求matlab离散化程序

已经知道y=0.0485x^2+0.0259x，0《x《8代表总数的曲线，现在我要求画z(i)=y(i+1)-y(i)的图，

x=0:0.1:8
y=0.0485*x.^2+0.0259*x
z=diff(y) %就是y(i+1)-y(i)
plot(z)

clear all; close all; ts=0.001; j=1/147; q=22/147; sys=tf(1,[j,q,0]); dsys=c2d(sys,ts,'z'); [num,den]=tfdata(dsys,'v'); r_1=0;r_2=0; u_1=0;u_2=0; y_1=0;y_2=0; error_1=0; ei=0; for k=1:1:1000 time(k)=k*ts; s=1; if s==1 %step signal rin(k)=1.0; elseif s==2 %sine signal rin(k)=1.0*sin(1*2*pi*k*ts); end yout(k)=-den(2)*y_1-den(3)*y_2+num(2)*u_1+num(3)*u_2; error(k)=rin(k)-yout(k); derror(k)=(error(k)-error_1)/ts; ap=22;bp=8.0;cp=0.8; kp(k)=ap+bp*(1-sech(cp*error(k))); ad=0.5;bd=2.5;cd=6.5;dd=0.30; kd(k)=ad+bd/(1+cd*exp(dd*error(k))); ai=1;ci=1; ki(k)=ai*sech(ci*error(k)); ei=ei+error(k)*ts; u(k)=kp(k)*error(k)+kd(k)*derror(k)+ki(k)*ei; %update parameters r_2=r_1;r_1=rin(k); u_2=u_1;u_1=u(k); y_2=y_1;y_1=yout(k); error_1=error(k); end figure(1); plot(time,rin,'k',time,yout,'k'); xlabel('time(s)');ylabel('rin,yout'); figure(2); plot(time,rin-yout,'k');ylabel('error'); xlabel('time(s)');ylabel('error'); figure(3); plot(time,derror,'k'); xlabel('time(s)');ylabel('derror'); m=1; if m==1 figure(4); subplot(311); plot(error,kp,'k');xlabel('error');ylabel('kp'); subplot(312); plot(error,kd,'k');xlabel('error');ylabel('kd'); ad+bd/(1+cd) subplot(313); plot(error,ki,'k');xlabel('error');ylabel('ki'); elseif m==2 figure(5); subplot(311); plot(time,kp,'k');xlabel('time(s)');ylabel('kp'); subplot(312); plot(time,kd,'k');xlabel('time(s)');ylabel('kd'); subplot(313); plot(time,ki,'k');xlabel('time(s)');ylabel('ki'); end