Add example for debugging

stddev.c

 /* Compile with:
-export LDLIBS="`pkg-config --libs gsl`"
-export CFLAGS="`pkg-config --cflags gsl` -g -Wall -std=gnu11 -O3"
-make stddev
+CFLAGS="-g -Wall -std=gnu11 -O3" make stddev
 */
 #include <math.h>
 #include <stdio.h> //size_t
@@ -12,14 +10,14 @@ meanvar mean_and_var(const double *data){
     long double avg = 0,
           avg2 = 0;
     long double ratio;
-    size_t cnt= 0;
+    size_t count= 0;
     for(size_t i=0;  !isnan(data[i]); i++){
-        ratio = cnt/(cnt+1.0);
-        cnt   ++;
+        ratio = count/(count+1.0);
+        count ++;
         avg   *= ratio;
         avg2  *= ratio;
-        avg   += data[i]/(cnt +0.0);
-        avg2  += pow(data[i], 2)/(cnt +0.0);
+        avg   += data[i]/(count +0.0);
+        avg2  += pow(data[i], 2)/(count +0.0);
     }
     return (meanvar){.mean = avg,
                     .var = avg2 - pow(avg, 2)}; //E[x^2] - E^2[x]

stddev_bugged.c

+/* Compile with:
+CFLAGS="-g -Wall -std=gnu11 -O3" make stddev
+*/
+
+
+//WARNING: This program is incorrect, intended for a debugging example.
+//See stddev.c for the correct version.
+
+
+
+#include <math.h>
+#include <stdio.h> //size_t
+
+typedef struct meanvar {double mean, var;} meanvar;
+
+meanvar mean_and_var(const double *data){
+    long double avg = 0,
+          avg2 = 0;
+    long double ratio;
+    size_t count= 0;
+    for(size_t i=0;  !isnan(data[i]); i++){
+        ratio = count/(count+1);
+        count ++;
+        avg   *= ratio;
+        avg2  *= ratio;
+        avg   += data[i]/(count +0.0);
+        avg2  += pow(data[i], 2)/(count +0.0);
+    }
+    return (meanvar){.mean = avg,
+                    .var = avg2 - pow(avg, 2)}; //E[x^2] - E^2[x]
+}
+
+int main(){
+    double d[] = { 34124.75, 34124.48,
+                   34124.90, 34125.31,
+                   34125.05, 34124.98, NAN};
+
+    meanvar mv = mean_and_var(d);
+    printf("mean: %.10g var: %.10g\n", mv.mean, mv.var*6/5.);
+
+    double d2[] = { 4.75, 4.48,
+                    4.90, 5.31,
+                    5.05, 4.98, NAN};
+
+    mv = mean_and_var(d2);
+    mv.var *= 6./5;
+    printf("mean: %.10g var: %.10g\n", mv.mean, mv.var);
+}