random number

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Views: 379 | Replies: 4 | Solved

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Join Date: Nov 2007

Posts: 73

Reputation: emilio is an unknown quantity at this point

Rep Power: 1

Solved Threads: 0

emilio

Offline

Junior Poster in Training

random number

May 12th, 2008

i'm trying to get random numbers in a loop like this

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syntax Syntax (Toggle Plain Text)

#include <stdio.h>
#include <time.h>
 
int getRand ( int a , int b ) 
{	
  return a+rand()%(b-a+1);
}
 
int main()
{
srand ( time( NULL ) ) ;
int i;
 
for(i=0 ; i<6 ; i++)
printf("%d\n",getRand(0,9));
 
return 0;
}

the problem is that i keep getting the same number

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Join Date: May 2008

Posts: 345

Reputation: Radical Edward has a spectacular aura about

Rep Power: 3

Solved Threads: 56

Radical Edward Radical Edward is offline

Offline

Posting Whiz

Re: random number

May 12th, 2008

The same number or some numbers are being repeated? Your code works just fine when Edward tests it, but repeated numbers are expected unless you go out of your way to avoid them with something like a random shuffle:

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(Toggle Plain Text)

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

static int *values;
static int index;

int initRandUnique(int lo, int hi)
{
  values = malloc((hi - lo) * sizeof *values);

  if (values == NULL)
    return 0;

  {
    int limit = hi - lo;
    int i;

    for (i = 0; i < limit; ++i)
      values[i] = lo++;

    for (i = 0; i < limit - 1; ++i) {
      int r = i + rand() % (limit - i);
      int temp = values[r];
      values[r] = values[i];
      values[i] = temp;
    }
  }

  return 1;
}

int getRandUnique() 
{
  return values[index++];
}

int main()
{
  int i;

  srand(time(NULL));
  initRandUnique(0, 9);

  for (i = 0; i < 6; ++i)
    printf("%d\n", getRandUnique());

  return 0;
}

If at first you don't succeed, keep on sucking until you do succeed.

•

Join Date: Nov 2007

Posts: 73

Reputation: emilio is an unknown quantity at this point

Rep Power: 1

Solved Threads: 0

emilio

Offline

Junior Poster in Training

Re: random number

May 13th, 2008

ok.
i wanted to use something simple cause my actual program is quite difficult
i need to do something like this:

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syntax Syntax (Toggle Plain Text)

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
 
 int sig,pid;
 
int getRand ( int a , int b )
{
return a+rand()%(b-a+1);
}
 
int main()
 {
      pid = getpid();
      srand ( time( NULL ) ) ;
       int i,n;
 
     for(i=0 ; i<6 ; i++)
      {
      if(getpid() == pid)
         sig = fork();
 
      if(sig == 0)
      {
       n = getRand(1,9);
      printf("%d\n",n);
      sleep(n);
      exit(0); 
      }
 
 
return 0;
      }

at this point it gives me the same random number.

•

Join Date: May 2008

Posts: 345

Reputation: Radical Edward has a spectacular aura about

Rep Power: 3

Solved Threads: 56

Radical Edward Radical Edward is offline

Offline

Posting Whiz

Re: random number

May 15th, 2008

That's just a fancy way of doing this:

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(Toggle Plain Text)

static unsigned seed;

int getRand(int a, int b) 
{
  srand(seed);	
  return a + rand() % (b - a + 1);
}

int main(void)
{
  seed = time(NULL);

  for (i = 0; i < 10; ++i)
    printf("%d\n", getRand(1, 9));

  return 0;
}

You initialize the seed once, and each child process uses the same seed but restarts the sequence from the beginning. In other words, each child process is printing the first number of the sequence, so all of the numbers are going to be the same. You need to reseed rand after spawning a new process:

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(Toggle Plain Text)

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

int sig,pid;

int getRand ( int a , int b )
{
  return a+rand()%(b-a+1);
}

int main()
{
  pid = getpid();

  int i,n;

  for(i=0 ; i<6 ; i++)
  {
    if(getpid() == pid)
      sig = fork();

    if(sig == 0)
    {
      srand ( time( NULL ) );
      n = getRand(1,9);
      printf("%d\n",n);
      sleep(n);
      exit(0); 
    }
  }

  return 0;
}

If at first you don't succeed, keep on sucking until you do succeed.