statistics.c

统计方面的程序

/* statistics.c -- general purpose statistics functions
 * 
 * Author:           Jon Hamkins
 * Last Revised:     Tue Jan 30 09:51:45 PST 2001
 */

/* mean of a vector a[b..e] */
double
mean(double a[], int b, int e)
{     
   double m = 0.; 
   int i; 

   for (i=b; i<=e; i++) m += a[i];
   return(m/(double)(e-b+1));
}     

/* variance of a vector a[b..e] */
/* Note: this function normalizes by N-1 where N is the array length.
 * This gives the best unbiased estimate of the variance if the array
 * elements are samples from a normal distribution.  If the array
 * elements are the possible values in an equal-probability
 * distribution, one should normalize by N instead. */
double   
var(double a[], int b, int e)
{     
   double v = 0., m;
   int i;

   m = mean(a,b,e);
   for (i=b; i<=e; i++) v += (a[i]-m)*(a[i]-m);
   return(v/(double)(e-b));
}