• dsimcha <dsimcha yahoo.com> Apr 20 2009
• Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> Apr 20 2009
• dsimcha <dsimcha yahoo.com> Apr 20 2009

dsimcha <dsimcha yahoo.com> writes:

Maybe I'm doing something subtly wrong here, but I've checked all the obvious
stuff.

import std.stdio, std.random, std.array;

import dstats.base : Perm;

void main() {
    uint[int[]] counts;
    immutable int[] foo = [1,2,3];

    // Make sure every permutation is represented in counts.
    foreach(perm; Perm!int(foo.dup)) {
        counts[perm.dup] = 0;
    }

    foreach(i; 0..10_000) {
        int[] result;
        foreach(elem; randomCover(foo, Random(unpredictableSeed))) {
            result ~= elem;
        }
        counts[result]++;
    }
    foreach(k, v; counts) {
        writeln(k, "\t", v);
    }
}

Output:

2 3 1   0
3 2 1   0
1 3 2   0
3 1 2   0
1 2 3   4929
2 1 3   5071

Note that when I use randomShuffle instead of randomCover, I get a fairly
uniform distribution on the permutation space.

Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:

dsimcha wrote:
 Maybe I'm doing something subtly wrong here, but I've checked all the obvious
 stuff.
 
 import std.stdio, std.random, std.array;
 
 import dstats.base : Perm;
 
 void main() {
     uint[int[]] counts;
     immutable int[] foo = [1,2,3];
 
     // Make sure every permutation is represented in counts.
     foreach(perm; Perm!int(foo.dup)) {
         counts[perm.dup] = 0;
     }
 
     foreach(i; 0..10_000) {
         int[] result;
         foreach(elem; randomCover(foo, Random(unpredictableSeed))) {


Here you reseed the random number generator every pass through the outer 
loop. Although unpredictableSeed is designed to be unpredictable, it is 
_not_ random.

You'd need to have each cover use the same random generator. Here's 
where a shortcoming in the design of randomCover becomes evident: 
randomCover stores a copy of its generator, which makes it difficult to 
support what you need. I will look into a fix.


Andrei

dsimcha <dsimcha yahoo.com> writes:

== Quote from Andrei Alexandrescu (SeeWebsiteForEmail erdani.org)'s article
 dsimcha wrote:
 Maybe I'm doing something subtly wrong here, but I've checked all the obvious
 stuff.

 import std.stdio, std.random, std.array;

 import dstats.base : Perm;

 void main() {
     uint[int[]] counts;
     immutable int[] foo = [1,2,3];

     // Make sure every permutation is represented in counts.
     foreach(perm; Perm!int(foo.dup)) {
         counts[perm.dup] = 0;
     }

     foreach(i; 0..10_000) {
         int[] result;
         foreach(elem; randomCover(foo, Random(unpredictableSeed))) {


 loop. Although unpredictableSeed is designed to be unpredictable, it is
 _not_ random.
 You'd need to have each cover use the same random generator. Here's
 where a shortcoming in the design of randomCover becomes evident:
 randomCover stores a copy of its generator, which makes it difficult to
 support what you need. I will look into a fix.
 Andrei


Actually, I meant to mention in the original post that I thought of this already
and it's not the problem.  When I re-seed the generator every time but use
randomShuffle, I get something pretty uniform looking.

Furthermore, even after I allow a few more clock ticks to go by and re-run the
program, I still get the same thing when using randomCover.  I also tried things
like keeping a Mersenne Twister outside the loop and using successive iterations
of that to seed the generator that I pass to randomCover.

I actually did find the real problem, though.  It's really simple and I've filed
it in Bugzilla.  See http://d.puremagic.com/issues/show_bug.cgi?id=2865 .

Also, since I remember the discussion a while back on how to make randomCover as
efficient as possible, why are we using bool[] instead of BitArray for it?