Monty Hall Project - c++

Question

I am currently in the process of writing a C++ Monty Hall Problem Simulation and have run into some trouble. The error I keep getting is:
source.cpp(23): error C4700: uninitialized local variable 'doorReveal' used
source.cpp(25): error C4700: uninitialized local variable 'doorSwitch' used
source.cpp(52): error C4700: uninitialized local variable 'stayWin' used
source.cpp(56): error C4700: uninitialized local variable 'switchWin' used
I can't seem to figure out what is wrong. The project is supposed to simulate the wins by first staying with the original door choice on the first 100 tries and then switching when door is revealed on the next 100 tries. Thank you all for your help in advance.
#include <iostream>
#include <time.h>
using namespace std;
int main()
{
int doorChoice;
int prizeDoor;
int doorReveal;
int doorSwitch;
int count;
int switchWin;
int stayWin;
srand((unsigned int) time(0));
for (count = 0; count <= 200; count++)
{
prizeDoor = (rand() % 3) + 1;
doorChoice = (rand() % 3) + 1;
cout << "The prize door is door number " << prizeDoor << " ." << endl;
cout << "The door the contestant chose was door " << doorChoice << endl;
doorReveal != prizeDoor || doorChoice;
cout << "The host revealed door number " << doorReveal << " ." << endl;
doorSwitch != doorChoice || doorReveal;
while (count < 101)
{
if (doorChoice == prizeDoor)
{
cout << "Great Job! You won!" << endl;
}
else {
cout << "Not this time!" << endl;
}
}
while (count < 201)
{
if (doorSwitch == prizeDoor)
{
cout << "You switched and won!" << endl;
}
else {
cout << "You switched and lost!" << endl;
}
}
if (doorChoice == prizeDoor)
{
stayWin++;
}
if (doorSwitch == prizeDoor)
{
switchWin++;
}
count++;
}
cout << "Your win percentage when staying was " << stayWin << "%!" << endl;
cout << "Your win percentage when switching was " << switchWin << "%!" << endl;
return 0;
}

Answer 1

Problem 1: operator!=
operator != doesn't do what you think it does.
Did you mean
doorReveal = !(prizeDoor || doorChoice);
cout << "The host revealed door number " << doorReveal << " ." << endl;
doorSwitch = (doorChoice || doorReveal);
I sense another logic issue in determining the doorReveal. I'll have to think about that later.Edit: see problem 5
Problem 2: while
There's also a problem with the while loops:
while(count < 101)
// ...
while(count < 201)
They're infinite loops, because count isn't increased during the loop. I think you meant if there, instead of while.
Problem 3: Initiliazing switchWin and stayWin
These variables are only being incremented. Like #KonradRudolph suggested,
declare your variables where they are first needed
initialize them
while you're at it, mark them const as appropriate
Problem 4: rand()%3 is biased
You should probably use a uniform distribution.
See
http://eternallyconfuzzled.com/arts/jsw_art_rand.aspx
I'll leave using std::uniform_int_distribution as an exercise for the reader here, as it is likely beyond the scope of your course. Remember to use it though, in any real life code.
Problem 5: Fix your door 'derivation'
Booleans aren't sets. Even if they were, you'd be stuck to binary sets. I propose the following model:
enum doors { door1 = 1, door2 = 2, door3 = 4, any = door1|door2|door3 };
so you can say:
doors const doorReveal = doors(~(prizeDoor | doorChoice) & any);
doors const doorSwitch = doors(~(doorChoice | doorReveal) & any);
Fixing that results in a program that appears to work:
#include <iostream>
#include <time.h>
using namespace std;
enum doors { door1 = 1, door2 = 2, door3 = 4, any = door1|door2|door3 };
static inline std::ostream& operator<<(std::ostream& os, doors val) {
switch(val) {
case door1: return os << "door #1";
case door2: return os << "door #2";
case door3: return os << "door #3";
case any: return os << "any door";
}
return os << "OOPS";
}
int main()
{
unsigned switchWin = 0;
unsigned stayWin = 0;
srand((unsigned int) time(0));
for(int count = 0; count <= 200; count++)
{
doors const prizeDoor = doors(1 << rand() / ( RAND_MAX / 3 ));
doors const doorChoice = doors(1 << rand() / ( RAND_MAX / 3 ));
cout << "The prize door is door number " << prizeDoor << " ." << endl;
cout << "The door the contestant chose was door " << doorChoice << endl;
doors const doorReveal = doors(~(prizeDoor | doorChoice) & any);
doors const doorSwitch = doors(~(doorChoice | doorReveal) & any);
cout << "The host revealed door number " << doorReveal << " ." << endl;
if(count < 101)
{
if(doorChoice == prizeDoor)
{
cout << "Great Job! You won!" << endl;
}
else
{
cout << "Not this time!" << endl;
}
};
if(count < 201)
{
if(doorSwitch == prizeDoor)
{
cout << "You switched and won!" << endl;
}
else
{
cout << "You switched and lost!" << endl;
}
};
if(doorChoice == prizeDoor)
{
stayWin++;
}
if(doorSwitch == prizeDoor)
{
switchWin++;
};
count++;
}
cout << "Your win percentage when staying was " << stayWin << "%!" << endl;
cout << "Your win percentage when switching was " << switchWin << "%!" << endl;
return 0;
}