rand() not so random....

Everytime I run this program, that is supposed to display a random number between 0 and 20, it outputs 20. I would really appreciate if somebody could point out were I'm going wrong.

Code:

---

#include <iostream.h>
#include <cstdlib.h>


using namespace std;

int main()
{

 int mynumber = rand() % 10 + 20;

 cout <<  mynumber;

 cin.get();

}

---

srand( time(0) );

before rand();

FAQ

Unrelated, but use <iostream>, not <iostream.h>

Your program is using default seed values to generate rands. To produce real random values the best method is using the ctime library; this way since no two time values are the same you get perfectly random numbers.

Add this to your code:

Code:

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#include <ctime>
srand((unsigned)time(0));

---

Quote:

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To produce real random values the best method is using the ctime library; this way since no two time values are the same you get perfectly random numbers.

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Not quite. Since time is ever-changing, using the time is a good way to generate a random number seed, such as that passed to srand. Using the time for a random number is a bad idea.

And anyway, rand() is a pseudo-random number generator -- it doesn't generate perfectly random numbers. It's very hard for computers to generate random numbers in general, and non-pseudo-random numbers in particular.

For most cases pseudo-random numbers a sufficient however.

[edit]

Quote:

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that is supposed to display a random number between 0 and 20

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Well, it won't.

Code:

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int mynumber = rand() % 10 + 20;

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Read that as "Generate a random number, divide it by 10 and take the remainder (which will be from 0 to 9), then add 20." Accordingly, you should get a number from 20+0 to 20+9 -- 20-29 inclusive.

If you want a random number from 0 to 20 inclusive use

Code:

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rand() % 21

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[/edit]

rand() is also pretty poor randomness on most compilers; most use some version of a linear congruential generator which is one of the worst (in terms of randomness) performing PRNGs. If you're making a professional program, try something like boost's mersenne twister PRNGs. Of course rand() works fine for simple things and testing, or where you don't care too much about how random things are.

You might want to be eternally confuzzled by Prelude concerning rand() before you take dwks' advice.

What you have to do is seed the random number generator. Just do a google search for seed and rand() and you will get a pile of great links...

here is a great way to generate random numbers:

Code:

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int getRandomInt( int iMin, int iMax )
{
        return static_cast<int>( (iMax - iMin) * double(rand())/(RAND_MAX-1) + iMin );
}

---

Change it to use whatever type you like

Quote:

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Originally Posted by avalanche333

here is a great way

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Not so great... read link posted by laserlight

avalanche's method is basically the one that laserlight's link recommended, apart from the fact it should be +1 and not -1 added to RAND_MAX.

However, any LCG will always have a stronger degree of serial correlation than many other PRNGs. This: http://upload.wikimedia.org/wikipedia/en/3/38/Randu.png is an example of a particularly poor (and once, a highly popular) LCG; when it generates triples of random points, all the points lie on one of fifteen different planes in the 3d space. Most LCGs are not as bad as that, but they all have the same phenomena, caused by a stronger than desired correlation between one output and the next subsequent output.

Thanks for all the help everyone. I've been trying out a few of the above and most work very well. Thanks again.

>Not so great... read link posted by laserlight
Same to you. ;) A solution doesn't have to be exactly the same to follow the same principles.

>apart from the fact it should be +1 and not -1 added to RAND_MAX
+1.0 would be more likely to avoid integer overflow if RAND_MAX is the same as INT_MAX.