Preallocating a Matrix(For loop, Vectorization)?
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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
1 Comment
Stephen23
on 9 May 2019
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).
Accepted Answer
James Tursa
on 9 May 2019
Edited: James Tursa
on 9 May 2019
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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