Statistics

#include <stdio.h>
static double sum(double *x, int n);
static double square_sum(double *x, int n);
static double multiply_sum(double *x, double *y, int n);
static void input_n(int *n);
static void input_x_y(double *x, double *y, int n);
int main(void);
static double sum(double *x, int n) {
  int i;
  double s = 0.;
  for (i = 0; i < n; i++)
    s += x[i];
  return s;
}
static double square_sum(double *x, int n) {
  int i;
  double s = 0.;
  for (i = 0; i < n; i++)
    s += x[i] * x[i];
  return s;
}
static double multiply_sum(double *x, double *y, int n) {
  int i;
  double s = 0.;
  for (i = 0; i < n; i++)
    s += x[i] * y[i];
  return s;
}
static void input_n(int *n) {
  printf("n = ");
  scanf("%d", n);
}
static void input_x_y(double *x, double *y, int n) {
  int i;
  printf("input x y:\n");
  for (i = 0; i < n; i++) {
    printf("%d ? ", i);
    scanf("%lf%lf", &x[i], &y[i]);
  }
}
int main(void) {
  int n;
  double x[BUFSIZ], y[BUFSIZ], sumx, sumy, sumxx, sumxy, aa, a0, a1;
  input_n(&n);
  input_x_y(x, y, n);
  sumx = sum(x, n);
  sumy = sum(y, n);
  sumxx = square_sum(x, n);
  sumxy = multiply_sum(x, y, n);
  aa = n * sumxx - sumx * sumx;
  a0 = (sumxx * sumy - sumx * sumxy) / aa;
  a1 = (n * sumxy - sumx * sumy) / aa;
  printf("a0 = %g\n", a0);
  printf("a1 = %g\n", a1);
  return 0;
}