Check whether a server is in an allow-list

easySets and tuples

An allow-list is a set of names that are permitted, and everything else is blocked. A set is the right tool for it: asking "is this name in the set?" stays quick no matter how many names the set holds, while a list has to check its items one by one.

Common mistake: comparing the name to the whole set.

TEXT

Wrong: if name == allow_list:

This asks whether the name is equal to the entire set, which is never true. To ask "is it inside the set?", use the membership check: in in Python, .contains() in Java, .count() in C++.

Your task: given the allow-list {"web-01", "web-02", "db-01"}, check the names "web-02" and "cache-09". Print each name followed by allowed or blocked.

Expected output:

TEXT

web-02: allowed
cache-09: blocked

Approach

Loop over the names and check each one against the set:

Python: name in allow_list
Java: allowList.contains(name), using a HashSet. Order does not matter here, since you only look names up.
C++: allowList.count(name) > 0, using an unordered_set. count gives 1 if the name is in the set and 0 if not.

DSA connection: checking a hash set takes roughly the same time however big the set gets, which is why it beats scanning a list.

allow_list = {"web-01", "web-02", "db-01"}
for name in ["web-02", "cache-09"]:
    if name in allow_list:
        print(f"{name}: allowed")
    else:
        print(f"{name}: blocked")

Problem statement

Approach