indexing nearest number problem

2011 6 11
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Hi all, I'm hoping for some help on this problem.
I have a matrix A 384x320. every point represents a depth. I also have a vector b 1x50, which is depths from 0-500.
I want to find the index of b, that corresponds to the nearest depth within matrix A.
For example: A[1,1] = 256 b [1:10:500]
So the corresponding nearest index on b that is closest to 256 of A, would be index 26 (or b 251).
I've tried a few loops, but nothing working...this is the closest I can come, but not correct.
for i=1:384 for j=1:320 x=nearest(b<=A(i,j)); end end
Thank you in advance for your time! Corinne

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Jan
Jan 2011-6-11

2 个投票

Get the index for a single element of A:
A = rand(384, 320);
b = rand(1, 50);
[absDiff, Index] = min(abs(A(1, 1) - b));
Get the index for all elements at once: EDITED: Get MIN over 2nd dimension - thanks Matt Fig!
[absDiff, Index] = min(abs(bsxfun(@minus, A(:), b)), [], 2);
Index = reshape(Index, size(A));
If b is equidistant the calculation can be made much more efficient by scaling the values of A and a simple ROUND.

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Corinne
Corinne 2011-6-11
Thanks Jan! works great!
I took the first part and ran it in a loop to go through all the elements. I know not the fastest way, but a little easier to understand at first.
When I run through all the elements above, Index only ends up being 1x50...
Matt Fig
Matt Fig 2011-6-11
If I am not mistaken, Jan simply made a dimensional error.
[absDiff, Index] = min(abs(bsxfun(@minus, A(:), b)),[],2);
Index = reshape(Index, size(A));
Corinne
Corinne 2011-6-11
Thanks Matt. That fixed the problem.

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Teja Muppirala
Teja Muppirala 2011-6-12

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For this problem, I suspect HISTC may be a better (faster and more memory efficient) alternative to BSXFUN.
A = rand(384,320);
b = sort(rand(1,50));
tic
[~,Index1] = histc(A,[-Inf interp1(1:numel(b),b,0.5 + (1:numel(b)-1)) Inf]);
toc
tic
[absDiff, Index2] = min(abs(bsxfun(@minus,A(:),b)),[],2);
Index2 = reshape(Index2,size(A));
toc
isequal(Index1,Index2)
For larger data, BSXFUN will give poor performance or run out of memory, like this:
A = rand(1000,1000);
b = sort(rand(1,10000));
tic
[~,Index1] = histc(A,[-Inf interp1(1:numel(b),b,0.5 + (1:numel(b)-1)) Inf]);
toc
tic
[absDiff, Index2] = min(abs(bsxfun(@minus,A(:),b)),[],2);
Index2 = reshape(Index2,size(A));
toc

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Jan
Jan 2011-6-12
What about: interp1(b, 1:numel(b), A, 'nearest');
But unfortunately Matlab's INTERP1 is not implemented efficiently, in opposite to HISTC.

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