Accessing multidimensional array with pairwise indices
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Hi everybody
Probably a stupid question but I can't get my head around it. Suppose A is a matrix with dimensions (20,20,1000). I have two vectors x and y with pairwise indices (lets say 5x1). Is there an easy & fast (vectorized) way to extract A(x(1),y(1),:), A(x(2),y(2),:) ... A(x(5),y(5),:)? The only solution I came up with would be sub2ind for the first two dimensions and repmat to expand a binary mask of the indices to the third dimension but that's probably not ideal performance- and memory-wise?
Any ideas?
Thanks in advance
2 Kommentare
James Tursa
am 27 Mär. 2015
How do you want the extracted data (a bunch of 1000 element "vectors") arranged in the result variable?
Antworten (5)
Jon
am 30 Mär. 2015
I think your for loop is only getting data off the top page of the array. When I run your code, B is [ni 1], but all the others results are [ni 2000]. You need to change the last index in A from '1' to ':'. You can also speed up your loop by initializing B before the loop.
B=zeros(numel(ix),size(A,3));
for i=1:numel(ix)
B(i,:)=A(ix(i),iy(i),:);
end
Also, I found the error in my snippet. The third line should be
ind2=bsxfun(@plus,ind,0:numel(A1):numel(A)-numel(A1));
With these changes, I get the following results for ni=1000.
forloop =0.074519
arrayfun=0.086363
bsxfun1 =0.040455
bsxfun2 =0.077431
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dpb
am 27 Mär. 2015
One solution; not sure if it's the fastest or not...returns an array of columns length as number of elements i the index arrays by the depth of the planes in the 3D array. Miniature example...
>> x=rand(3,3,2) % small data array
x(:,:,1) =
0.1524 0.9961 0.1067
0.8258 0.0782 0.9619
0.5383 0.4427 0.0046
x(:,:,2) =
0.7749 0.0844 0.8001
0.8173 0.3998 0.4314
0.8687 0.2599 0.9106
>> ix=[3 1];iy=[2 2]; % x,y coordinates to select
>> squeeze(cell2mat(arrayfun(@(i,j) x(i,j,:),ix,iy,'uniformoutput',0)))
ans =
0.4427 0.2599
0.9961 0.0844
>>
0 Kommentare
Jon
am 27 Mär. 2015
I think sub2ind is the recommended approach in most cases. If you want to avoid repmat, you can do something like this.
>> A=rand(3,3,2)
A(:,:,1) =
0.35166 0.54972 0.7572
0.83083 0.91719 0.75373
0.58526 0.28584 0.38045
A(:,:,2) =
0.56782 0.5308 0.12991
0.075854 0.77917 0.56882
0.05395 0.93401 0.46939
>> ix=[3 1];iy=[2 2];
% Pull out the top page of the array for sizing
>> A1=A(:,:,1)
% get the indices of the rows/columns you want
>> ind=sub2ind(size(A1),ix,iy));
% extend the indices to cover all pages of the 3D array
>> ind2=bsxfun(@sum,ind',0:numel(A1):numel(A)-numel(A1));
% pull out the elements of the array you want
>> B=A(ind2)
B =
0.28584 0.93401
0.54972 0.5308
You can even put everything in one line if you want.
0 Kommentare
Roger Stafford
am 28 Mär. 2015
If, as you have indicated, x and y are column vectors, do this:
B = reshape(A(bsxfun(@plus,(x+20*(y-1)).',(20*20)*(0:999).')),1,[]);
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