If variance is great, summing the values of an array can be problematic. The problem can often be solved by sorting the array before summing.

sum.c

double sum_d(const double *x, size_t length) {
  double sum = 0.0;
  for (size_t i = 0; i < length; ++i) {
    sum += x[i];
  }
  return sum;
}

double srtsum_d(const double *x, size_t length) {
  // Copy the array & sort it with quicksort.
  double *sorted = (double*)malloc(length * sizeof(double));
  memcpy((void*)sorted, (void*)x, length * sizeof(double));
  qsort((void*)sorted, length, sizeof(double), compare_double_asc);
  const double sum = sum_d(sorted, length); // The actual answer
  free(sorted); // Free memory
  return sum;
}