Thread: Monty Hall Problem

Did you just do a search and replace? Cause now you have several variables being "assigned" with the comparison operator.

Code:

int d;

d = 5; /* assignment */

if (d == 5) ; /* comparison */

You need to fix all these mistakes first. Secondly you still have places where your braces are all wrong. Lines 33-41 are all misaligned and the braces are not right. Heres how that part should look:

Code:

    if (choice==1)
        newdoor = choice;
    else if (choice==2)
        newdoor = originaldoor;

    if (newdoor==1)
        printf("You have won the prize!\n");
    else
        printf("You lose!\n");

Also, your "game" function reaches it's end without returning a value, yet it is declared to return an int (despite the fact you don't use the return value in the main() function). You also need to use "game(void)" in your declaration to be standards compliant.

You should be listening to the warnings your compiler is telling you and fixing EVERY one. Also make sure all warnings are turned up to the MAX. You are definitely getting warnings if not errors.

Okay so i have fixed all those errors but I still do not get correct results. I also did not get any errors or warnings for my earlier code.

Post your current code.

Code:

#include <stdio.h>
#include <stdlib.h>


void game()


{
    int door[3]={0,0,0};
    int choice, i, newdoor, originaldoor, openeddoor, randomnumber;
    if (originaldoor==1)
    {
     openeddoor=rand() % 3;
    }
    else
    {
        openeddoor=0;
    }
    printf("Door %d does not have the prize\n", openeddoor);
    printf("Would you like to switch your door? 1 for yes 2 for no.\n");
    scanf("%d" , &choice);


    while ((choice<0) || (2<choice))


    {
        printf("You entered an invalid number\n");
        break;
    }
    if (choice==1)
        newdoor = choice;
    else if (choice==2)
        newdoor = originaldoor;


    if (newdoor==1)
        printf("You have won the prize!\n");
    else
        printf("You lose!\n");




}
int main(void)
{
    int door[3]={0, 0, 0};
    int x, prize, randomnumber, originaldoor, choice, newdoor, y;
    prize=1;


    while (1)
    {
    srand (time(NULL));
    rand() % 3 + 1;


    door[randomnumber] = 1;


    printf("Which door do you pick 1, 2, or 3? 0 to exit.\n");
    scanf("%d", &originaldoor);
    game(originaldoor);


    if ((0>=originaldoor) || (originaldoor>3))
    {
        break;
    }


    }


return(0);
}

Look at your game function. You don't have any parameters in your definition, yet you pass in an integer in main.

You might want to make your game function like so..

Code:

void game(int originaldoor)

Lose the local variable originaldoor in your function, you want to check the value passed in from main, not a local variable.

Yes that makes a lot of sense, don't know why I did that.. But now when I run the program I always get the result of winning? Is there something wrong with my random number generator then?

Actually i get that if you change your door you always win.

How would I get the program to find the only remaining door if the prize is in the door you choose before the first door is opened?

Originally Posted by tysawyers13

Is there something wrong with my random number generator then?

Yes you are calling srand() each iteration of your while() loop, you only need to call that once at the beginning of the program (in main)

Originally Posted by nonpuz

Yes you are calling srand() each iteration of your while() loop, you only need to call that once at the beginning of the program (in main)

Yes i have changed that, but now I get a winning answer an unlikely amount of times?

In the posted code above, you don't actually assign the random number you generate to the variable 'randomnumber'. You call rand() and then the value just gets thrown away. Also, I'm not sure if this was mentioned before, but you're +1 in the rand() call is wrong. Your array is of size 3, that means the highest index is 2, so rand() % 3 is all you need; that will give you values in the range 0, 1, 2, which is the indexes for your array of size 3.

Another point is that your "winning door" is ALWAYS door 1, regardless of where you put the prize. You need to change the if check to make it check the selected prize door, not just 2 every time. Once you assign the value to 'randomnumber' you just use that variable to index which door is the 'prize' door in your comparison later. This means you probably have to pass the winning door number to your door check function, as well as the door the user has chosen.

So I must change the last if statement? I thought that if new door would be equal to 1 then it should be true?\

Code:

if (choice==1)
        newdoor = choice;
    else if (choice==2)
        newdoor = originaldoor;
    if (newdoor==0)
        printf("You lose!\n");
    else
        printf("You have won the prize!\n");

I mean if new door was equal to 0 then you lost.

If new door does NOT equal the door with the prize in it, then you lose. So you need to compare newdoor with the winning door (which may or may not be zero).