A problem with random number generation

I am taking a course on programming, and we're using C++. We had an assignment where, at some point, we needed to code a function that would return a random number in an [upper, lower] interval. I used the following:

lower + (int) (upper * (rand() / (RAND_MAX + 1.0)));

I did not forget to change srand by using srand((unsigned int) time(0)).

However, I get the same value every time! I asked my professor for help and he, after some investigation, found out that the first number generated by 开发者_运维知识库rand() isn't that random... The higher order bits remained unchanged, and since this implementation uses them, the end result isn't quite what I expected.

Is there a more elegant, yet simple solution than to discard the first value or use remainders to achieve what I want?

Thanks a lot for your attention!

~Francisco

EDIT: Thank you all for your input. I had no idea rand() was such a sucky RNG :P

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Given that rand() is not a very strong random number generator, the small amount of bias added by the standard approach is probably not an issue: (higher-lower) needs to be smaller than MAX_RAND of course.

lower + rand() % (higher-lower+1);

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fixed off by one error.

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rand() is not a good random-number generator. In addition to the problem you observed it's period length can be very short.

Consider using one of the gsl random number generators.

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Depending on what OS your are using you may have random() available in addition to rand(). This generates much better pseudo-random numbers than rand(). Check <stdlib.h> and/or man 3 random.

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Your code is good, but you should substitute

lower + (int) upper * (rand() / (RAND_MAX + 1.0));

with

lower + (int) (upper - lower + 1)*(rand() / (RAND_MAX + 1.0));

The following code works nicely on my machine:

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

#define lower 10
#define upper 20

int main(void)
{
    int i;
    int number;

    srand(time(0));
    for(i=0; i<10; i++)
    {
        number = lower + (int) (upper - lower + 1)*(rand() / (RAND_MAX + 1.0));
        printf ("%d\n", number);
    }

    return 0;
}    

Of course, since time(0) gives the current time in seconds, two executions within the same second give the same result.

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C++0x random number library (also available in TR1 and Boost) finally solves some nasty issues of rand. It allows getting real randomness (random_device) that you can use for proper seeding, then you can use a fast and good pseudo random generator (mt19937), and you may apply a suitable distribution to that (e.g. uniform_int for min-max range with equal probability for each value).

It also does not use global hidden state like rand() does, so there won't be any issues in multi-threaded programs.

Due to all the modularity it is a bit more difficult to use than simply calling rand, but still the benefits greatly outweigh the steeper learning curve.