threep.m

牛顿插值法

function r=threep(x,y)
    nx=length(x);
    for i=1:nx
        if(i~=nx)&(i~=1)
            t1=x(i)+0.1;
            t2=x(i)-0.1;
            for k=1:3
                x1(k)=x(i+k-2);
                y1(k)=y(i+k-2);
            end
            s1=lagr(x1,y1,t1);
            s2=lagr(x1,y1,t2);
            r(i)=(s1-s2)/(t1-t2); 
        end
        if i==nx;
            t1=x(i)-0.2;
            t2=x(i)-0.1;
            for k=1:3
                x1(k)=x(nx-3+k);
                y1(k)=y(nx-3+k);
            end
            s1=lagr(x1,y1,t1);
            s2=lagr(x1,y1,t2);
            r(i)=(s1-4*s2+3*y(i))/(x(i)-t1);
        end
        if i==1;
           t1=x(i)+0.2;
           t2=x(i)+0.1;
           for k=1:3
                x1(k)=x(k);
                y1(k)=y(k);
           end
           s1=lagr(x1,y1,t1);
           s2=lagr(x1,y1,t2);
           r(i)=(-3*y(i)+4*s2-s1)/(t1-x(i));
        end
    end