请教ployfit,ployval 两个命令如何使用

jingrenzhi

请教大家,这两个命令在MATLAB中是怎么用?
ployfit,ployval谢谢

[ 本帖最后由 eight 于 2007-9-2 21:15 编辑 ]

twomao

好像写错了吧

[ 本帖最后由 lxq 于 2007-9-2 15:28 编辑 ]

twomao

POLYFIT Fit polynomial to data.
    POLYFIT(X,Y,N) finds the coefficients of a polynomial P(X) of
    degree N that fits the data, P(X(I))~=Y(I), in a least-squares sense.

    [P,S] = POLYFIT(X,Y,N) returns the polynomial coefficients P and a
    structure S for use with POLYVAL to obtain error estimates on predictions.
    P is a row vector of length N+1 containing the polynomial coefficients
    in descending powers, P(1)*X^N + P(2)*X^(N-1) +...+ P(N)*X + P(N+1).
    If the errors in the data, Y, are independent normal with constant
    variance, POLYVAL will produce error bounds which contain at least 50% of
    the predictions.

    The structure S contains the Cholesky factor of the Vandermonde
    matrix (R), the degrees of freedom (df), and the norm of the
    residuals (normr) as fields.   

    [P,S,MU] = POLYFIT(X,Y,N) finds the coefficients of a polynomial
    in XHAT = (X-MU(1))/MU(2) where MU(1) = mean(X) and MU(2) = std(X).
    This centering and scaling transformation improves the numerical
    properties of both the polynomial and the fitting algorithm.

lxq

help polyfit polyval

一个简单的多项式拟合例子
x=[0.5:0.1:2];
y=[0.39664 0.24624 0.13724 0.063503 0.018869 -0.0028101 -0.007685 -0.0019063 0.0083751 0.017009 0.017844 0.0047299 -0.028484 -0.087948 -0.17981 -0.31023];
b=polyfit(x,y,3)%%三次拟合
x1=0.46:.01:2.04;
f=polyval(b,x1);
plot(x,y,'ko',x1,f,'markerfacecolor','k','linewidth',2)

jingrenzhi

谢谢,上面两位老兄!

jingrenzhi

我这里有段程序,请大家帮我看看!拟合多项式阶数是多少?采样频率是多少?从程序中能看出吗?要是看不出,该怎么加上?谢谢!
clear
clc
close all hidden
fni=input('wenjian:','s');
fid=fopen (fni,'r');
sf=fscanf (fid,'%f',1);
m=fscanf (fid,'%d', 1);
fno=fscanf (fid,'%s',1);
x=fscanf(fid,'%f',inf);
status=fclose (fid);
n=length(x);
t=(0:1/sf:(n-1)/sf)';
a=polyfit(t,x,m);
y=x-polyval(a,t);
subplot(2,1,1);
plot(t,x);
grid on;
subplot(2,1,2);
plot(t,y);
grid on;

lxq

拟何阶数m
采样频率sf
从你程序上看 都是从文件中读取的

jingrenzhi

谢谢!我还是不太明白,我要拟合的阶数是3,采样频率是500,怎么在程序中看出?或者我在运行程序时怎么输入?说明一下我的文件是一组采集得到的数!

octopussheng

你拟合的阶数从这里看a=polyfit(t,x,m);
还有m=fscanf (fid,'%d', 1);
应该是要求自己输入的吧！

lxq

从程序中不能看出啊
但是可以直接定义的
m=3;
sf=500;

jingrenzhi

怎么定义?能教我吗?
不会是直接写上吧?

jingrenzhi

现在有一个问题,我变换了m和sf的值,对同一组数据处理,根本没有变化!

兆佳芷宁

为什么我的matlab找不到ployfit函数？
能给个帮助吗？

eight

原帖由 兆佳芷宁 于 2008-3-28 11:12 发表

                               
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为什么我的matlab找不到ployfit函数？
能给个帮助吗？

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