Trouble with Die Roll Program

Question

I wrote this program that rolls a pair of dice 20000 times. The rules are:

If a 7 or 11 is rolled, then the player wins
If a 2, 3, or 12 is rolled then the player loses.
If neither of the above is rolled, then the sum is stored and the die are rolled again

Second roll rules:

If a 7 is rolled, then the player wins
If the same sum as before is rolled, then the player loses
If not, the player rolls again until either of those two conditions are met.

At the end it calculates a percentage of games won. My problem is that I should be getting a percentage of games won around 39%-60% according to a colleagues, but every time I run the program, I get around 20% and I don't understand why. Am I doing something wrong? Can someone help me please?

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

main()
{   
int dieOne, dieTwo, keyValue, value, wins = 0, firstWins = 0, subWins = 0, loss = 0, count=0;
double percentWin, percentFirstWins, percentSubWins;
srand(time(NULL));

do{
dieOne = rand() % 6 + 1;
dieTwo = rand() % 6 + 1;
value = dieOne + dieTwo;
count++;


if (value==7||value==11) {
    firstWins++;
    wins++;
}

else if (value== 2||value==3||value==12) {
    loss++;
}

else {
    do{ 
    keyValue = value;

    dieOne = rand() % 6 + 1;
    dieTwo = rand() % 6 + 1;
    value = dieOne + dieTwo;
    count++;

    if (value==7) {
    subWins++;
    wins++;
    }

    else if (value = keyValue) { 
        loss++;
    }

    } while ( value != 7 && value != keyValue );
}
} while (count <= 20000);

    percentWin = (double) wins/count * 100;
    percentFirstWins = (double) firstWins/count * 100;
    percentSubWins = (double) subWins/count * 100;
    printf("You won %.1lf percent of your games! \nYou won %.1lf percent of games on the first roll.\nYou won %.1lf percent of games on the second roll.\n", percentWin, percentFirstWins, percentSubWins );

    system("pause");
}

There are 6 possible ways to roll a 7 and two possible ways to roll an 11.
That's a total of 8 possible ways to win on the first roll.
There are 36 (6^2) possible ways to roll the dice. This means you have an 8 in 36 chance to win on the first roll or around 22% of the time.

Moreover, we have 2 possible ways to roll a 2, and 2 possible ways to roll a 3, and 2 possible ways to roll a 12. So that means there is a 1 in 6 chance (6/36) of losing on the first roll, or about 17% of the time.

So the remaining 61% of wins and losses have to come from the second roll. Whatever the case, my program is generating a very high losing streak, whereas the programs of my colleagues seem to generate a 40-60 percent of total wins. Where am I going wrong?

Oops. Okay, so the probability of losing is about 11%. With a little more research, I found the percentage should be closer to 49%. Either way, that doesn't explain why my program isn't giving me that percentage. Ohhh I hope someone can help me... — user1695758
Logic errors in the code aside, note that 'rand() % 6' is not going to be perfectly uniform, although that's probably not a big enough factor to explain the discrepancy you're seeing. — twalberg

Cody Cody · Accepted Answer · 2012-09-24T23:11:56

This may be what's causing your problem:

else if (value = keyValue) {

You're assigning, rather than comparing. This will always evaluate to true, thus artificially inflating your losses. You should be using == here instead of =.

Also, for rolls that are neither a win nor a loss, you're not incrementing either the wins or the loss counter for either of those, but you are adding to count. So you're going to end up with a situation where wins + loss != count. When you calculate the win percentage, try calculating it as wins / (loss + wins).

Trouble with Die Roll Program

1 Answers