rand() (not really random)

[Lukezzz]

I use this random generator for numbers but it seems that this generator isn´t really random because it seems to generate the same "Random" sequence chain every time I restart the application.

Is there a better way to generate random numbers than this, that is more random ?

for( int i = 0; i < 200; i++ )
{
				 
	int RandomNum = rand() % 100;
			 
}

[siddhant3s]

1.Seed using srand()
Visit http://www.daniweb.com/forums/thread1769.html
2.Computer can only generate pseudo random number. Understand the difference. Please visit http://www.eternallyconfuzzled.com/arts/jsw_art_rand.aspx

[BlackJavaBean]

There are libraries that generate better pseudo-random numbers. The rand() function in the standard libraries is - as you've seen - a poor substitute if you really need random numbers. I ended up searching Google last time I needed a random number generator. Someone on here may have more info though. If all else fails, try looking for "stochastic library, random number generator library" or "pseudo-random number generator". I can't remember what I searched for (it was a couple of years ago, sorry)

[Lukezzz]

wow, thank you for these advices, I will googling around for this and also check these links to see what I can find and do...

Regards..

[Narue]

>but it seems that this generator isn´t really random
Any deterministic method for generating random numbers won't be "really" random. That's why they're called pseudorandom numbers. They're sufficiently patternless to appear random for many purposes. "Real" random numbers only come from truly random real world sources, such as the decay of a radioactive material or atmospheric noise.

>because it seems to generate the same "Random"
>sequence chain every time I restart the application
That's a good thing. Imagine trying to debug your program when you can't reproduce the same sequence, ever. The seed for rand defaults to 1, and you can call srand to change it. Here's a demo of seeding based on the current time:

#include <climits>
#include <cstdlib>
#include <ctime>
#include <iomanip>
#include <iostream>

namespace JSW {
    using namespace std;

    // Replaces the conventional but non-portable
    //    (unsigned)time(NULL)
    // as a seed for srand with a portable and
    // equivalent quality seed generator based 
    // on the current time
    unsigned time_seed()
    {
        time_t now = time ( 0 );
        unsigned char *p = (unsigned char *)&now;
        unsigned seed = 0;

        for ( size_t i = 0; i < sizeof now; i++ )
            seed = seed * ( UCHAR_MAX + 2U ) + p[i];

        return seed;
    }

    // Just for output formatting
    size_t digits ( int value )
    {
        if ( value == 0 )
            return 1;

        size_t n = 0;

        for ( n = 0; value != 0; n++ )
            value /= 10;

        return n;
    }

    void sleep ( double seconds )
    {
        // This busy-wait method is not recommended, but 
        // it's portable until you try fractions of a second
        time_t start = time ( 0 );

        while ( difftime ( time ( 0 ), start ) < seconds )
            ;
    }
}

int main()
{
    using namespace std;

    const int batches = 10;
    const int numbers = 10;

    // Generate M batches of N random 
    // numbers, reseeding before each batch
    for ( int i = 0; i < batches; i++ ) {
        srand ( JSW::time_seed() );

        for ( int j = 0; j < numbers; j++ )
            cout<< setw ( JSW::digits ( RAND_MAX ) ) << rand() <<' ';

        cout<<'\n';
        JSW::sleep ( 2 );
    }
}

[Lukezzz]

Thanks Narue and others. I think I will go with the srand() it seems to to do the nessecary randomness for this purpose.
I came up with this example:

#include <ctime>

srand((unsigned)time(nullptr)); 
int RandomNum = 0;

for( int i = 0; i < 200; i++ ) 
{ 
     RandomNum = rand() % 100; 
}