想用差分方程实现对y=sin(pi*t)的跟踪，文件保存为：zuijin2t ，程序如下：
clear;
clc;
n=100;
pi=3.14;
for t=1:n
    T=t/100;
    y_d(t)=sin(pi*t/100);
end
t1=0.01;
y1=0.04;
for t=1:n
    y(t+1)=y(t)+pi*cos(pi*t/100)*t1;
end
T=t/100;
t=1:n
plot(t,yd(t),'b');hold on;
plot(t,y(t),'r');hold on;
可是，无论怎样改，总提示：
??? Undefined function or method 'y' for input arguments of type 'double'.
Error in ==> zuijin2t at 12
    y(t+1)=y(t)+pi*cos(pi*t/100)*t1;