stddev_bugged.c

/* Compile with:
CFLAGS="-g -Wall -std=gnu11 -O3" make stddev_bugged
*/


//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);
}