Matrix Indexing from an Array of Numbers

Question

Lauren Hopkins 2016-2-26

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评论： Lauren Hopkins 2016-2-26

Hello:

I have a matrix Y = 55x236 of values and a separate matrix Z = [24 38 46 52 66] I would like to extract all of the rows of Y but ONLY the columns starting at the values in Z (plus the next 4 columns). So in other words I would like to extract from Y, columns 24-28, 38-42, 46-50, 52-56, and 66-70.

So far I have been doing this with a for loop but this seems like something that could be done much easier. I have tried something like A = Y(:, (Z:Z+4)) but this only gives me columns 24-28. If I have to index into Z to do this, wouldn't that still require a loop?

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Answer 1

Roger Stafford 2016-2-26

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https://ww2.mathworks.cn/matlabcentral/answers/270089-matrix-indexing-from-an-array-of-numbers#answer_211279

编辑：Roger Stafford 2016-2-26

在 MATLAB Online 中打开

Assuming Z is a row vector

Y2 = Y(:,bsxfun(@plus,Z,(0:4).'));

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Lauren Hopkins 2016-2-26

Perfectly elegant; thanks. Also introduced me to bsxfun which I'm definitely going to use a lot in the future. Cheers!

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Answer 2

dpb 2016-2-26

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在 MATLAB Online 中打开

One soluion; may be some "more better"...

idx=cell2mat(arrayfun(@colon,Z,Z+4,'uniform',0)));  % column indices vector
A=Y(:,idx);

On can dispense with the temporary idx of course by folding into the indexing expression...

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Lauren Hopkins 2016-2-26

This in-and-of itself is very interesting as I wasn't sure there was a 'one-line' piece of code that could extend the indices into ranges. And it certainly does the job. Thank you for your response!

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