About preallocating for speed

There is a " for loop" in my program and Matlab gives me a suggestion to consider "Preallocating" for speed. I want to learn how to do it. This is my code:

 A = [];
for i = 1:size(P,2)
    Ai = build_matrix(P(:,i));
    *A* = [A; Ai];
end

there is a red line under A on Bold saying that The size of the indicated variable or array appears to be changing with each loop iteration. Could you guys tell me what should I do to solve it. Thanks!!

Answer 1

[James Tursa]

This depends on what the size and class of the matrix returned by build_matrix( ) is. E.g., suppose it returns an MxN double matrix. Then the pre-allocation and the assignments would look like this:

A = zeros(size(P,2)*M,N);  % pre-allocate result
for i = 1:size(P,2)
    A(1+(i-1)*M:i*M,:) = build_matrix(P(:,i));  % modify the way you do the assignment
end