# CS 144 Assignment 2. Monty Hall Problem solution

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## Description

This assignment will give you practice decomposing a problem into smaller subproblems
whose solutions you implement with functions. Submit a C++ program that simulates the
Monty Hall Puzzle to demonstrate empirically whether it’s better to stay with an original
door choice or to switch doors (or it doesn’t matter).
Choose the right door and win the new car!
Monty Hall was a popular game show host on American television. In one segment of his
show, you are a contestant from the studio audience. He shows you three doors,
Door #1, Door #2, and Door #3. Hidden behind one door is a brand new car, and behind
each of the other two doors is a goat.
Monty asks you to pick a door. Of course, you want to pick the right door and win the car.
Monty knows which door hides the car. After
you’ve picked a door, he opens one of the
other doors and shows you a goat.
Monty then offers you a chance to stay with
the door you originally picked, or you can
switch your choice to the remaining third door.
Would staying or switching be better?
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The simulation
Write a C++ program to simulate the above scenario. The program should:
1. Randomly pick a door to hide the car.
2. Acting as you, the contestant, randomly pick a door as your first door choice.
3. Acting as Monty, open a door to reveal a goat. Since Monty (i.e., the program)
knows which door hides the car:
a. As the contestant, if your first door choice was correct, the other two
doors each hides a goat. Monty randomly chooses one of the two doors
to open.
b. If your first door choice was incorrect (your door hides a goat), Monty
opens the other door that hides a goat.
4. This leaves a third door. You, the contestant, don’t know whether it hides the car
or a goat. Should you stay with your first door as your original choice, or should
you switch to the third door as your second door choice? Your program should
keep track of whether you win by staying with your first door choice or you win if
you switch to your second door choice.
Run the simulation 100 times. How many times do you win by staying with your first door
choice vs. switching to a second door choice? Does it make a difference? What is the
ratio of second door choice wins to first door choice wins?
Stay or switch?
Many people, including some prominent mathematicians, intuitively believed that it
shouldn’t make any difference whether to stay with your first door choice or to switch to a
second choice. Either way, the probability of winning the car is one third. Or is it?
A statistician can explain the results using conditional probabilities.
Seehttps://en.wikipedia.org/wiki/Monty_Hall_problem. But this simulation will
demonstrate empirically which strategy is better.
Random number generation
As described in the above scenario, each simulation involves up to three random
numbers. Use the predefined srand function to seed the random number generator at
the start, then subsequently use the predefined rand function to generate the next
random number. The generated pseudo-random numbers will be nonnegative integers.
Since the random numbers represent door numbers, you will need to modify the
generated random numbers to be either 1, 2, or 3.
srand: http://www.cplusplus.com/reference/cstdlib/srand/
rand: http://www.cplusplus.com/reference/cstdlib/rand/
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Sample output
Due to the random nature of the output, ignore CodeCheck’s comparison of your
program’s output to the master. Here’s sample output with 10 simulations (your program
should do 100 simulations):
For each simulation, your output should show:
• which door hides the car (randomly generated)
• which door is your first choice (randomly generated)
• which door Monty opens (possibly random)
• which door is your second choice (the remaining unopened third door)
• whether the first or second choice wins
For many, the results are counterintuitive. A statistician can explain the results using
conditional probabilities. See https://en.wikipedia.org/wiki/Monty_Hall_problem
Go to the above CodeCheck URL for a partially written program MontyHall.cpp. Your
program must be decomposed into functions. The provided function declarations are
suggestions only. You are not obligated to use these declarations, and you can add
other functions. Put your function declarations before the main, and the function
definitions after the main.
# Car First Opened Second Win Win
here choice door choice first second
1 3 1 2 3 yes
2 2 1 3 2 yes
3 2 2 1 3 yes
4 2 2 3 1 yes
5 1 1 3 2 yes
6 3 2 1 3 yes
7 2 2 1 3 yes
8 1 3 2 1 yes
9 2 3 1 2 yes
10 1 3 2 1 yes
4 wins if stay with the first choice
6 wins if switch to the second choice
Win ratio of switch over stay: 1.5
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Submission into Canvas
this zip file into Canvas. You can submit as many times as you want until the deadline,
and the number of submissions will not affect your score. Only your last submission will
Due to the random nature of the output, ignore CodeCheck’s comparison of your
program’s output to the master.
Submit the signed zip file from CodeCheck into Canvas:
Assignment #2. Monty Hall Problem.
Note: You must submit the signed zip file that you download from CodeCheck, or your
submission will not be graded. Do not rename the zip file.
Rubric
Criteria Max points
Good output
• Correct output values.
• Correct output format.
30
• 20
• 10
Good program design
• Good functional decomposition.
• Good choice of function names.
• Good use of function arguments.
• Good comments that describe what each function does,
what its parameters are, and what it returns.
50
• 20
• 10
• 10
• 10
Good program style
• Descriptive variable names.