三次样条插值函数 matalab通用程序

817huang

function []=spline3(X,Y,dY,x0,m)
N=size(X,2);
s0=dY(1); sN=dY(2);
interval=0.025;
disp('x0为插值点')
x0
h=zeros(1,N-1);
for i=1:N-1 h(1,i)=X(i+1)-X(i); end
d(1,1)=6*((Y(1,2)-Y(1,1))/h(1,1)-s0)/h(1,1);
d(N,1)=6*(sN-(Y(1,N)-Y(1,N-1))/h(1,N-1))/h(1,N-1);
for i=2:N-1
d(i,1)=6*((Y(1,i+1)-Y(1,i))/h(1,i)-(Y(1,i)-Y(1,i-1))/h(1,i-1))/ (h(1,i)+h(1,i-1)); end
mu=zeros(1,N-1); md=zeros(1,N-1);
md(1,N-1)=1; mu(1,1)=1;
for i=1:N-2
u=h(1,i+1)/(h(1,i)+h(1,i+1)); mu(1,i+1)=u;
md(1,i)=1-u; end
p(1,1)=2; q(1,1)=mu(1,1)/2;
for i=2:N-1
p(1,i)=2-md(1,i-1)*q(1,i-1); q(1,i)=mu(1,i)/p(1,i); end
p(1,N)=2-md(1,N-1)*q(1,N-1);
y=zeros(1,N); y(1,1)=d(1)/2;
for i=2:N y(1,i)=(d(i)-md(1,i-1)*y(1,i-1))/p(1,i); end
x=zeros(1,N); x(1,N)=y(1,N);
for i=N-1:-1:1 x(1,i)=y(1,i)-q(1,i)*x(1,i+1); end
fprintf ('M为三对角方程的解\n'); M=x;
fprintf ('\n');
syms t;
digits (m);
for i=1:N-1
pp(i)=M(i)*(X(i+1)-t)^3/(6*h(i))+M(i+1)*(t-X(i))^3/(6*h(i))+(Y(i)-M(i)*h(i)^2/6)*(X(i+1)-t)/h(i)+(Y(i+1)-M(i+1)*h(i)^2/6)*(t-X(i))/h(i);
pp(i)=simplify(pp(i)); coeff=sym2poly(pp(i));
if length(coeff)~=4
tt=coeff(1:3); coeff(1:4)=0; coeff(2:4)=tt; end
if x0>X(i)&x0<X(i+1) L=i;
y0=coeff(1)*x0^3+coeff(2)*x0^2+coeff(3)*x0+coeff(4);
end
val=X(i):interval:X(i+1);
for k=1:length(val)
fval(k)=coeff(1)*val(k)^3+coeff(2)*val(k)^2
+coeff(3)*val(k)+coeff(4); end
if mod(i,2)==1 plot(val,fval,'r+');
else plot(val,fval,'b.'); end
hold on
clear val fval
ans=sym(coeff,'d');
ans=poly2sym(ans,'t');
fprintf('在区间[%f,%f]内\n',X(i),X(i+1));
fprintf('三次样条函数S(%d)=',i);
pretty(ans); end
fprintf ('x0所在区间为[%f,%f]\n',X(L),X(L+1));
fprintf ('函数在插值点x0=%f的值为\n',x0);
y0

lihuaxin18

求各输入参数含义。