💡 Problem Formulation: Run-length encoding is a simple form of data compression where sequences of the same data value are stored as a single data value and a count. The challenge is to implement a Python iterator class that decodes a run-length encoded string. Given the input string '2a5b3c', the desired output would be an iterator that produces ['a', 'a', 'b', 'b', 'b', 'b', 'b', 'c', 'c', 'c'] when iterated over.
Method 1: Basic Iterator Class Using Regex
Implementing a basic iterator class to decode run-length encoded strings involves using regular expressions to identify numeric and character pairs in the input string. The iterator will maintain a state of the current count and character to return on each iteration.
Here’s an example:
import re
class RunLengthIterator:
def __init__(self, encoded):
self.encoded = encoded
self.pattern = re.compile(r'(\d+)(\D)')
self.matches = self.pattern.finditer(self.encoded)
self.current_match = None
self.counter = 0
def __iter__(self):
return self
def __next__(self):
if not self.current_match or self.counter == 0:
self.current_match = next(self.matches)
self.counter = int(self.current_match.group(1))
self.counter -= 1
return self.current_match.group(2)
Output when used in a for loop:
decoded = []
for char in RunLengthIterator('2a5b3c'):
decoded.append(char)
print(decoded)
['a', 'a', 'b', 'b', 'b', 'b', 'b', 'c', 'c', 'c']
This code snippet demonstrates how to build a run-length iterator class that utilizes regex to split the encoded data into count-character pairs and then iterate over these pairs accordingly, reducing the count each time the relevant character is returned.
Method 2: Using Iterator with Built-in Itertools
The itertools library provides a combination of iterators that can simplify the process of decoding. By combining the groupby and chain functions, we can build a more Pythonic approach to decoding run-length encoded strings.
Here’s an example:
from itertools import chain
class RunLengthIterator:
def __init__(self, encoded):
self.encoded = encoded
self.pairs = ((int(encoded[i]), encoded[i+1]) for i in range(0, len(encoded), 2))
def __iter__(self):
return chain.from_iterable([char] * num for num, char in self.pairs)
# Example usage
decoded = list(RunLengthIterator('2a5b3c'))
print(decoded)
['a', 'a', 'b', 'b', 'b', 'b', 'b', 'c', 'c', 'c']
Here, RunLengthIterator leverages itertools to create a more streamlined and readable approach. The encoded string is first transformed into a sequence of pairs representing counts and characters. These pairs are then flattened into a single iterator that emits the characters the correct number of times.
Method 3: Generator-Based Approach
A generator-based approach to making an iterator class can make the code leaner and more readable. Generators are a simple way of creating iterators and provide a powerful tool for working with sequences of data.
Here’s an example:
class RunLengthIterator:
def __init__(self, encoded):
self.encoded = encoded
def __iter__(self):
return self.decode_gen()
def decode_gen(self):
i = 0
while i < len(self.encoded):
count = int(self.encoded[i])
char = self.encoded[i+1]
for _ in range(count):
yield char
i += 2
# Example usage
decoded = list(RunLengthIterator('2a5b3c'))
print(decoded)
['a', 'a', 'b', 'b', 'b', 'b', 'b', 'c', 'c', 'c']
This method defines an iterator class that utilizes a generator function decode_gen. This generator is cleverly designed to yield the next-character in the decoded sequence each time the iterator is used, making it elegant and efficient.
Method 4: Explicit State Management
In situations where using libraries or built-in Python features is not preferred, we can manage the state of the iterator explicitly. This involves parsing the encoded string character by character and keeping track of the current character and its count manually.
Here’s an example:
class RunLengthIterator:
def __init__(self, encoded):
self.encoded = encoded
self.index = 0
self.current_char = ''
self.current_count = 0
def __iter__(self):
return self
def __next__(self):
while self.current_count == 0 and self.index 0:
self.current_count -= 1
return self.current_char
raise StopIteration
# Example usage
decoded = []
for char in RunLengthIterator('2a5b3c'):
decoded.append(char)
print(decoded)
['a', 'a', 'b', 'b', 'b', 'b', 'b', 'c', 'c', 'c']
This approach manually parses the encoded string, updating the current_char and current_count during iteration. Although it lacks the elegance of regexp or itertools, this method gives you granular control over the iteration process.
Bonus One-Liner Method 5: Comprehension With Repetition Operator
Python’s list comprehensions and the repetition operator can be used to one-line decode a run-length encoded string. This approach provides a compact, albeit less readable, solution.
Here’s an example:
class RunLengthIterator:
def __iter__(self):
return (char for i in range(0, len(self.encoded), 2)
for char in self.encoded[i+1] * int(self.encoded[i]))
# Example usage
encoded = '2a5b3c'
decoded = list(RunLengthIterator(encoded))
print(decoded)
['a', 'a', 'b', 'b', 'b', 'b', 'b', 'c', 'c', 'c']
This method defines an ultra-compact iterator that decodes the string using a nested generator expression, capitalizing on Python’s powerful inline repetition operator to multiply characters.
Summary/Discussion
- Method 1: Basic Iterator Class Using Regex. Strengths: readable, uses powerful regex for parsing. Weaknesses: regex can be slower for very long strings and less straightforward for those unfamiliar with it.
- Method 2: Using Iterator with Built-in Itertools. Strengths: concise and makes use of Python’s standard library. Weaknesses: requires understanding of itertools and less explicit control over iteration.
- Method 3: Generator-Based Approach. Strengths: lean, Pythonic, and takes full advantage of generators. Weaknesses: less explicit than a class-based iterator, may be more complex for beginners.
- Method 4: Explicit State Management. Strengths: offers full control over iteration process, doesn’t rely on Python-specific features. Weaknesses: more verbose and manual management of states.
- Bonus One-Liner Method 5: Comprehension With Repetition Operator. Strengths: compact one-liner. Weaknesses: can be considered less readable, especially for those who are new to Python’s comprehensions.