# RosettaCode Monty Hall

## Reference

Statement of the Monty Hall problem on RosettaCode: here.

Deadline for adding to RosettaCode page: 31 Aug 2012; submitter:

## Eiffel code

Here's an initial implementation, just to get things started. I just typed this straight into the wiki, so it might not even compile.

Please feel free to edit this in place, I won't be offended.

class MONTY_HALL feature play local games: INTEGER random: RANDOM doors: ARRAYED_LIST [BOOLEAN] chosen, shown: INTEGER stayWins: INTEGER do games := 1000000 create random.make across 1 |..| games as plays loop create doors.make_filled (3) -- False is a goat, True is a car random.forth doors [random.item \\ 3 + 1] := True -- Put a car behind a random door random.forth chosen := random.item \\ 3 + 1 -- Pick a door, any door from shown := chosen until shown /= chosen and not doors [shown] loop random.forth shown := random.item \\ 3 + 1 end if doors [chosen] then -- If you would have won by staying, count it stay_wins := stay_wins + 1 end end out ("Staying wins " + stay_wins + " times.%N") out ("Switching wins " + games - stay_wins + " times.%N") end end

Note that the implementations in many other languages maintain a separate variable to count the number of switch wins. They calculate whether switching wins at each step via some funky logic that relies on zero-based array indexing, which would be inconvenient in Eiffel. But we don't need to do that at all anyway, because calculating it at each step is completely redundant: we can just do a final subtraction at the end, right?