Random numbers in C

loke137 · 03-05-2003, 01:10 PM

I am trying to generate two random numbers using random(), but it always returns 4 and 7 for me. I am using % in the rnd() function so I get only numbers between 1 and max. Thanks for the help
Here is the code:

Code:

#include <stdio.h>

int main()
{

   int rnd( int max) {
      return ( random () % max) +1;
   }

    int valor1, valor2, max;
    max = 30;
    valor1 = rnd(max);
    valor2 = rnd(max);

  return 0;

}

Palin · 03-05-2003, 01:19 PM

If I remember correctly you have to set the random seed which will change the values that you will recieve from the random function.

loke137 · 03-05-2003, 01:20 PM

How can you tell me how is it done? I have tried it

Palin · 03-05-2003, 01:25 PM

ok in the <stdlib.h>

the ones that I found through the kdevelop documentation.

void srand( unsigned int )
will set the random seed

int rand( void )
will return a random integer

loke137 · 03-05-2003, 01:42 PM

thx for the help dude!!!
I am also a LNO refugee, eheheh!!!

Hko · 03-06-2003, 03:40 PM

The function random() (and also rand() ) have a very large sequence of random number, from which they return the next in the sequence every time random() (or rand() ) is called.

The "problem" is that when your program starts random() starts at the same place in the sequence of random numbers, so every time you run your program you get the same "random" numbers.

The solution is that you can tell the random system where in the sequence to start. To do this, use the srandom() function (or resp. the srand() function if you use rand() instead of radom() ).

So you need to call srandom() with a number that is not the same every time you run your program. One common way is to use the time() function, which return the numbers of seconds since 00:00:00 UTC, January 1, 1970.
The current time (in seconds) -as you can imagine- is different every time you start the program. So call "srandom(time(0))" just once at the start of the program.

Here's your program fixed:

Code:

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

int rnd( int max) {
     return ( random () % max) +1;
}

int main()
{
     int valor1, valor2, max;
     max = 30;

     srandom(time(0));

     valor1 = rnd(max);
     valor2 = rnd(max);

     printf("valor1 = %d\n", valor1);
     printf("valor2 = %d\n", valor2);

     return 0;

}

loke137 · 03-06-2003, 08:07 PM

Thanks!!

stephstellar · 03-07-2003, 04:56 PM

The other thing about the sequence is that the second value you called will always be the next one in the sequence from the first one. For most things it won't be a problem, but if you are writing some dumb dice simulator for some dumb tutorial (has anyone on a computing course NOT written a dice simulator?) and you want the numbers to match up now and again, you'll have to do the whole thing over again to generate the second number, etc. What fun.

Greg Marks · 09-10-2010, 08:02 PM

I would suggest one minor emendation to the program:

Code:

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

int rnd(int max) {
     int temp, value;
     do {
          temp = random ();
          value = temp % max + 1;
     } while (temp >= RAND_MAX - RAND_MAX % max);
     return value;
}

int main()
{
     int valor1, valor2, max;
     max = 30;

     srandom(time(0));

     valor1 = rnd(max);
     valor2 = rnd(max);

     printf("valor1 = %d\n", valor1);
     printf("valor2 = %d\n", valor2);

     return 0;

}

The added "while" condition prevents the returned value from being (slightly) biased if the range of output values of random() is not a multiple of the modulus.

H_TeXMeX_H · 09-11-2010, 10:37 AM

If I were using a Linux system and needed good random numbers I'd use /dev/random or /dev/urandom as a seed. I wrote a program like this some time ago, here:

Code:

// includes
#include <stdio.h>
#include <stdlib.h>

// defines
#define NUM_RAND 10

// main
int main(void)
{
  // init vars
  unsigned  seed = 0; // random seed

  // ! init vars
  FILE *urandom; // file handle for /dev/urandom
  unsigned int i; // all-purpose counter

  urandom = fopen("/dev/urandom", "rb"); // open /dev/urandom as readonly binary
  if (urandom == NULL) // did it open ?
  {
    perror("!!! ERROR: could not open /dev/urandom for reading !!!");
    exit(1); // fail
  }

  // read the one seed from urandom
  if (fread(&seed, sizeof(int), 1, urandom) < 1) // did it read ?
  {
    perror("!!! ERROR: failed to read from /dev/urandom !!!");
    exit(1); // fail
  }

  // we are now finished reading from /dev/urandom, close it
  fclose(urandom);

  // print the seed just to make sure
  printf("The seed obtained from /dev/urandom is %u\n", seed);

  // seed rand with seed
  srand(seed);

  // generate some random numbers
  for (i = 1; i <= NUM_RAND; i++)
  {
    printf("Random number #%d, is %d\n", i, rand());
  }

printf("%d\n",sizeof(int));

  // success
  return 0;
}

I'm not a very good programmer, but it seems to work.