/**
* <p>函数功能：最小二乘法曲线拟合</p>
* @param x 实型一维数组，长度为 n 。存放给定 n 个数据点的 X 坐标
* @param y 实型一维数组，长度为 n 。存放给定 n 个数据点的 Y 坐标
* @param n 变量。给定数据点的个数
* @param a 实型一维数组，长度为 m 。返回 m-1 次拟合多项式的 m 个系数
* @param m 拟合多项式的项数，即拟合多项式的最高次数为 m-1.
* 要求 m<=n 且m<=20。若 m>n 或 m>20 ，则本函数自动按 m=min{n,20} 处理.
* <p>Date:2007-12-25 16:21 PM</p>
* @author qingbao-gao
* @return
*/
public static double[] PolyFit(double x[], double y[], int n, double a[], int m)
{
int i, j, k;
double z, p, c, g, q = 0, d1, d2;
double []s=new double[20];
double []t=new double[20];
double[] b=new double[20];
double[]dt=new double[3];
for (i = 0; i <= m-1; i++)
{
a[i] = 0.0;
}
if (m > n)
{
m = n;
}
if (m > 20)
{
m = 20;
}
z = 0.0;
for (i = 0; i <= n-1; i++)
{
z = z+x[i]/(1.0 *n);
}
b[0] = 1.0;
d1 = 1.0 * n;
p = 0.0;
c = 0.0;
for (i = 0; i <= n-1; i++)
{
p = p+(x[i]-z);
c = c+y[i];
}
c = c/d1;
p = p/d1;
a[0] = c * b[0];
if (m > 1)
{
t[1] = 1.0;
t[0] = -p;
d2 = 0.0;
c = 0.0;
g = 0.0;
for (i = 0; i <= n-1; i++)
{
q = x[i]-z-p;
d2 = d2+q * q;
c = c+y[i] *q;
g = g+(x[i]-z) *q * q;
}
c = c/d2;
p = g/d2;
q = d2/d1;
d1 = d2;
a[1] = c * t[1];
a[0] = c * t[0]+a[0];
}
for (j = 2; j <= m-1; j++)
{
s[j] = t[j-1];
s[j-1] = -p * t[j-1]+t[j-2];
if (j >= 3)
for (k = j-2; k >= 1; k--)
{
s[k] = -p * t[k]+t[k-1]-q * b[k];
}
s[0] = -p * t[0]-q * b[0];
d2 = 0.0;
c = 0.0;
g = 0.0;
for (i = 0; i <= n-1; i++)
{
q = s[j];
for (k = j-1; k >= 0; k--)
{
q = q *(x[i]-z)+s[k];
}
d2 = d2+q * q;
c = c+y[i] *q;
g = g+(x[i]-z) *q * q;
}
c = c/d2;
p = g/d2;
q = d2/d1;
d1 = d2;
a[j] = c * s[j];
t[j] = s[j];
for (k = j-1; k >= 0; k--)
{
a[k] = c * s[k]+a[k];
b[k] = t[k];
t[k] = s[k];
}
}
dt[0] = 0.0;
dt[1] = 0.0;
dt[2] = 0.0;
for (i = 0; i <= n-1; i++)
{
q = a[m-1];
for (k = m-2; k >= 0; k--)
{
q = a[k]+q *(x[i]-z);
}
p = q-y[i];
if (Math.abs(p) > dt[2])
{
dt[2] = Math.abs(p);
}
dt[0] = dt[0]+p * p;
dt[1] = dt[1]+Math.abs(p);
}
return a;
}
/**
* <p>对X轴数据节点球平均值</p>
* @param x 存储X轴节点的数组
* <p>Date:2007-12-25 20:21 PM</p>
* @author qingbao-gao
* @return 平均值
*/
public static double ave(double []x)
{
double ave=0;
double sum=0;
if(x!=null)
{
for(int i=0;i<x.length;i++)
{
sum+=x[i];
}
System.out.println("sum-->"+sum);
ave=sum/x.length;
System.out.println("ave"+ave+"x.length"+x.length);
}
return ave;
}
/**
* <p>由X值获得Y值</p>
* @param x 当前X轴输入值,即为预测的月份
* @param xx 当前X轴输入值的前X数据点
* @param a 存储多项式系数的数组
* @param m 存储多项式的最高次数的数组
* <p>Date:2007-12-25 PM 20:07</p>
* <P>Author:qingbao-gao</P>
* @return 对应X轴节点值的Y轴值
*/
public static double getY(double x,double[]xx,double[]a,int m)
{
double y=0;
double ave=ave(xx);
double l=0;
for(int i=0;i<m;i++)
{
l=a[0];
if(i>0)
{
y+=a[i]*Math.pow((x-ave),i );
System.out.println(i+"--|-->"+y+"--a[i]--"+a[i]);
}
System.out.println("a[0]|"+a[0]);
}
System.out.println("l--|"+(l));
return (y+l);
}
//--------------------------------------------测试代码
public static void main(String []args)throws DBException
{
double []x={200401,200402,200403,200404,200405,200406,200407,200408,200409,2004010,2004011,2004012,200501,200502,200503,200504};
double []y={51,51,53,53,54,55,57,60,63,64,66,66,69,71,72,75};
double[]a=new double[20];
double[]aa= PolyFit(x, y, 16,a, 3);
double yy=0;
System.out.println("拟合-->"+getY(200505,x,aa,3));
}
测试结果为：拟合-->72.38898870320554
效果还可以。