Coding Grandmaster · project 18 of 46 · premium, free for now

Permission flags

With READ = 4, WRITE = 2 and EXEC = 1, write grant(perms, flag), revoke(perms, flag), has(perms, flag) and describe(perms) which returns text like r-x (r, w and x, or - for a missing one).

Learn

Real systems pack many yes/no settings into one number, a bit field. File permissions work this way: READ is 4, WRITE is 2, EXECUTE is 1 (in the classic scheme), and 6 means read plus write. Granting a permission is OR, revoking is AND with the inverse, and checking is AND.

Your task

With READ = 4, WRITE = 2 and EXEC = 1, write grant(perms, flag), revoke(perms, flag), has(perms, flag) and describe(perms) which returns text like r-x (r, w and x, or - for a missing one).

Stretch challenge

Add a function that turns a text like rw- back into the number 6.

← Every subset Counting outcomes →

Python

Press Run to see what your program prints.

Keep going in Coding Grandmaster

Seeing the bits

Print each of 5, 12 and 255 as 8 binary digits, and write count_ones(n) that counts the 1 bits using & and >>.

Start →14

One-line bit tricks

Write is_power_of_two(n) using n & (n - 1), is_odd(n) using n & 1, and print the powers of two up to 200 using a list comprehension.

Start →15

XOR magic

Swap a and b with three XOR lines (no temporary variable), and write lonely(numbers) that returns the number that appears only once by XOR-ing everything together.

Start →16

Set, clear and toggle

Write set_bit(n, i), clear_bit(n, i), toggle_bit(n, i) and test_bit(n, i) (True or False) using shifts and bitwise operators.

Start →17

Every subset

Write subsets(items) returning a list of every subset as a list, in the order of the counter 0 to 2^n − 1, using bit tests. Print how many subsets four items have, and the subsets of two items.

Start →

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FAQ

Permission flags: common questions

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