How can I sum an expression over the indices of an array?

A simplification of the type of code I require for a more complex situation is:

A=[1*x, 2*x, 3*x, 4*x];

for y=2:length(A)
    syms t
    symsum(A(y)-A(t-1), t, 2, y)
end

However, this is resulting in the error:

Error using sym/subsindex (line 1558)
Indexing input must be numeric, logical or ':'.

Error in example (line 5)
    symsum(A(y)-A(t-1), t, 2, y)

I was expecting the results below:

l = 2 gives A(2) - A(1) = x

l = 3 gives (A(3) - A(1)) + (A(3) - A(2))= 3*x

l = 4 gives (A(4) - A(1)) + (A(4) - A(2)) + (A(4) - A(3))= 6*x

The more complex situation for which I am using this code is:

syms n qq

for y=2:length(t)
    for u=1:length(x)
        Del_sig(y,u)=materials(material_barrier).gamma*(embSurf(u)-surf(u));
        for v=1:length(z)
            sumOverTime = symsum((Del_sig(qq,u)-Del_sig(qq-1,u))/materials(layers(1,1)).E*(d-z(v)-(4*d/(n^2*pi^2))*(1-cos(n*pi))*(cos(n*pi*z(u)/(2*d))-cos(n*pi/2))*exp(-n^2*pi^2*c_v*(t(y)-t(qq-1))/(4*d^2))),qq,2,y);
            rho1(v,u)=symsum(sumOverTime,n,1,Inf);
        end
    end
end

I need to compute a sum over qq in terms of n, and then a sum over n. All other variables have been defined earlier; c_v , d, materials(material_barrier).gamma and materials(layers(1,1)).E are scalars; t, x, z, embSurf and surf are arrays.

Any help would be much appreciated.

Answer 1

[Azzi Abdelmalek]

A=[1, 2, 3, 4];
syms x out
for y=2:numel(A)
  out(y-1)= sum(A(y)-A(y-1:-1:1))*x
end