py_string_sculptor — Alternating Case Across Letters

Difficulty: medium

Transform a string by alternating the case of alphabetic characters only. Non-alphabetic characters remain unchanged and are ignored for the alternation. The first alphabetic character is lowercase, the second uppercase, the third lowercase, and so on.

def string_sculptor(text: str) -> str:

Examples

>>> string_sculptor("hello")
"hElLo"

>>> string_sculptor("Hello World")
"hElLo wOrLd"

>>> string_sculptor("aBc123def")
"aBc123DeF"

>>> string_sculptor("Python3.9!")
"pYtHoN3.9!"

>>> string_sculptor("")
""

Solution

Download py_string_sculptor.py

def string_sculptor(text: str) -> str:
    result = []
    idx = 0
    for c in text:
        if c.isalpha():
            result.append(c.lower() if idx % 2 == 0 else c.upper())
            idx += 1
        else:
            result.append(c)
    return "".join(result)

How It Works

Goal: Lowercase the 1st letter, uppercase the 2nd, lowercase the 3rd, etc. — counting only letters, leaving everything else alone.

Approach: Maintain a separate index idx that advances only on alphabetic characters, so non-letters never throw off the alternation rhythm.

Step by step:

1. Iterate over each character of the input.
2. If it's alphabetic, lowercase it when idx is even and uppercase it when odd. Increment idx.
3. Otherwise, append the character unchanged.
4. Join the list back into a string.

Key concept: Decoupling the input index from a "logical" index — when you only count certain elements, keep a separate counter that ignores the rest.