Preallocating a Matrix(For loop, Vectorization)?
显示 更早的评论
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 个评论
Stephen23
2019-5-9
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).
采纳的回答
James Tursa
2019-5-9
编辑：James Tursa
2019-5-9
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.
更多回答（0 个）
类别
在 帮助中心 和 File Exchange 中查找有关 Matrix Indexing 的更多信息
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!
Translated by 
