Following on from

this thread I thought, "enough with making LEDs blink, time for some probability!".

Taking the Monty Hall Problem from:

http://marilynvossavant.com/game-show-problem/Suppose you’re on a game show, and you’re given the choice of three doors. Behind one door is a car, behind the others, goats. You pick a door, say #1, and the host, who knows what’s behind the doors, opens another door, say #3, which has a goat. He says to you, "Do you want to pick door #2?" Is it to your advantage to switch your choice of doors?

Putting aside the mathematical treatment, let's try some code:

/*

Monty-Hall problem.

Author: Nick Gammon

Date: 25 August 2013

See: http://en.wikipedia.org/wiki/Monty_hall_problem

and: http://marilynvossavant.com/game-show-problem/

*/

const bool change = false; // whether or not to switch doors

const int seed = 1234;

const unsigned long rounds = 2000;

unsigned long wins;

void doOneRound ()

{

bool car [3];

for (byte i = 0; i < 3; i++)

car [i] = false;

// choose which door has the car

car [random (0, 3)] = true;

// contestant makes a random choice

byte choice = random (0, 3);

// host chooses another door

byte hosts_choice;

// don't choose contestant's door

// also don't reveal the car

do

{

hosts_choice = random (0, 3);

} while (hosts_choice == choice || car [hosts_choice]);

// change choice if desired

if (change)

{

if (choice != 0 && hosts_choice != 0)

choice = 0;

else if (choice != 1 && hosts_choice != 1)

choice = 1;

else

choice = 2;

} // end if

// did we win the car?

if (car [choice])

wins++;

} // end of doOneRound

void setup ()

{

Serial.begin (115200);

Serial.println ();

randomSeed (seed);

// do as many rounds as desired

for (unsigned long round = 0; round < rounds; round++)

doOneRound ();

Serial.print ("You won ");

Serial.print (100UL * wins / rounds);

Serial.println ("% of the time");

} // end of setup

void loop () { }

Before running it, do you predict:

You won 33% of the time

You won 50% of the time

You won 66% of the time

<something else>

(Or thereabouts, depending on the exact numbers from the random number generator).

You can change the line:

const bool change = false; // whether or not to switch doors

Make it "true" to change your decision about whether to switch doors, and see if it makes a difference.

Also see:

http://en.wikipedia.org/wiki/Monty_hall_problem