Sampling a large matrix using column indices

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Jacob Moses
Jacob Moses 2012-12-26
I am trying to sample a large(17x16602) matrix for use as a kernel for a kernel kmeans algorithm. Because the matrix is very large(and full), I decided sample it randomly. I now have a list of 1000 column indices which I want to use to extract data from my matrix. I have tried all of the simple indexing techniques, but all I am getting are the values within the first row that I want. What I have so far is:
L=length(x);
IDX_RAND=randperm(L);
IDX_SAMP=IDX_RAND(1:1000);
What I want is to use the column values in IDX_SAMP to extract a 17x1000 matrix for use in my kernel. The desired result would return the column value in IDX_SAMP and preserve the trailing 16 rows in that column. Any help or clarification requests are appreciated. Thanks!

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Walter Roberson
Walter Roberson 2012-12-26
If you are looking for the value of the same columns for each of the rows, then
x(:, IDX_SAMP)
Note: I advise you to use L = size(x,2) instead of L = length(x) as length() does not always choose the same dimension.
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Jacob Moses
Jacob Moses 2012-12-26
Thank you, this is exactly what I needed. And I'll keep in mind using size for this project.

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