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categories: TenLiners,Mar,2009,DonaldM

The Monty Hall Problem

Donald 'Paddy' McCarthy has a nice Awk solution to the Monty Hall Problem, which he describes as follow:

  • The contestant in in front of three doors that he cannot see behind..
  • The three doors conceal one prize and the rest being booby prizes, arranged randomly.
  • The Host asks the contestant to choose a door.
  • The host then goes behind the doors where only he can see what is concealed, then always opens one door, out of the other s not chosen by the contestant, that must reveal a booby prize to the contestant.
  • The host then asks the contestant if he would like either to stick with his previous choice, or switch and choose the other remaining closed door.

It turns out that if the contestant follows a strategy of always switching when asked, then he will maximise his chances of winning. Donald's simulator shows that:

  • A strategy of never switching wins 1/3rd of the time.
  • A strategy of randomly switching wins 1/2 of the time.
  • A strategy of always switching wins 2/3rds of the time.


	doors = 3
	iterations = 10000
	# Behind a door: 
	EMPTY = "empty"; PRIZE = "prize"
	# Algorithm used
    KEEP = "keep"; SWITCH="switch"; RAND="random"; 
function monty_hall( choice, algorithm ) { # Set up doors
  for ( i=0; i<doors; i++ ) {
		door[i] = EMPTY
	door[int(rand()*doors)] = PRIZE # One door with prize

  chosen = door[choice]
  del door[choice]

  #if you didn't choose the prize first time around then
  # that will be the alternative
	alternative = (chosen == PRIZE) ? EMPTY : PRIZE 

	if( algorithm == KEEP) {
		return chosen
	if( algorithm == SWITCH) {
		return alternative
	return rand() <0.5 ? chosen : alternative
function simulate(algo){
	prizecount = 0
	for(j=0; j< iterations; j++){
		if( monty_hall( int(rand()*doors), algo) == PRIZE) { 
			prizecount ++ 
	printf "  Algorithm %7s: prize count = %i, = %6.2f%%\n", \
		algo, prizecount,prizecount*100/iterations
	print "\nMonty Hall problem simulation:"
	print doors, "doors,", iterations, "iterations.\n"

Sample Output

gawk -f montyHall.awk

Monty Hall problem simulation:
3 doors, 10000 iterations.

  Algorithm    keep: prize count = 3411, =  34.11%
  Algorithm  switch: prize count = 6655, =  66.55%
  Algorithm  random: prize count = 4991, =  49.91%
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