Unit Step Function and Subscript Indices Error

I have been asked to write a code to stem plot discrete-time functions. I have the values the for 'a' and 'n', but not the values for 'u'.

I have included a hint which was given to me in the code for finding the values of 'u'. When i follow the hint i get a 1x41 matrix with 0's and 1's inside.

This produces error "Subscript indices must either be real positive integers or logicals." So i set u = 1:20 and still get the same error.

I suppose my issue is obtaining the correct values for 'u' and removing the above error. I hope i have provided enough supporting information.

a1 = 0.8;
a2 = 1;
a3 = 1.5;

n = -20:20;

%I have been given hint "The unit step function y = u(n) may be computed

%in Matlab using the command y = (n>=0), where n is a vector of values of

%time indices.

u = 1:20;

aa = (a1.^n)*(u(n) - u(n-10))
ab = (a2.^n)*(u(n) - u(n-10))
ac = (a3.^n)*(u(n) - u(n-10))

[M]

You get the error

"Subscript indices must either be real positive integers or logicals."

because you defined

u=1:20

which is an array from 1 to 20, such that

u(i)=i, i=1...20

but you want to access

u(n-10), with n = -20:20

so you ask MATLAB to use the value of u(-30), which does not exist.

[Vaidyanathan Thiagarajan]

Yes, but you cannot even write 'u(n)' as 21 out of 41 indices (as n = -20:20) are negative. In MATLAB, array indices can only be positive integers (i.e. >= 1).