var.c

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#include <math.h>
#include <grass/imagery.h>
/* sum: sum of x
 * sum2: sum of x squared
 * n:    number of points
 */

double I_variance(double sum, double sum2, int n)
{
    if (n < 2)
        return ((double)0.0);
    else
        return ((sum2 - sum * sum / n) / (n - 1));
}

double I_stddev(double sum, double sum2, int n)
{
    if (n < 2)
        return ((double)-99.0);
    else
        return sqrt(I_variance(sum, sum2, n));
}