String Iteration (zip, enumerate, reverse)

String iteration walks characters in order—for ch in s, enumerate, zip(s, t), or index loops. Reverse walks use reversed(s) or negative stepping on indices.

Why this shows up in the real world

Diff algorithms pair forward and reverse passes. Checksums combine each byte in traversal order.

Core idea (explained for students)

Parallel iteration: for a, b in zip(s, t):. Different lengths: zip truncates; use zip_longest from itertools when you need padding rules.

Try this in Python

def pairwise_xor_digits(a: str, b: str) -> list[int]:
    return [int(x) ^ int(y) for x, y in zip(a, b)]


print(pairwise_xor_digits("101", "110"))

Common mistakes

• Mutating an index while iterating forward.
• Assuming O(1) per-character access cost in languages where strings are UTF-16 or ropes—Python is fine, but know your platform.

Key takeaways

• Prefer enumerate when you need position for slices or neighbor checks.
• For huge files, iterate lines or chunks instead of loading one giant string.