拉格朗日算法求救！！！

拉格朗日算法：
function s=lag(x,y,t)
syms p;
n=length(x);
s=0;
for (k=1:n)
    la=y(k);
    for(j=1:k-1)
        la=la*(p-x(j))/(x(k)-x(j));
    end;
    for(j=k+1:n)
        la=la*(p-x(j))/(x(k)-x(j));
    end;
    s=s+la;
    simplify(s);
end
if(nargin==2)
    s=subs(s,'p','x');
    s=collect(s);
    s=vpa(s,6);
else
    m=length(t);
    for i=1:m
        temp(i)=subs(s,'p','t(i)');
    end
    s=temp;
end

主程序：
x=[pi/4,pi/6,pi/3,pi/2];
y=[cos(pi/4),cos(pi/6),cos(pi/3),cos(pi/2)];
t=[-40*pi/180,47*pi/180,53*pi/180,79*pi/180,174*pi/180];
disp('角度')
du=[-40 47 53 79 174]
disp('插值结果')
yt=lag(x,y,t)
disp('cos函数值')
yreal=[cos(-40*pi/180)
    cos(47*pi/180)
    cos(53*pi/180)
    cos(79*pi/180)
    cos(174*pi/180)]'
disp('插值与函数值误差')
dy=yt-yreal
yt=lag(x,y);
ezplot(yt,[-pi/4,pi])
hold on
ezplot('cos(t)',[-pi/4,pi]);
grid
hold off

警告内容：
Support of strings that are not valid variable names or define a number will be removed in a future release. To create symbolic expressions, first create
symbolic variables and then use operations on them.
> In sym>convertExpression (line 1536)
  In sym>convertChar (line 1441)
  In sym>tomupad (line 1198)
  In sym (line 177)
  In sym/subs>inputchk (line 229)
  In sym/subs>mupadsubs (line 134)
  In sym/subs (line 125)
  In lag (line 23)
  In lag_main (line 7)