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#python#algorithms#strings

Python: pseudo-encryption by shuffling indices

Swap even and odd index characters N times — plus a decrypt function that reverses it.

The problem

Given a string S and an integer N, the algorithm:

  1. Take characters at odd indices (1, 3, 5…)
  2. Append characters at even indices (0, 2, 4…)
  3. Repeat steps 1–2 exactly N times

Examples for encrypt:

  • encrypt("012345", 1) → "135024"
  • encrypt("012345", 2) → "304152"
  • encrypt("012345", 3) → "012345" — after 3 steps we're back to the start!

For odd-length strings it works the same — e.g. encrypt("01234", 2) → "32104".

Approach — encrypt

Each iteration splits the string into two lists by index parity, joins odd + even, and repeats.

Approach — decrypt

Reverse operation: the first half of the encrypted string holds odd-index characters, the second half holds even-index ones. Interleave them back: even[i], odd[i], even[i+1], odd[i+1]…

Solution

def encrypt(encrypted_text, n):
    result = encrypted_text
 
    while n > 0:
        even_indexes = []
        odd_indexes = []
        for index, item in enumerate(result):
            if index % 2 == 0:
                even_indexes.append(item)
            else:
                odd_indexes.append(item)
 
        result = "".join(odd_indexes + even_indexes)
        n -= 1
 
    return result
 
 
def decrypt(text, n):
    result = text
 
    while n > 0:
        middle = len(result) // 2
        combined = []
        odd_part = result[:middle]
        even_part = result[middle:]
 
        for i in range(max(len(odd_part), len(even_part))):
            combined.append(even_part[i])
            if i < len(odd_part):
                combined.append(odd_part[i])
 
        result = "".join(combined)
        n -= 1
 
    return result

Examples

encrypt

Input N Output
"012345" 1 "135024"
"012345" 2 "304152"
"012345" 3 "012345"
"01234" 2 "32104" 
print(encrypt("012345", 1))  # 135024
print(encrypt("012345", 2))  # 304152
print(encrypt("012345", 3))  # 012345
print(encrypt("01234", 2))   # 32104

decrypt

print(decrypt("s eT ashi tist!", 2))  # This is a test!

Summary

Simple index shuffling in a loop — fun fact: for "012345", the third encryption restores the original. Writing decrypt helps you understand the reverse operation.