gather data from different sections of each column

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Christopher
Christopher 2015-11-18
评论： Christopher 2015-11-23
Consider the code:
totmods = 10000;
numselect = 1000;
A = rand(100,totmods); % numbers
B = randi(90,10,1000); % start row index
C = randi(totmods,numselect,1); % column index
D = A(B:B+9,C);
The resulting matrix D should be the 10x1000 matrix wherein each column is a selection of 10x1 data starting from the index B for each column in A. However, the code does not work correctly. The code uses only B(:,1) as the row index for all the elements gathered from A in the line "D = A(B:B+9,C);"
How can this be corrected without using a loop?
  1 个评论
Guillaume
Guillaume 2015-11-18
编辑：Guillaume 2015-11-18
The code uses only B(:,1) is slightly incorrect. The code actually only uses B(1, 1) because that's the documented behaviour of the colon operator:
If you specify nonscalar arrays, MATLAB interprets j:i:k as j(1):i(1):k(1)
I have no idea which values of B (considering it is 10x1000 matrix) you're trying to use for each row of your D output.

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采纳的回答

Thorsten
Thorsten 2015-11-18
You only need numelselect values of rows:
B = randi(90,numselect,1);
Then use
for i = 1:numel(B), D(:,i) = A(B(i):B(i)+9, C(i)); end
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Thorsten
Thorsten 2015-11-18
编辑：Thorsten 2015-11-18
Try
D = A(bsxfun(@plus, bsxfun(@plus, B, 0:9), (C-1)*size(A,1))');
This computes the linear indices into A based on B and C.

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更多回答（2 个）

Guillaume
Guillaume 2015-11-18
totmods = 10000;
numselect = 1000;
A = reshape(1:100*totmods, 100, totmods);
B = randi(size(A, 1) - 9, numselect, 1); %column vector
C = randi(totmods, 1, numselect); %row vector to make it easier later
rowmatrix = bsxfun(@plus, B, 0:9)'; %matrix of row indices
D = A(sub2ind(size(A), rowmatrix, repmat(C, 10, 1)))
Stephen23
Stephen23 2015-11-18
编辑：Stephen23 2015-11-18
Preallocate the output before allocating to it:
http://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html
http://blogs.mathworks.com/loren/2012/11/29/understanding-array-preallocation/
As numselect gets larger this will have a very significant effect on the calculation speed. Try something this:
B = randi(90,numselect,1);
D = nan(10,numel(B));
for k = 1:numel(B)
D(:,k) = A(B(k):B(k)+9, C(k));
end

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