Basic Vectorization Question (speed required)
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Matlab2010
le 22 Nov 2013
Commenté : Matlab2010
le 22 Nov 2013
I have a 2D matrix of data. I have an index to reduce that 2D matrix to a 1D matrix.
How can I do it without using a for loop? Ie by vectorization as it needs to be super quick.
thanks
Z = rand(5,100); %matrix of data
IDY = randi([1 5], [1,100]); %index into that matrix
X = NaN(size(Z,2),1); %pre allocate
X = Z(IDY,:); %this fails as it does not generate a 1D matrix (vector) as required
this is how I can do it using a loop
X2 = NaN(size(Z,2),1);
for i = 1: size(X,1)
X2(i) = Z(IDY(i),i);
end
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Azzi Abdelmalek
le 22 Nov 2013
Z = rand(5,100); %matrix of data
IDY = randi([1 5], [1,100]); %index into that matrix
X = NaN(size(Z,2),1); %pre allocate
X = Z(IDY,:)
X3=Z(sub2ind(size(Z),IDY,1:size(Z,2)))
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Image Analyst
le 22 Nov 2013
To get all of Z in row-major order, do:
X = Z(:);
To get random ordering, like your code attempted to do, try this:
Z = rand(5,100) %matrix of data
randomIndexes = randperm(numel(Z)) %index into that matrix
X = Z(randomIndexes)
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