On Sat, Feb 18, 2012 at 02:52:15AM +0100, Alf P. Steinbach wrote: > On 18.02.2012 02:39, H. S. Teoh wrote: > >// Outputs a randomly selected line from standard input with equal > >// likelihood. > >import std.random; > >import std.stdio; > > > >void main() { > > auto n = 0; > > string choice; > > foreach (line; stdin.byLine()) { > > n++; > > if (uniform(0,n) == 0) > > choice = line.idup; > > } > > writeln(choice); > >} > > > > > >P.S. Proof that the probability any line is selected is exactly 1/n > >(where n is the total number of lines read) is left as an exercise > >for the reader. ;-) > > Assuming that by "any" you mean "any particular", you would have to > read all the lines first. Otherwise, if the code selects the first > line with probability 1/K, then I can just input some other number of > lines.
But therein lies the trick. The algorithm self-adapts to the number of lines it reads. It does not know in advance how many lines there are, but guarantees that by the end of the input, the probability that any particular line is selected is exactly 1/n, where n is the number of lines read. The key lies in the way uniform() is called. :) (This has been tested by running the program repeatedly on the same input and analysing the number of occurrences of each line. The proof of the algorithm is left as an exercise for the reader. ;-) ) > >P.S.S. The .idup is a bit ugly, but necessary, since apparently byLine() > >calls readln() with a static buffer, so choice will be silently > >overwritten if the .idup is omitted. > > That sounds ominous. One should never have to be aware of low level > details in order to do simple string assignment or initialization, > when the source already is a string. Does one really have to do that > in D? [...] Well, there are two issues here. One is that stdin.byLine() returns char[], which cannot be assigned to string directly. Two is that the first version of the code didn't have the .idup (I used char[] instead of string for choice), and the result was garbage in the output due to said overwriting of buffer. Now I agree that one shouldn't need to know how byLine() is implemented in order to be able to use it, but this is the way the current Phobos implementation is, unfortunately. However, the one upside to all this is that the type system, in a sense, forces you to do the right thing: byLine() returns char[] which cannot be assigned to string, so naturally you have to write .idup, which automatically also avoids the buffer overwriting issue. Using string instead of char[] is a logical choice in this case (pun intended ;-)), because it's something whose value you want to retain until the next time it's modified. So using string for 'choice' naturally leads to needing an .idup in the loop. The system isn't perfect, but it's pretty good. T -- Political correctness: socially-sanctioned hypocrisy.