"Gaijinco" <ga******@gmail.com> wrote in message
news:11**********************@z14g2000cwz.googlegr oups.com...
Why is it better:
rand() / (RAND_MAX / N + 1)
than:
srand(time(0));
rand() % N;
The problem with random integers in a desired interval is that when you
start doing tricks like the above the distribution ceases to be uniform.
Here's an example revealing this problem:
#include <stdio.h>
#define MY_RAND_MAX 15 // my RAND_MAX for myrand()
#define N 10 // the N in myrand()%N
#define M 1000 // iterations count
// My fake rand() function,
// mimicking uniform distribution.
// THIS IS A BAD random generator,
// BUT A GOOD means to show the problem
// of the rand()%N approach to generate
// uniformly distributed numbers in the range
// smaller than [0,RAND_MAX].
int myrand()
{
static int seed = 0;
int res = seed;
// generate numbers in the range [0,MY_RAND_MAX]
// like so: 0,1,2,...,MY_RAND_MAX-1,MY_RAND_MAX,0,1,2,...
if (++seed > MY_RAND_MAX)
seed = 0;
return res;
}
int main()
{
int i;
int aCnt[N];
// zero up counters of each of the numbers
// that will be generated at random:
for (i=0; i<N; i++)
aCnt[i] = 0;
// generate numbers at random and count
// how many time each of them was generated:
for (i=0; i<M; i++)
aCnt[myrand()%N]++;
// print out the statistics:
printf ("Statistics:\n"
"+--------+-------------------+\n"
"| Number | Times Encountered |\n"
"+--------+-------------------+\n");
for (i=0; i<N; i++)
printf ("| %-6d | %-17d |\n", i, aCnt[i]);
printf ("+--------+-------------------+\n");
return 0;
}
Alex
