MATLAB: Symbolic array pre-allocation

symbolic array preallocationSymbolic Math Toolbox

Symbolic array pre-allocation only allocates pointers as I understood it from reading the forum. This is causing a major slowdown in code we are looking at
a=sym(zeros(50000,1));
Filling the area with relatively simple expression e.g. '3+sqrt(5)/sqrt(7)' a serious slowdown is noted around the 9000th index.
Would it be possible to allocate memory by defining the sym array (in the definition) with large expressions '3+sqrt(1)+sqrt(2)+sqrt(3)+sqrt(4)+sqrt(5)+sqrt(6)+sqrt(7)' for all elements 1:50000, so that effectively a memory allocation is forced at the definition step. Is this possible in the definition of sym? I tried repmat, but didn't get it too work.

Best Answer

My question is in which form do you have the 50000 entries you want to fill in. As strings in a cell array A? Then
B = cellfun(@sym, A, 'UniformOutput', false);
list1 = [B{:}];
is better. Or really 50000 times the same string? Then avoid calling sym(..) on it in a loop.
All sym vectors are internally vectors of pointers. You cannot pre-allocate the memory these pointers point to ...
Related Question