111.绘制正态分布曲线

#include "stdio.h"
#include "math.h"
#include "graphics.h"
double lgam1(x) /*Gamma函数的计算*/
double x;
{ 
  int i;
    double y,t,s,u;
    static double a[11]={ 0.0000677106,-0.0003442342,
    0.0015397681,-0.0024467480,0.0109736958,
    -0.0002109075,0.0742379071,0.0815782188,
    0.4118402518,0.4227843370,1.0};
    if (x<=0.0)
  { printf("err**x<=0!\n"); return(-1.0);}
    y=x;
    if (y<=1.0)
  {
    t=1.0/(y*(y+1.0)); y=y+2.0;}
    else if (y<=2.0)
  { 
    t=1.0/y; y=y+1.0;}
    else if (y<=3.0) t=1.0;
    else
  { 
    t=1.0;
        while (y>3.0)
    { y=y-1.0; t=t*y;}
  }
    s=a[0]; u=y-2.0;
    for (i=1; i<=10; i++)
    s=s*u+a[i];
    s=s*t;
    return(s);
}  
double lgam2(a,x) /*不完全Gamma函数*/
double a,x;
{ 
  int n;
    double p,q,d,s,s1,p0,q0,p1,q1,qq;
    if ((a<=0.0)||(x<0.0))
  { if (a<=0.0) printf("err**a<=0!\n");
  if (x<0.0) printf("err**x<0!\n");
  return(-1.0);
  }
    if (x+1.0==1.0) return(0.0);
    if (x>1.0e+35) return(1.0);
    q=log(x); q=a*q; qq=exp(q);
    if (x<1.0+a)
  { 
    p=a; d=1.0/a; s=d;
        for (n=1; n<=100; n++)
    { 
      p=1.0+p; d=d*x/p; s=s+d;
      if (fabs(d)<fabs(s)*1.0e-07)
      { 
        s=s*exp(-x)*qq/lgam1(a);
                return(s);
      }
    }
  }
    else
  { 
    s=1.0/x; p0=0.0; p1=1.0; q0=1.0; q1=x;
        for (n=1; n<=100; n++)
    { 
      p0=p1+(n-a)*p0; q0=q1+(n-a)*q0;
            p=x*p0+n*p1; q=x*q0+n*q1;
            if (fabs(q)+1.0!=1.0)
      { 
        s1=p/q; p1=p; q1=q;
                if (fabs((s1-s)/s1)<1.0e-07)
        { 
          s=s1*exp(-x)*qq/lgam1(a);
                    return(1.0-s);
        }
                s=s1;
      }
            p1=p; q1=q;
    }
  }
    printf("a too large !\n");
    s=1.0-s*exp(-x)*qq/lgam1(a);
    return(s);
}
double lerrf(x) /*误差函数*/
double x;
{ 
  double y;
    if (x>=0.0)
    y=lgam2(0.5,x*x);
    else
    y=-lgam2(0.5,x*x);
    return(y);
}
double lgass(a,d,x) /*正态分布函数*/
double a,d,x;
{
  double y;
    if (d<=0.0) d=1.0e-10;
    y=0.5+0.5*lerrf((x-a)/(sqrt(2.0)*d));
    return(y);
}
main()
{ 
  int i;
  double j;
    double a, d;
  int gdriver = DETECT, gmode;
  clrscr();
  printf("This program will draw the Normal Distribution Graph.\n");
  printf("Please input the mathematical expectation (Alpha): ");
  scanf("%lf", &a );
  printf("Please input the variance (Sita >0): ");
  scanf("%lf", &d );
  /*registerbgidriver( EGAVGA_driver );*/
  initgraph( &gdriver, &gmode, "e:\\tc\\bgi" );
  setbkcolor( BLUE );
  moveto( 50, 430 );
  lineto( 590, 430 );
  outtextxy( 600, 425, "X");
  moveto( 200, 50 );
  lineto( 200, 450 );
  outtextxy( 200, 30, "Y" );
  outtextxy( 185, 435, "O");
  setcolor( RED );
  moveto( 51, 430 - 100 * lgass( a, d, -150.0 ) );
  for( i = 51; i <= 590; i++ )
  {
    j = 430 - 360 * lgass( a, d, (double)(i-200) );
    lineto( i, j );
  }
  getch();
  closegraph();
}