Why am I getting an Indexing error while using sum

Hi, I am trying to plot this equation in Matlab:

e_t refers to a vector of randomly generated values, by putting e_t, I am simply trying to pull out the vector with such index.

This is my code:

clear;

alpha = 0.5;
n = 100;

e=randn(n,1);
e(1) = 0;

t = 1:1:n;
syms k

f(k) = (alpha.^(k-1)).*(e(k));

Error using indexing
Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments must be symbolic variables, and function body must be sym expression.

err = zeros(n,1);
tot = zeros(n,1);

for i=1:1:n
	err(i) = subs(f,k,1:i);
	tot(i) = (alpha.^(i-1)) + sum(err(i));
end

plot(t,tot)

I am currently getting "Error using indexing". I am new to matlab and appreciate any help. Thank you

[Dyuman Joshi]

Using symbolic variables as indices is not allowed. You have used the symbolic variable k as an index to the array e, that's why you get the error.

You can do this without using symbolic variables. Using a function handle instead would be much more efficient.

Also, how are e_t and alpha related to/dependent on i?

[Star Strider]

I am not certain what you want to do, however the symsum function may be necessary to get where you want to go.

clear;

alpha = 0.5;
n = 100;

e=randn(n,1);
e(1) = 0;

% t = 1:1:n;
syms k t 
sympref('AbbreviateOutput',false);

e = sym('e',[1 n]);

f(k) = alpha.^(t-1) + symsum(e*alpha^(t-1), t, 1, n)

f(k) = 

f = vpa(simplify(f, 500), 5)

f(k) = 

return                  % STOP HERE

err = zeros(n,1);
tot = zeros(n,1);

for i=1:1:n
	err(i) = subs(f,k,1:i);
	tot(i) = (alpha.^(i-1)) + sum(err(i));
end

plot(t,tot)

.

Answer 1

[Walter Roberson]

e=randn(n,1);
e(1) = 0;

e is a vector.

syms k

k is a symbolic variable

f(k) = (alpha.^(k-1)).*(e(k));

With e being a vector, e(k) is a request to index e at the location designated by symbolic variable k. However, symbolic variables can never be used as indices.

You should construct the definite vector

k = 1 : n;
f = (alpha.^(k-1)).*(e(k));

and then index it as needed.

Question:

t = 1:1:n;

what is that for? You never use it.