# Preallocating a Matrix(For loop, Vectorization)?

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ManvsMachine 2019 年 5 月 9 日
Commented: ManvsMachine 2019 年 5 月 9 日
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 件のコメント

Stephen Cobeldick 2019 年 5 月 9 日
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).

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### 採用された回答

James Tursa 2019 年 5 月 9 日

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.

#### 3 件のコメント

ManvsMachine 2019 年 5 月 9 日
i uderstand thankyou ,could i make that 2line of code more efficeint?
Stephen Cobeldick 2019 年 5 月 9 日
",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 年 5 月 9 日
got it,appreciate that.thanks

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