GRASS Programmer's Manual  6.4.2(2012)
var.c
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00001 #include <math.h>
00002 #include <grass/imagery.h>
00003 /* sum: sum of x
00004  * sum2: sum of x squared
00005  * n:    number of points
00006  */
00007 
00008 double I_variance(double sum, double sum2, int n)
00009 {
00010     if (n < 2)
00011         return ((double)0.0);
00012     else
00013         return ((sum2 - sum * sum / n) / (n - 1));
00014 }
00015 
00016 double I_stddev(double sum, double sum2, int n)
00017 {
00018     if (n < 2)
00019         return ((double)-99.0);
00020     else
00021         return sqrt(I_variance(sum, sum2, n));
00022 }
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