Character Set Membership Check (Allowed Alphabets)

Set membership asks whether a character belongs to an allowed alphabet—ch in allowed with a set for O(1) average lookups after you pay the build cost.

Why this shows up in the real world

Base64 validators ensure only A-Za-z0-9+/= appear. Hex color pickers restrict to [0-9a-fA-F] per slot.

Core idea (explained for students)

ALLOWED = set('abc'); ch in ALLOWED. For static ASCII classes, sometimes a boolean chain is faster to type in contests, but sets scale when the alphabet grows.

Try this in Python

BASE64 = set("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=")


def is_base64_char(ch: str) -> bool:
    return len(ch) == 1 and ch in BASE64


print(is_base64_char("Z"), is_base64_char("?"))

Common mistakes

• Building a new set inside a tight loop—hoist construction outside.
• Case sensitivity: 'a' in {'A'} is false unless normalized.

Key takeaways

• Precompute frozen sets for immutable allowed alphabets in hot paths.
• Pair membership checks with normalization (lower) when rules ignore case.