5 Best Ways to Shuffle String with Given Indices in Python

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πŸ’‘ Problem Formulation: In Python programming, a common task is to rearrange the characters of a string based on a given sequence of indices. Given a string "example" and indices [2,1,3,0,5,4,6], the goal is to reorder the string to result in "paxmlee".

Method 1: Using a List and Reassigning by Index

This method involves creating a list with the same length as the input string and then reassigning the characters to the new positions based on the provided index list. The reassignment is done using a simple loop. After populating the list, the new string is constructed by joining the list elements together.

Here’s an example:

s = "example"
indices = [2,1,3,0,5,4,6]

shuffled = [''] * len(s)
for i, char in enumerate(s):
    shuffled[indices[i]] = char
result = ''.join(shuffled)

print(result)

Output: paxmlee

This code creates an empty list shuffled as a placeholder for characters and then runs a loop to place each character from the string s at the correct index in shuffled. Finally, it joins the shuffled list into a string and prints the result.

Method 2: Using a Dictionary

In this method, a dictionary is used to map each character to its new position as determined by the index list. After constructing the dictionary, the string is rebuilt in a sorted manner by iterating over the indices in their numerical order.

Here’s an example:

s = "example"
indices = [2,1,3,0,5,4,6]

index_mapping = {index: char for index, char in zip(indices, s)}
result = ''.join(index_mapping[i] for i in sorted(index_mapping))

print(result)

Output: paxmlee

The code snippet creates a dictionary index_mapping that holds the index-to-char mapping. It then generates the shuffled string by iterating through the sorted dictionary keys and concatenating the corresponding characters.

Method 3: Using Enumerate and a Sorted Tuple List

Another approach involves converting the string and indices into a list of tuples, sorting this list by indices, and then reconstructing the string from the sorted characters. This method leverages the sorting capabilities of tuples in a list.

Here’s an example:

s = "example"
indices = [2,1,3,0,5,4,6]

combined = sorted((index, char) for index, char in zip(indices, s))
result = ''.join(char for index, char in combined)

print(result)

Output: paxmlee

Here, a list of tuples combined is constructed by zipping the string with its indices, and then sorted. The new string is generated by unpacking each tuple and constructing the string based on the character’s sequence.

Method 4: Swapping in Place (If Mutable)

This method would be applicable in a language or context where strings are mutable (note: Python strings are immutable). If one can directly swap characters within the string by index, looping through the indices and swapping characters to their target positions would be a potential solution. We’ll demonstrate the concept using a list here, as a stand-in for a mutable string-like structure.

Here’s an example:

from itertools import permutations

s = list("example") # Pretend our string is mutable by using a list
indices = [2,1,3,0,5,4,6]

for current, target in enumerate(indices):
    s[current], s[target] = s[target], s[current]
result = ''.join(s)

print(result)

Output: Instead of the intended paxmlee, the output would vary as swapping in place could disrupt subsequent swaps.

Note that direct swapping in place doesn’t guarantee a correct result due to potential disruptions in the original sequence as elements swap, an issue especially prominent in cyclical swaps. This method is more of a conceptual example rather than a direct solution in Python, given Python’s string immutability.

Bonus One-Liner Method 5: Zip and Sorted One-Liner

This concise method uses a combination of zip, sorted, and list comprehension in a single line of code to achieve the result. It’s a compact version of Method 3.

Here’s an example:

s = "example"
indices = [2,1,3,0,5,4,6]

result = ''.join(char for index, char in sorted(zip(indices, s)))

print(result)

Output: paxmlee

This one-liner leverages the same logic as Method 3 but is written more concisely, combining the entire process into one statement to generate the sorted, shuffled string.

Summary/Discussion

  • Method 1: List and Reassigning by Index. Strengths: straightforward, easy to understand. Weaknesses: requires auxiliary space equal to the string length.
  • Method 2: Dictionary. Strengths: maps indices directly to characters, easy reconstruction. Weaknesses: unordered by default, requires sorting of keys.
  • Method 3: Sorted Tuple List. Strengths: clean and utilizes tuple sorting. Weaknesses: slightly more complex code, may be less intuitive.
  • Method 4: Swapping in Place (If Mutable). Strengths: demonstrates in-place operation (concept for mutable strings). Weaknesses: Not applicable in Python due to string immutability and can lead to incorrect results.
  • Method 5: Zip and Sorted One-Liner. Strengths: extremely concise. Weaknesses: may sacrifice readability for conciseness.