Why is cumsum not working with symbolic vectors?

Question

I wrote a small Octave script involving cumsum on symbolic vectors, which I expected to work but unfortunately, it does not. Here's the code:

pkg load symbolic
n = 3;
syms q x
q = sym('q', [n 1]);
x = sym('x', [n 1]);
cumsum(q - x)

Instead of the expected result, I get:

error: cumsum: wrong type argument 'class'

Why does this happen? Would it work with Matlab Symbolic Toolbox? (I don't have it so I cannot test, unfortunately.)

Answer 1

That's just because cumsum written in Octave is not supported for symbolic elements as indicated by the error message. Your code gives the following in MATLAB:

ans =
                     q1 - x1
           q1 + q2 - x1 - x2
 q1 + q2 + q3 - x1 - x2 - x3

---

You can do something like this:

t1 = q-x;
t2 = triu(ones(numel(t1)));
sum(repmat(t1,1,3).*t2).'
% repmat is necessary here because implicit expansion is also not 
% supported for matrices of class sym in Octave

Above code in Octave and MATLAB gives the following respectively:

ans = (sym 3×1 matrix)

  ⎡            q₁₁ - x₁₁            ⎤
  ⎢                                 ⎥
  ⎢      q₁₁ + q₂₁ - x₁₁ - x₂₁      ⎥
  ⎢                                 ⎥
  ⎣q₁₁ + q₂₁ + q₃₁ - x₁₁ - x₂₁ - x₃₁⎦

ans =
                     q1 - x1
           q1 + q2 - x1 - x2
 q1 + q2 + q3 - x1 - x2 - x3