#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>

#define PI  3.14159265358979323846264

double Rand(void)
{
  return (double)rand() / (RAND_MAX + 1);
}

/*
 * ボックス＝ミューラー法
 */
double NormalRandom(double mu, double sigma)
{
  double t, u, r;
  static int sw = 0;
  static double rs;
  
  if (sw == 0) {
    t = sqrt(-2.0 * log(1.0 - Rand()));
    u = 2.0 * PI * Rand();
    r = t * cos(u);
    rs = t * sin(u);
    sw = 1;
  } else {
    r = rs;
    sw = 0;
  }
  
  return sigma * r + mu;
}

int main(void)
{
  int i, n, ix, *histo;
  double mu, sigma, x, s1 = 0.0, s2 = 0.0;
  
  srand((unsigned)time(NULL));
  printf("個数 n = ");
  scanf("%d", &n);
  printf("平均 μ = ");
  scanf("%lf", &mu);
  printf("分散 σ = ");
  scanf("%lf", &sigma);
  
  if ((histo = (int *)malloc(sizeof(int) * n)) == NULL)
    exit(1);
  
  for (i = 0; i < n; i++) {
    x = NormalRandom(mu, sigma);
    ix = (int)floor(2.0 * x) + 10;
    if (ix >= 0 && ix < 20)
      histo[ix]++;
    s1 += x;
    s2 += x * x;
  }

  for (i = 0; i < 20; i++)
    printf("%4.1f -- %4.1f: %5.1f%%\n", 0.5 * (i - 10), 0.5 * (i - 9), 100.0 * histo[i] / n);
  s1 /= n;
  s2 = sqrt((s2 - n * s1 * s1) / (n - 1));
  printf("平均 %g  標準偏差 %g\n", s1, s2);
  
  free(histo);
  
  return 0;
}