how to write a code second order diffrentiation code in MTLAB

hello eveyone, please can someone help me out......I am trying to write a matlab code that can perform second order differentiation. below is my attempted code but it failed work. Thanking you in advance.

clear all;
close all;
st = -1;
inc=0.001;
en =1;
t=st:inc:en;
y= t.^3;
x=zeros(1,(length(i)-1));
for i=1:(length(t)-1);
    x(1,i)=(y(1,i+2)-2*y(1,i+1)+y(1,i))./(inc.^2);
end
t1=st:inc:(en-inc);
figure(2)
plot(t1,x);
title('After differentiation')

Answer 1

[KSSV]

st = -1;
inc=0.001;
en =1;
t=st:inc:en;
y= t.^3;
x=zeros(1,(length(i)-2));
for i=1:(length(t)-2)
    x(i)=(y(i+2)-2*y(i+1)+y(i))./(inc^2);
end
t1=st:inc:(en-inc);
figure(2)
plot(t1(1:end-1),x);
title('After differentiation')

[Eshovo Moses]

Hi KSSV, thanks for your comment... but the suggestions did not wotrk either. therewas no plot

[Eshovo Moses]

YEs it works

[Eshovo Moses]

Thank you KSSV