MATLAB: Preallocating a Matrix(For loop, Vectorization)

preallocation

I hear that preallocating a matrix before the for loop is efficeint , but why do i get the error " The value assigend to the variable might be unused", when used before vectorization ? Example code

%% Vectorization method

Y4=repmat(A,N,1);

Y4 = cell2mat(arrayfun(@(i) A^i, (1:N)', 'Uni', false)

%% Normal Multiplication method

Y5=repmat(A,N,1);

for i=1:N

Y5((i-1)*Dim+1:i*Dim,:)=A^i;

%disp i

end

Best Answer

In these lines:

Y4=repmat(A,N,1);
Y4 = cell2mat(arrayfun(@(i) A^i, (1:N)', 'Uni', false)

The first line assigns something to Y4. The second line wipes out what you just assigned to Y4 in the first line and replaces it with something else from the second line. In other words, the first line is useless and the value assigned to Y4 in the first line is unused.

Related Solutions

MATLAB: How do i do Matrix reordering (cutting into blocks)

It's not exactly clear what the actual size of your matrix is (maybe you could clarify), but for what you have posted maybe this is what you want:

result = [A(:,1:4);A(:,5:8)];

EDIT:

Based on your comment below, it would be simpler to reshape the cell array first before you use cell2mat. E.g.,

A = [1 1 0 0 ; 1 1 1 0 ; 0 1 2 0; 1 0 0 1];
Q = cell2mat(arrayfun(@(i) A^i, (1:3)', 'Uni', false)); % transpose the 1:3

MATLAB: Concatenate arrays within a loop,

Easy, given any number of arrays in a cell array C. You could use a loop like this, although it is inefficient because the output array is not preallocated:

C = {[1,2],[3,4],...} % cell array of arrays.
A = C{1};
for k = 2:numel(C)
    A = [A;C{k}];
end

Simpler and more efficient would be to concatenate them all at once, without a loop, e.g.:

A = vertcat(C{:})

Whatever you do, do NOT try to access variable names dynamically in a loop! Read this to know why:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Using an array (e.g. a cell array) and indexing is much simpler, neater, much more efficient, less buggy, and easier to debug.

Best Answer

Related Solutions

MATLAB: How do i do Matrix reordering (cutting into blocks)

MATLAB: Concatenate arrays within a loop,

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