Loop Principal Component Analysis

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stefan strasser
stefan strasser on 21 Jun 2012
Hello,
I would like to do a principal component analysis. The matrix is of 50x50 dimension. I don't want Matlab to run the PCA on the whole 50x50 Matrix but perform it from rows 1-10 then from 11-20 and so forth up until row 50. I tried around using loops. However, I did not arrive at my desired result which is a matrix that consists of 5 rows and 50 colums. They first row would contain the coefficients of the first component retrieved from the 1-10 pca, the second row would contain the first components for the 11-20 pca and so on…. Any chance somebody could give me a hint? Thank you very much Stefan

Answers (2)

Kevin Holst
Kevin Holst on 21 Jun 2012
I'd suggest looking at the help page a little more for PCA:
"COEFF = princomp(X) performs principal components analysis (PCA) on the n-by-p data matrix X, and returns the principal component coefficients, also known as loadings. Rows of X correspond to observations, columns to variables. COEFF is a p-by-p matrix, each column containing coefficients for one principal component. The columns are in order of decreasing component variance."
EDIT
by first component of the 1-10 analysis, do you mean the first row of the coefficients? If so, I think this will work for you:
for i = 1:size(x,1)/10 % this assumes the number of rows is ALWAYS a multiple of 10
[COEFFnew,SCOREnew,latentnew] = princomp(x((i-1)*10+1,i*10)); % if you don't need SCOREnew or latentnew, you can just leave those off
coeff(i,:) = COEFFnew(1,:);
end
coeff should be a 5x50 for your example of a 50x50 matrix, but the same code will also expand to a 1000x1000
  2 Comments
stefan strasser
stefan strasser on 21 Jun 2012
%pca looped
this is what i had in mind as a loop…BUT: how to proceed so that i receive the desired matrix?
for t=11:10:50
[COEFFnew,SCOREnew,latentnew] = princomp(x(((t-10):(t-1)),:))
end

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George McFadden
George McFadden on 17 Nov 2012
The easiest way would be to divide your matrix in to 5 matrices, each 10x50. Then do a for loop that calls each matrix.
Example:
%variable =50x50
v{1}=variable(1:10,:);v{2}=variable(11:20,:); %etc.
%then perform loop
for i=1:length(v)
[coeff,score,latent,explained]=pca(v{i});
end
  1 Comment
George McFadden
George McFadden on 17 Nov 2012
Oops! You would have to assign the pca results to a new variable for each iteration. So...
Example: %variable =50x50 v{1}=variable(1:10,:);v{2}=variable(11:20,:); %etc. %then perform loop for i=1:length(v) [coeff{i},score{i},latent{i},explained{i}]=pca(v{i}); end

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