Preallocating a Matrix(For loop, Vectorization)?

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ManvsMachine 2019-5-9

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评论： ManvsMachine 2019-5-9

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

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Stephen23 2019-5-9

在 MATLAB Online 中打开

Y4 = repmat(A,N,1); % This is NOT preallocation!
Y4 = ....

You are simply replacing one array with another array, which has nothing to do with preallocation. Preallocation requires specifying an entire array in memory before accessing parts of it using indexing (note that you are not using indexing into Y4).

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

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Answer 1

James Tursa 2019-5-9

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https://ww2.mathworks.cn/matlabcentral/answers/461071-preallocating-a-matrix-for-loop-vectorization#answer_374236

编辑：James Tursa 2019-5-9

在 MATLAB Online 中打开

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.

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Stephen23 2019-5-9

编辑：Stephen23 2019-5-9

",could i make that 2line of code more efficeint?"

Not really: both cell2mat and arrayfun are relatively slow. The fastest solution is likely by using an expilcit loop with a preallocated output array.

ManvsMachine 2019-5-9