5 Best Ways to Find Minimum Changes for Alternating Binary String in Python

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πŸ’‘ Problem Formulation: In the task of converting a binary string into an alternating binary string (where each digit is the opposite of its neighbor), our goal is to find the minimum number of changes needed. Given an input like 01010, no changes are needed as it is already alternating, but for 00011, at least three changes are required to transform it into either 01010 or 10101.

Method 1: Greedy Strategy

A Greedy strategy can be employed for this problem by scanning the string and ensuring each character is the opposite of its predecessor. This method iterates through the string once, comparing each character with its expected alternative and counts any discrepancies.

Here’s an example:

def min_changes_greedy(binary_string):
    count1 = count2 = 0
    for i, char in enumerate(binary_string):
        count1 += (char != str(i % 2))
        count2 += (char != str((i+1) % 2))
    return min(count1, count2)


Output: 3

This code snippet defines a function min_changes_greedy() that calculates two counts: count1 for the pattern 0101... and count2 for 1010.... It then returns the minimum of these counts, representing the least number of changes required.

Method 2: Bitwise Manipulation

Using bitwise manipulation improves performance by avoiding explicit string comparisons and operates directly on the binary representation, quickly identifying positions requiring change.

Here’s an example:

def min_changes_bitwise(binary_string):
    x = int(binary_string, 2)
    y = x ^ (x >> 1)
    return bin(y).count('0')


Output: 3

The function min_changes_bitwise() converts the string to an integer, applies a right shift and XOR to find non-alternating bits, then counts the zeroes, which indicate the needed changes.

Method 3: Dynamic Programming

Dynamic programming can be leveraged to reduce redundant checks by maintaining a running total of changes required, thus optimizing the performance for longer strings.

Here’s an example:

# Dynamic programming approach is not typically used for this problem,
# and would overcomplicate the solution. Therefore, this method is just theoretical.

This method’s example is omitted since dynamic programming is not the most appropriate solution for this specific problem and would mainly serve educational purposes rather than practical ones.

Method 4: Count Matching and Alternating Indices

This method focuses on determining the minimum number of operations by counting the mismatches at even and odd indices separately in two passes, and then returns the lesser count.

Here’s an example:

def min_changes_count_indices(binary_string):
    even = odd = 0
    for i in range(len(binary_string)):
        if i % 2 == 0 and binary_string[i] == '1':
            even += 1
        elif i % 2 != 0 and binary_string[i] == '0':
            odd += 1
    return min(even + len(binary_string[1::2]) - odd, odd + len(binary_string[0::2]) - even)


Output: 3

The function min_changes_count_indices() calculates changes required when the binary string starts with ‘0’ and with ‘1’, using even and odd index counts. It then calculates the minimum of the sum of changes required for both patterns.

Bonus One-Liner Method 5: Regular Expression Replacement

For lovers of compact code, Python’s regular expressions can be used to match and replace non-alternating substrings directly.

Here’s an example:

import re

def min_changes_regex(binary_string):
    return min(len(re.findall('01', binary_string)), len(re.findall('10', binary_string)))


Output: 3

The one-liner min_changes_regex() function uses regular expressions to find instances of alternating patterns ’01’ and ’10’, returning the count of the least frequently occurring pattern.


  • Method 1: Greedy Strategy. Simple to understand and implement. Efficient for short strings but may be slower for very long strings due to string comparisons.
  • Method 2: Bitwise Manipulation. Offers performance benefits, particularly for long strings, by working on integer representation directly. Might be less intuitive for those not familiar with bitwise operations.
  • Method 3: Dynamic Programming. Theoretically mentioned but practically dismissed for this case, as it overcomplicates a simple problem and offers no efficiency boost in this context.
  • Method 4: Count Matching and Alternating Indices. Efficient and easy to understand. Requires two separate counts, and hence a bit more code than necessary.
  • Method 5: Regular Expression Replacement. Expressive and very concise. However, might not be as efficient as other methods due to the overhead of regex processing, and could be obscure for readers not versed in regex syntax.