Andrei Alexandrescu Wrote: > I'm writing a range to select a random sampling of a forward range. The > number of elements in the forward range is already known. > > The strategy is: at any moment, I know I need to select k random samples > out of n. To implement popFront, I generate a random number in [0, n - > k]. That's the probability that the next item in the input will be part > of the selection. If the random number is 0, then the element got > selected. Otherwise, I must ignore that guy so I popFront the input > range, decrease n by one, and repeat. > > This seems reasonable but somehow the selection comes skewed: the > elements towards the beginning of the range are less represented than > the ones towards the end. Where am I wrong? > > > Andrei
You have your probability wrong. The selection probability is k/n. Pick a number in [0,n) and pick the element if the result is in [0,k). If you do select an item, decrement k. Always decrement n when popping, regardless of if the element is selected.
