How to do matrix Preallocation?

I have a matrix called 'ResultMtx'. its size is

size(ResultMtx)
ans =
   792     5

I try to do Preallocation of the matrix by writing:

   ResultMtx = zeros(792,5);  

Then later in my program I calculate values for the matrix and define it like this:

    ResultMtx = [ResultMtx; m, o, r, t, Diff_irr];
                end
                 ResultMtx = [ResultMtx;  nan nan nan nan nan];

But then the result of my matrix is all zeros. what am i doing wrong?

Answer 1

[Stephen23]

"what am i doing wrong?"

You are concatenating the new data onto the bottom of your preallocated matrix, rather than using indexing to allocate that data to the matrix. You need to use indexing, for example where k is the loop iteration:

ResultMtx(k,:) = [m,o,r,t,Diff_irr];
%        ^^^^^ indexing!

or

ResultMtx(k,:) = NaN;
%        ^^^^^ indexing!

This approach is shown in the documentation:

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html