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Author Topic: Random() and Randomize implementation  (Read 3861 times)

joselitosn

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Random() and Randomize implementation
« on: April 01, 2007, 07:11:18 AM »
I found a macro on the web to use random() and randomize(), but it does'nt seem to work.

It's likely to be a problem with the pseudo-random number generator.

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

/****** snippet from comp.lang.c ******/
#ifndef __TURBOC__
   #define random( num ) ( int ) ((( rand()) * ( long ) ( num )) / ((( long ) RAND_MAX ) + 1 ))
   #define randomize() srand(( unsigned ) time( NULL ) | 1 )
#endif

int main( void )
{
   unsigned long long int x, y, z; // 64 bit, right?
   
   randomize();

   x = RAND_MAX;
   printf("That's RAND_MAX : %llu", x );
   getchar();
   
   printf("This number should be random in the interval from 0 to 52 : %d", random( 52 ));
   getchar();

/****** now i do the same as the random macro ******/

   y = rand() * 52; // 52 is the max number i want
   x += 1;
   z = (long long int) ((long double)y / x); // convert double for division and convert back? Error?

   printf("The result of %llu divided by %llu results in %llu ?", y, x, z );
   getchar();

   return( 0 );
}

Tested with Ch Interpreter and lcc-win32, worked fine in both. The visible difference is the RAND_MAX which is 32767.

Also is there any snippet for the sound() and delay() functions?
« Last Edit: April 01, 2007, 08:13:58 AM by joselitosn »

Offline frankie

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  • Posts: 2096
Re: Random() and Randomize implementation
« Reply #1 on: April 01, 2007, 12:29:31 PM »
Yuor problem is due to the fact that other compilers returns a short as result (16bits integer max positive value=32768). PellesC returns a real 32 bits int that is equivalent to long (32 bits).
To let work the code just change the casting to long:
Code: [Select]
/****** snippet from comp.lang.c ******/
#ifndef __TURBOC__
   #define random( num ) ( int ) ((( (long long)rand()) * ( long long ) ( num )) / ((( long long ) RAND_MAX ) + 1 ))
   #define randomize() srand(( unsigned ) time( NULL ) | 1 )
#endif

int main( void )
{
   unsigned long long int x, y, z; // 64 bit, right?
int i;
   
   randomize();

   x = RAND_MAX;
   printf("That's RAND_MAX : %llu (0x%x)", x, x );
   getchar();
   
for (i=0; i<52; i++)
   printf("This number should be random in the interval from 0 to 52 : %d\n", random( 52 ));
   getchar();

/****** now i do the same as the random macro ******/

   y = (long long)rand() * 52; // 52 is the max number i want
   x += 1;
   z = (long long int) ((long long)y / x); // convert double for division and convert back? Error?

   printf("The result of %llu divided by %llu results in %llu ?", y, x, z );
   getchar();

   return( 0 );
}
Or if you prefer reduce the rand output to 16 bits. The last will let increase the frequency for lower values in requested interval.
It is better to be hated for what you are than to be loved for what you are not. - Andre Gide

joselitosn

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Re: Random() and Randomize implementation
« Reply #2 on: April 01, 2007, 07:43:25 PM »
I thought it should be an implicit type conversion. But now i understood. Thanks!

EDIT: If i got it right "unsigned int" should have the same size as "unsigned long int" in Pelles C compiler, so i should use "unsigned long int" for compatibility (unless i can change the RAND_MAX or the max number rand() can give).

If i choose to use "unsigned long int", "unsigned long long int" need to be used only for "rand() * num" as this results into a larger number.

Should this be the optimal solution for compatibility with all compilers?
Quote
#ifndef __TURBOC__
   #define randomize() srand((unsigned int)time((time_t *)NULL))
   #define random(num) (unsigned long int)((((unsigned long long int)rand())*((unsigned long int)(num)))/(((unsigned long int)RAND_MAX)+1))
#endif

Later i'm going to find a PRNG library, but now i can only use that.
« Last Edit: April 02, 2007, 03:09:25 AM by joselitosn »