, character):
    first_occurrence = string.find(character)
    last_occurrence = string.rfind(character)
    return first_occurrence == last_occurrence or last_occurrence - first_occurrence == string.count(character) - 1

print(check_occurrences_find("hello", "l"))  # Expected output: True
print(check_occurrences_find("loop", "o"))   # Expected output: False

This code snippet checks if the first and last occurrence of the character are equal or the distance between them equals the count of the character minus one, implying they are contiguous.

Method 4: Using str.count and string slicing

This method leverages the power of slicing and str.count() to determine the presence and grouping of the target character within the string.

Here’s an example:

def check_occurrences_slice(string, character):
    count = string.count(character)
    first_occurrence = string.find(character)
    return string[first_occurrence:first_occurrence + count] == character * count

print(check_occurrences_slice("hello", "l"))  # Expected output: True
print(check_occurrences_slice("loop", "o"))   # Expected output: False

The function defined in the code uses slicing to grab the segment of the string where the target character is expected to be contiguous, and compares it to a string composed of the target character, repeated as many times as it’s counted in the original string.

Bonus One-Liner Method 5: Using set intersection

Python’s set allows for concise checks by looking at the intersection of the string’s partitions around the character.

Here’s an example:

check_occurrences_set = lambda s, c: {c} == set(s.split(c, 1)[1].partition(c)[0])

print(check_occurrences_set("hello", "l"))  # Expected output: True
print(check_occurrences_set("loop", "o"))   # Expected output: False

This one-liner checks if splitting the string on the target character results in exactly one other instance, indicating that all occurrences are grouped together.

Summary/Discussion

  • Method 1: Iteration. Easy to understand. Can be inefficient for very long strings due to manual traversal.
  • Method 2: Using regex. Very powerful and concise. May be overkill for simple cases and slightly less performant than some other methods.
  • Method 3: Using str.find and str.rfind. Simple built-in methods. Assumes only one character check and doesn’t easily extend to larger substrings.
  • Method 4: Using str.count and string slicing. Combines counting and indexing for a clever solution. Requires multiple passes on the string (for count and find).
  • Method 5: Using set intersection. Extremely concise. Assumes that the character occurs at least twice and could be less intuitive to understand.