## Let’s Make A Deal!

### July 24, 2009

Let’s Make A Deal! was a television game show that originated in the United States in the 1960s and has since been produced throughout the world. In one of the games, prizes were placed behind three doors, and the contestant was asked to pick a door; one door hid an automobile, the other two doors hid goats. Then the host, who knows what is behind the doors, opened one of the doors that the contestant did not pick, revealing in every case a goat, and asks if you want to switch doors. Is it to your advantage to switch your choice of doors?

Your task is to write a program that simulates a large number of games and determines whether or not it is to your advantage to switch doors. What are the odds that you will win the automobile by switching? When you are finished, you are welcome to read or run a suggested solution, or to post your solution or discuss the exercise in the comments below.

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### 10 Responses to “Let’s Make A Deal!”

1. cwbowron said
```#lang scheme

(define DOOR-COUNT 3)

(define (car? n lst) (eq? (list-ref lst n) 'car))

(define (get-door-layout n)
(let loop ([n n] [doors (list)])
(cond
[(= 0 n) doors]
[(or (member 'car doors)
(not (= 0 (random n))))
(loop (sub1 n) (cons 'goat doors))]
[else
(loop (sub1 n) (cons 'car doors))])))

(define (get-random-door door-count criteria-function)
(let ([door (random door-count)])
(if (criteria-function door)
door
(get-random-door door-count criteria-function))))

(define (get-revealed-door doors contestant-selection)
(get-random-door (length doors)
(lambda (n)
(and (not (= n contestant-selection))
(eq? (list-ref doors n) 'goat)))))

(define (get-switch-door door-count unavailable-doors)
(get-random-door door-count (lambda (n) (not (member n unavailable-doors)))))

(define (make-a-deal n (show-debug-info #f))
(let ([success-count-stick 0]
[success-count-switch 0])
(for ((trial (in-range n)))
(let ([doors (get-door-layout DOOR-COUNT)]
[selected-door (random DOOR-COUNT)])
(let ([revealed-door (get-revealed-door doors selected-door)])
(let ([selected-door-switch (get-switch-door DOOR-COUNT (list selected-door revealed-door))])
(when show-debug-info
(printf "DOORS: ~A~%" doors)
(printf "SELECTED DOOR: ~A~%" selected-door)
(printf "REVEALED DOOR: ~A~%" revealed-door)
(printf "SWITCH DOOR: ~A~%" selected-door-switch))
(cond [(car? selected-door doors)
[(car? selected-door-switch doors)
(printf "Success Rate of Sticking:  ~A%~%" (exact->inexact (* 100 (/ success-count-stick n))))
(printf "Success Rate of Switching: ~A%~%" (exact->inexact (* 100 (/ success-count-switch n))))
(if (> success-count-switch success-count-stick)
(printf "Better off Switching~%")
(printf "Better off Sticking~%"))))

(make-a-deal 100000)
```
2. Jamie Hope said

I already knew that the correct answer is the probability of winning by switching is 2/3, but:

```#!r6rs

(import (rnrs)
(srfi :27))

(define (create-game) ; false = goat, true = car
(let ((v (make-vector 3 #f)))
(vector-set! v (random-integer 3) #t)
v))

(define (player-choice) (random-integer 3))

(define (host-choice g p) ; host always chooses a goat
(case p
((0) (if (vector-ref g 1) 2 1))
((1) (if (vector-ref g 0) 2 0))
((2) (if (vector-ref g 0) 1 0))))

(define (switch p h)
(case (+ p h)
((1) 2)
((2) 1)
((3) 0)))

(define (run-test n)
(let loop ((wins 0) (total 0))
(if (= total n) (/ wins total)
(let* ((g (create-game))
(p (player-choice))
(h (host-choice g p))
(s (switch p h)))
(if (vector-ref g s)
(loop (+ wins 1) (+ total 1))
(loop wins (+ total 1)))))))

(display (inexact (run-test 1000000)))
```

=> 0.66756

3. Mark VandeWettering KF6KYI said

Ugly, but it works.

```#!/usr/bin/env python

# code for the monty hall problem
# written by Mark VandeWettering as a challenge on
# the programming praxis website.

import random

NTRIALS=10000

def trial(switch=False):
# pick a random door...
door = random.randint(1, 3)
guess = random.randint(1, 3)
if not switch:
return door == guess
else:
# slightly clever, but the only way that you
# lose is if you were lucky enough (or unlucky
# enough) to guess the right door in the first
# place.   This gives away the entire problem.
return door != guess

cnt_noswitch = 0
cnt_switch = 0

for x in range(NTRIALS):
if trial():
cnt_noswitch = cnt_noswitch + 1
if trial(switch=True):
cnt_switch = cnt_switch + 1

print "By not switching, you won %d/%d rounds." % (cnt_noswitch, NTRIALS)
print "By switching, you won %d/%d rounds." % (cnt_switch, NTRIALS)
if cnt_switch > cnt_noswitch:
print "It appears you should switch."
else:
print "It appears you should not switch."
```
4. I suggest also reading a nice treatment of this exact problem in “The man who loved only numbers” by Paul Hoffman.

5. Brian Everett said

I’m just now learning Python and figured I’d play around with its list capabilities for this one…

Originally I was using list comprehension to generate a list of random numbers (to indicate which door the car is behind) and keeping the contestant’s choice constant (always picking door number 1), but I realized it might be slightly faster to randomize the contestant’s choice and assume the car is always located behind door number 1 – simply using the list as a looping mechanism.

```import random
trials = 100000
print "Switching will win: %2.2f%% of the time" % (len(filter(lambda trial: random.randint(1, 3) != 1, xrange(trials))) / float(trials) * 100)
```
6. […] Let’s Make A Deal! « Programming Praxis. […]

7. Greg R said

I made an attempt in PL/SQL (Oracle). I made the number of doors configurable (assuming that the host opens all doors except yours or the prize, or a random single door if you have chosen the prize door initially).

```declare
Runs  integer := 10000;
Doors integer := 3;

function RunMonteHaul
(
Runs    in  integer,
MaxDoor in  integer,
Swap    in  boolean
)
return integer
is
Prize     integer;
OtherDoor integer;
MyDoor    integer;
Wins      integer := 0;
i integer;
j integer;
begin

for i in 1 .. Runs loop
Prize := dbms_random.value(1,MaxDoor);
MyDoor := dbms_random.value(1,MaxDoor);
if Prize = MyDoor then
if Prize = MaxDoor then
OtherDoor := 1;
else
OtherDoor := MaxDoor;
end if;
else
OtherDoor := Prize;
end if;
if Swap then
MyDoor := OtherDoor;
end if;
if MyDoor = Prize then
Wins := Wins + 1;
end if;
end loop;
return Wins;
end RunMonteHaul;
begin
dbms_output.put_line('Runs  ' || Runs);
dbms_output.put_line('Doors ' || Doors);
dbms_output.put_line('Swap Wins    ' || RunMonteHaul(Runs,Doors,true));
dbms_output.put_line('No Swap Wins ' || RunMonteHaul(Runs,Doors,false));
end;

```

This produces the output

```Runs  10000
Doors 3
Swap Wins    6290
No Swap Wins 3725

Runs  10000
Doors 10000
Swap Wins    9999
No Swap Wins 1
```
8. Diego Giuliani said
```# Monty Hall Paradox simulation / Let's make a deal
# Simulates a number of games and output the total number of wins and looses,
# and the percentage of each, to determine if switching doors after the host has opened one
# is a better option.

from random import random,randint

def scrambleDoors(doors):
"""
Simulates the scramble of doors by choosing one to contain a Car instead of a Goat
"""
doors[randint(1,3)] = "Car"
return doors

def pick():
"""
Choose a door between 1 and 3
"""
return int(random() * 100) % 3 + 1

def openDoor(door_list,door):
"""
The host opens a door, wich must not reveal the price, and also is not the one the player has choosen
"""
return [i for i in door_list.keys() if (door_list[i] != "Car") & (i != door)][0]

def switchDoor(door_list,door):
return [i for i in door_list.keys() if i != door]

def play():
# 1 - Scramble doors
doors = scrambleDoors({1:"Goat",2:"Goat",3:"Goat"})

# 2 - Pick a door
door = pick()

# 3 - Open a door, and remove it from door list
od = openDoor(doors,door)
del doors[od]

# 4 - Switch door
newDoor = switchDoor(doors,door)

# 5 - Won?
return doors[newDoor[0]] == "Car"

def game(n = 100):
"""
n: Number of games to simulate
"""

win = 0
loose = 0
for i in range(0,n):
if (play()):
win +=1
else:
loose += 1
porcWin = (float(win) / n) *  100
porcLoose = (float(loose) / n) *  100
print """
Runs  : % s
Win   : %s  Percentage win  : %s
Loose : %s  Percentage Loose: %s
""" % (n,win,porcWin, loose,porcLoose)
```

Which produces the following output

```Runs  : 1000
Win   : 680  Percentage win  : 68.0
Loose : 320  Percentage Loose: 32.0
```
9. princess83 said

Yes, but when the game was played, did the host always open a door? Or did the host, for example, have the discretion to let the original choice go through. Unless we know this, it is meaningless to claim that we know the best strategy for the player.