Return the number of 1 bits in a nonnegative integer without converting it to a string.

n = 13
print(bin(n), bin(n & (n-1)))

Clearing the lowest set bit with n & (n-1) removes exactly one bit. The loop runs once per set bit. Before writing code, identify what each variable means after one iteration. Walk through the example and explain why the next transition preserves that meaning. Then check the smallest permitted input and a case with repeated values. A correct answer follows the contract for every input, including the boundary cases in the checks. Keep any input mutation within the stated rules; return a fresh result when the task asks for one.

Return the number of 1 bits in a nonnegative integer without converting it to a string.

The explanation opens here once every check passes.