Given an unknown length list, return a random item in it by scanning it only 1 time

Question

Given an unknown length list, return a random item in it by scanning it only 1 time.

My idea:

A similar algorithm is Reservoir Sampling (posted by others). But

Answer 1

See the Perl cookbook for the algorithm, which you'll easily adapt to C++.

Basically, scan the list once, and for each entry with index i you read, keep it if a random number between 0 and i+1 is less than 1.

The result is the last entry kept.