pre-allocation in loop

Hi all,

MATLAB is suggesting me to pre-allocate MATRICI_SIMULATE_nonan and colonna_i in the following loop:

A_nan = A~=0;
    
MATRICI_SIMULATE_nonan={};
    
for i = 1:N_RIPETIZIONI
     for j = 1:size(colonna_i,2)
         MATRICI_SIMULATE_nonan{i,j}=MATRICI_SIMULATE{i,j}.*A_nan;
         MATRICI_SIMULATE_nonan{i,j}(MATRICI_SIMULATE_nonan{i,j} == 0) = NaN;
      end
 
end

However I think I have already done it at least with MATRICI_SIMULATE_nonan. Am I missing something? Maybe I should use cell()?

Answer 1

[Sabin]

To preallocate memory for a cell array please check this doc page:

https://mathworks.com/help/matlab/matlab_prog/preallocate-memory-for-a-cell-array.html

In your case you can do:

MATRICI_SIMULATE_nonan{N_RIPETIZIONI,size(colonna_i,2)}={};

[Walter Roberson]

MATRICI_SIMULATE_nonan(N_RIPETIZIONI,size(colonna_i,2)) = {};

perhaps? Or

[MATRICI_SIMULATE_nonan{N_RIPETIZIONI,size(colonna_i,2)}] = deal({});

[Stephen23]

This answer is fragile, inconsistent, and gives no explanation why it assigns an empty cell array in the final cell.

Lets have a look at the content of that preallocated cell array, by trying the author's code:

N_RIPETIZIONI = 3;
colonna_i = rand(1,2);
MATRICI_SIMULATE_nonan{N_RIPETIZIONI,size(colonna_i,2)}={}

MATRICI_SIMULATE_nonan = 3×2 cell array
    {0×0 double}    {0×0 double}
    {0×0 double}    {0×0 double}
    {0×0 double}    {0×0 cell  }

Why should the last cell contain a 0x0 cell array? This is very odd, superfluous, and in some situations this inconsistency might cause problems. Best avoided.

Answer 2

[Stephen23]

"However I think I have already done it at least with MATRICI_SIMULATE_nonan."

There is nothing like preallocation in your code.

"Am I missing something?"

Preallocation requires creating an array of the final size, which your code does not do.

"Maybe I should use cell()?"

Yes, that would be the best way. For example, something like:

MATRICI_SIMULATE_nonan = cell(N_RIPETIZIONI,size(colonna_i,2));

Your code would be clearer with a temporary variable, e.g.:

A_nan = A~=0;
MATRICI_SIMULATE_nonan = cell(N_RIPETIZIONI,size(colonna_i,2)); % preallocate!
for ii = 1:N_RIPETIZIONI
    for jj = 1:size(colonna_i,2)
        tmp = MATRICI_SIMULATE{ii,jj}.*A_nan;
        tmp(tmp==0) = NaN;
        MATRICI_SIMULATE_nonan{ii,jj} = tmp;
    end
end