💡 Problem Formulation: Given a string ‘str’ and an integer ‘k’, the challenge is to reverse every ‘k’ characters of the first ‘2k’ characters in the string, leaving any remaining characters in their original order. For example, given the input ‘str = “abcdefg”, k = 2’, the desired output is ‘bacdfeg’.
Method 1: Using Slicing and Loops
This method utilises Python’s string slicing to reverse each segment of the string in a loop. Given parameters ‘str’ and ‘k’, it iterates over the string, reversing chunks of ‘k’ elements while leaving the next ‘k’ elements unchanged. This process is repeated for the length of the string.
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Here’s an example:
def reverse_string_k(str, k):
str_list = list(str)
for i in range(0, len(str), 2*k):
str_list[i:i+k] = reversed(str_list[i:i+k])
return ''.join(str_list)
print(reverse_string_k("abcdefg", 2))Output: ‘bacdfeg’
In this snippet, the reverse_string_k() function is defined to take two arguments, the string and k. Inside the function, the string is converted to a list to easily manipulate its characters. A loop is used to reverse each k-sized chunk of characters in steps of 2k, ensuring every alternate k-sized chunk is reversed. The result is then joined back into a string.
Method 2: Using List Comprehension
This approach leverages list comprehension for concise code. It builds a new string by concatenating reversed and unreversed segments produced by implicitly iterating over the string in slices of ‘2k’.
Here’s an example:
def reverse_string_k(str, k):
return ''.join(str[i:i+k][::-1] + str[i+k:i+2*k] for i in range(0, len(str), 2*k))
print(reverse_string_k("abcdefgh", 2))Output: ‘bacdfegh’
The reverse_string_k() function builds the result through concatenation of sliced strings inside a list comprehension. The slice str[i:i+k][::-1] reverses the first k characters and str[i+k:i+2*k] captures the next k characters as is, iterating over the string in increments of 2k.
Method 3: Using Recursion
This recursive solution splits the problem into smaller subproblems. If the string length is larger than ‘2k’, it processes the first ‘2k’ characters and recursively calls itself for the remaining string, until the entire string is reversed as needed.
Here’s an example:
def reverse_string_k(str, k):
if len(str) < k:
return str[::-1]
elif len(str) < 2*k:
return str[:k][::-1] + str[k:]
else:
return str[:k][::-1] + str[k:2*k] + reverse_string_k(str[2*k:], k)
print(reverse_string_k("abcdefgh", 2))Output: ‘bacdfegh’
The reverse_string_k() function applies string slicing and recursion. It handles three cases: strings shorter than k (fully reversed), strings between k and 2k (partial reverse), and strings longer than 2k where the function calls itself for the remaining substring beyond 2k characters.
Method 4: Using Iterable Unpacking
Iterable unpacking in Python enables swapping chunks of data efficiently. This method divides the string into k-sized chunks and reverses each pair of chunks (‘chunks[0]’ and ‘chunks[1]’) using this technique.
Here’s an example:
def reverse_string_k(str, k):
chunks = [str[i:i+k] for i in range(0, len(str), k)]
for i in range(0, len(chunks), 2):
chunks[i] = chunks[i][::-1]
return ''.join(chunks)
print(reverse_string_k("abcdefghi", 2))Output: ‘bacdfeghi’
In the code above, a list of chunks is created, and then iterable unpacking is used to reverse only the chunks at even indices (chunks[i]). The updated chunks are joined together to form the final string.
Bonus One-Liner Method 5: Using Itertools.Chain and Slicing
The itertools module provides a way to efficiently chain iterables. By using itertools.chain, this one-liner first reverses the necessary parts and then chains them together in a single readable line.
Here’s an example:
import itertools
def reverse_string_k(str, k):
return ''.join(itertools.chain(*[(str[i:i+k][::-1], str[i+k:i+2*k]) for i in range(0, len(str), 2*k)]))
print(reverse_string_k("abcdefgh", 2))Output: ‘bacdfegh’
The function constructs pairs of reversed and unreversed k-length segments using list comprehension and then flattens the list of pairs using itertools.chain to concatenate the elements into one string.
Summary/Discussion
- Method 1: Using Slicing and Loops. Straightforward and easily understandable. Relatively verbose in comparison to other methods.
- Method 2: Using List Comprehension. Compact and Pythonic. May sacrifice some readability for those unfamiliar with list comprehensions.
- Method 3: Using Recursion. Elegant for smaller strings but can lead to stack overflow for very large strings due to recursion limits.
- Method 4: Using Iterable Unpacking. Efficient swapping using unpacking technique. Might be confusing to beginners due to the advanced slicing and unpacking operations.
- Method 5: Using Itertools.Chain and Slicing. Very concise one-liner. Depends on an external module and requires understanding of iterator chaining, which may not be obvious for all.