Rolling two dice a given number of times C++ from a randomly generate number between 0 and 1

Question

I have written this code which will simulate the roll of 2 dice. When I run it with just the 2 dice rolling it seems to give me a random number between 2 and 12 (which is what I want). However when I try to simulate the roll multiple times (using a for loop) it always bins the data in the same vector space. If I run it for an extremely large number of times it does start to separate out a little bit. This points me towards thinking that it is the random seed that is the issue, however I'm not sure how to correct this.

//using rand to generate a uniformly distributed number between 0 and 1. Multiplying that number by 6, adding 1 to it and then rounding it down to simulate a dice throw. Rolling 2 dice together to get a total between 2 and 12, then keeping a tally of rolling 2 dice a given number of times.

#include <iostream>
#include <vector>
#include <iostream>
#include <stdlib.h>
#include <cstdlib>
#include <time.h>


using namespace std;

double rnd ()
{    
    //defining the variables as doubles
    double n;
    double y;

    //setting up the random number seed
    srand (time(NULL));

    //using the rand command to generate a pseudo-random number between 0 and 1
    n = ((double)rand()/(double)RAND_MAX);
    return n;
}

int dice ()
{
    double x = rnd()*6;
    int y = x+1;
    return y;
}


int two_dice ()
{
    int throw_1 = dice();
    int throw_2 = dice();
    int score = throw_1 + throw_2;
    return score;
}

int main ()
{
    //asking the user for the number of trials
    int n_trials;
    cout << "plese enter the number of trial that you wish to simulate \n";
    cin >> n_trials;

    //creating the vector to store the data in
    vector<int> dice_throws(11); //has 12 bins to store data in

    int sum_dice;

    //using a for loop to roll the dice multiple times
    for (int roll = 0; roll <= n_trials; roll++) {
        sum_dice = two_dice();
        dice_throws[sum_dice - 2]++;
    }


    for (int y = 0; y<dice_throws.size()+1; ++y) {
        cout << dice_throws[y] << "\n";
    }

    return 0;
}

Also, consider using the generators in [``](http://en.cppreference.com/w/cpp/numeric/random) instead, they are far better and of known quality. — Deduplicator, Oct 02 '14 at 10:12
[`std::uniform_int_distribution<> dist(1,6)`](http://en.cppreference.com/w/cpp/numeric/random/uniform_int_distribution) would come in darn handy for this task. — WhozCraig, Oct 02 '14 at 10:17
If `n` is 1, which it can be, then `dice ()` will return 7. You should divide by `RAND_MAX-1`. — David Schwartz, Oct 02 '14 at 10:23

Spikatrix · Accepted Answer · 2014-10-02T10:24:49.270

3

You get the same number because srand is called everytime the rnd function is called . Move the srand at the start of main and it should work fine !

edited Oct 02 '14 at 10:24

answered Oct 02 '14 at 10:18

Spikatrix

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Thanks, that has been bugging me for hours and I couldn't seem to find an answer. You can probably tell I'm extremely new to C++ – thewire247 Oct 02 '14 at 10:43

Rolling two dice a given number of times C++ from a randomly generate number between 0 and 1

1 Answers1