R2: A Probabilistic Programming System

using MicrosoftResearch.R2Lib;
using MicrosoftResearch.R2Lib.Distributions;

class MontyHall
{
public Tuple Infer()
{
double[] probabilities = new double[] { 1.0/3, 1.0/3, 1.0/3 };

// Set up the game.
int carDoorNum = Categorical.Sample(probabilities);

// Contestant picks a door.
int chosenDoorNum = Categorical.Sample(probabilities);

// Host picks a door.
int[] possibleOpenDoors;
int openedDoorNum = -1;
int i, j;
if (carDoorNum == chosenDoorNum)
{
possibleOpenDoors = new int[2];

j = 0;
for (i = 0; i < 3; ++i)
{
if (i != carDoorNum)
{
possibleOpenDoors[j] = i;
++j;
}
}

j = Categorical.Sample(new double[] { 0.5, 0.5 });
openedDoorNum = possibleOpenDoors[j];
}
else
{
for (i = 0; i < 3; ++i)
{
if ((i != carDoorNum) && (i != chosenDoorNum))
{
openedDoorNum = i;
}
}
}

bool chosenDoorContainsCar = false, otherDoorContainsCar = false;

// Does contestant win if s/he doesn't switch?
chosenDoorContainsCar = (chosenDoorNum == carDoorNum);

// Does contestant win if s/he switches?
for (i = 0; i < 3; ++i)
{
if ((i != chosenDoorNum) && (i != openedDoorNum))
{
otherDoorContainsCar = (i == carDoorNum);
}
}

return new Tuple(chosenDoorContainsCar,
otherDoorContainsCar);
}
}