MATLAB: What does the statement Ytraining = Y(:,1:230e3); mean? here, matrix Y is a matrix of dimensions 256×119025

dictionary learningdigital image processing

I am going through a couple of codes related to dictionary learning by patch extraction and couldnt understand the following line:
Ytraining = Y(:,1:230e3);
Code:
Im = double(imread('temp1.jpg'));
Y = getPatches2D( Im , p , []);
Y =  reshape(Y,[p^2,size(Y,3)]);
Y = bsxfun(@minus, Y, mean(Y));
Ytraining = Y(:,1:230e3);
Ytest = Y(:,230e3+1:233e3);
clear Y

Best Answer

"Y is a matrix of dimensions 256x119025"
Well, it may have been when it was computed in
Y = getPatches2D( Im , p , []);
but it was immediately turned into something else by
Y =  reshape(Y,[p^2,size(Y,3)]);
that is same number of points but p^2 rows and however many columns that turns out to be (has to be same as the computed size, size(Y,3), but I'd've written it as reshape(Y,p^2,[]) instead. That aside, the line you're wondering about simply takes the subset area of all p^2 rows, columns 1:230000 and assigns it to Ytraining.
Ytraining = Y(:,1:230e3);
Why that specific magic number would have to do with whatever the image was one must presume.
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