Building a matrix in a faster way

Question

Azza 2012-10-16

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https://ww2.mathworks.cn/matlabcentral/answers/50994-building-a-matrix-in-a-faster-way

Hi,

I am trying to build a matrix by giving each array in the matrix the same value in its first column. The value is [0;0;1]. My code look something like this:

yv = 1:-1:-1;
xv = -1:1:1;
for Y = 1:length(yv)
    for X = 1:length(xv)
          M(:,1,X,Y) =  [0;0;1]; 
      end
end

I was wondering if there is more efficient way to give the arrays for length (yv) and (xv) the value [0;0;1] instantly without using the for loop. My matrix in original is much larger than this and I need to make the code as faster to execute the data as possible.

Highly appreciate any help with this.

Best wishes

AA

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Answer 1

Matt J 2012-10-16

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https://ww2.mathworks.cn/matlabcentral/answers/50994-building-a-matrix-in-a-faster-way#answer_62129

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 d=[0;0;1];
 M=d(:,1,ones(1,length(xv)),    ones(1,length(yv)))

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Walter Roberson 2012-10-16

Which can also be written as

M = repmat(d, [1, 1, length(xv), length(yv)]);

Matt J 2012-10-16

Yes, although repmat does use mcode containing loops, and therefore can be slow.

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Answer 2

Azza 2012-10-17

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https://ww2.mathworks.cn/matlabcentral/answers/50994-building-a-matrix-in-a-faster-way#answer_62320

在 MATLAB Online 中打开

Many thanks Matt and Walter for your help. Both your answers are very valuable. The thing with my code is that I need to keep a counter in each line. For example the code with the for loop should look something like this:

for Y = 1:length(yv)

    for X = 1:length(xv)
        counter = 1;
        M(:,counter,X,Y)= [0;0;1];
        counter = counter;
       M(:,counter,X,Y) = A*Rflip*M(:,1,X,Y)+B;       
    end
end

Thus, the counter should change from value 1 to 2 accordingly.

I have also replicated the matrix for A, Rflip and B in order to accomodate the M value for the length of arrays of (xv) and (yv) similar to your methods. The original sizes of matrices A and Rflip were 3*3 for each element. So I managed to replicate the matrix to [3 3 3 3] for (xy) and (xv) While for B was 3*1 and I made it into [3 1 3 3]. When I tried to execute the line with the replicated matrices for A, Rflip and B {while excluding the counter} I got this error message:

??? Error using ==> mtimes Input arguments must be 2-D.

So would you kindly help me in giving the length of arrays for (xv) and (xy) the same value of M without using the lengthy for loop method while including the counter?

Best wishes

AA