5 Best Ways to Decode a Run Length Form of String into Normal Form in Python

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Decoding Run Length Encoded Strings in Python

πŸ’‘ Problem Formulation: Run-length encoding (RLE) is a simple form of data compression where runs of data are stored as a single data value and count. This article illustrates how to decode such a run-length encoded string back to its original form. For example, converting the encoded string “4A3B2C1D” into its decoded form “AAAABBBCCD”.

Method 1: Iterative Approach

The iterative approach involves looping over each character in the encoded string and building the decoded string by repeating characters according to their counts. This method is easy to understand and implement, and works well for small to medium size strings.

Here’s an example:

def decode_rle(s):
    decoded = ''
    count = ''
    for char in s:
        if char.isdigit():
            count += char
        else:
            decoded += char * int(count)
            count = ''
    return decoded
    
print(decode_rle("4A3B2C1D"))

Output: AAAABBBCCD

This function iterates over every character in the input string and builds the decoded string by appending the character repeated by the preceding number count. It resets the count after processing each character.

Method 2: Using Regular Expressions

This method introduces the power of regular expressions to match the pairs of counts and characters. The re.findall() method is used to find all occurrences of a specific pattern, which we then decode in a list comprehension.

Here’s an example:

import re

def decode_rle(s):
    return ''.join(char * int(count) for count, char in re.findall(r'(\d+)(\D)', s))

print(decode_rle("4A3B2C1D"))

Output: AAAABBBCCD

This code uses a regular expression pattern to extract pairs of counts and characters and a list comprehension to form the decoded string accordingly. It is concise but requires knowledge of regular expressions.

Method 3: Using itertools.groupby

Python’s itertools library has a groupby function which can be utilized to group elements in the string. By modifying the key function, we can decode the RLE by looking at sequences of digits and non-digits.

Here’s an example:

from itertools import groupby

def decode_rle(s):
    grouped = (''.join(g) for k, g in groupby(s, str.isdigit))
    return ''.join(next(grouped) * int(char) for char in grouped)

print(decode_rle("4A3B2C1D"))

Output: AAAABBBCCD

This code uses groupby to partition the string into groups of digits and non-digits and then decodes it in pairs. It’s a very Pythonic way to handle the problem but might be less intuitive for beginners.

Method 4: Using the zip Function

This approach combines list slicing and the zip function to pair counts with characters and then decodes the string. It’s an innovative approach that takes advantage of Python’s iterable unpacking.

Here’s an example:

def decode_rle(s):
    digits = s[::2]
    letters = s[1::2]
    return ''.join(int(d) * l for d, l in zip(digits, letters))

print(decode_rle("4A3B2C1D"))

Output: AAAABBBCCD

By slicing the input string into digits and letters separately and then zipping them together, this function compacts the process of decoding. It’s clean and effective for the correct RLE format.

Bonus One-Liner Method 5: Using Generator Expressions

The ultimate one-liner for RLE decoding in Python makes use of a generator expression nested within a call to join(). This method may sacrifice a bit of readability for brevity.

Here’s an example:

import re

print(''.join(char * int(count) for count, char in re.findall(r"(\d+)(\D)", "4A3B2C1D")))

Output: AAAABBBCCD

This one-line solution captures the essence of the regex-based method, compacting it into a single expression. It’s the shortest method but may be the hardest for beginners to decipher at a glance.

Summary/Discussion

  • Method 1: Iterative Approach. Straightforward. Good for beginners. Can become inefficient with very large strings.
  • Method 2: Using Regular Expressions. Concise. Requires understanding of regex. Efficient for varied formats.
  • Method 3: Using itertools.groupby. Pythonic. Compact. Might be confusing for beginners. Assumes correctly formatted input.
  • Method 4: Using the zip Function. Innovative. Uses list slicing efficiently. Depends on a strict RLE format with alternating counts and letters.
  • Method 5: Using Generator Expressions. Extremely concise. Least readable. Best for those already comfortable with regex and generator expressions.