PCA values

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Raviteja
Raviteja on 17 Sep 2011
Edited: Mouse on 8 Nov 2021 at 21:26
I have written following code to learn about PCA.
clear all
clc
load hald
hald=ingredients;
[m n]=size(hald);
mu=mean(hald);
mumat=repmat(mu,[m 1]);
hald_new=hald-mumat;
cov_mat=cov(hald_new);
[EVec EVal]=eig(cov_mat);
[sq iq]=sort(diag(EVal)','descend');
for i=1:length(iq)
EVec_sort(:,i)=EVec(:,iq(i));
end
EVal_sort=sq;
pca_data=EVec_sort'*hald_new';
And also I compared my results with matlab princomp() function. Results are like this..
My result
>> pca_data'
ans =
36.8218 6.8709 -4.5909 0.3967
29.6073 -4.6109 -2.2476 -0.3958
-12.9818 4.2049 0.9022 -1.1261
23.7147 6.6341 1.8547 -0.3786
-0.5532 4.4617 -6.0874 0.1424
-10.8125 3.6466 0.9130 -0.1350
-32.5882 -8.9798 -1.6063 0.0818
22.6064 -10.7259 3.2365 0.3243
-9.2626 -8.9854 -0.0169 -0.5437
-3.2840 14.1573 7.0465 0.3405
9.2200 -12.3861 3.4283 0.4352
-25.5849 2.7817 -0.3867 0.4468
-26.9032 2.9310 -2.4455 0.4116
Matlab result:
>> [pc,score,latent,tsquare] = princomp(ingredients);
>> score
score =
36.8218 -6.8709 -4.5909 0.3967
29.6073 4.6109 -2.2476 -0.3958
-12.9818 -4.2049 0.9022 -1.1261
23.7147 -6.6341 1.8547 -0.3786
-0.5532 -4.4617 -6.0874 0.1424
-10.8125 -3.6466 0.9130 -0.1350
-32.5882 8.9798 -1.6063 0.0818
22.6064 10.7259 3.2365 0.3243
-9.2626 8.9854 -0.0169 -0.5437
-3.2840 -14.1573 7.0465 0.3405
9.2200 12.3861 3.4283 0.4352
-25.5849 -2.7817 -0.3867 0.4468
-26.9032 -2.9310 -2.4455 0.4116
Why the second column giving wrong result (in signs)?
  1 Comment
Debarshi Chakraborty
Debarshi Chakraborty on 21 Jan 2020
how to plot this data?

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Accepted Answer

Andrew Newell
Andrew Newell on 17 Sep 2011
Eigenvectors are a little arbitrary. If A is a matrix and b is one of its eigenvectors, then so is b multiplied by any scalar. In particular, if
A*b = e*b
for some eigenvalue e, then
A*(-b) = e*(-b).
  3 Comments
Mouse
Mouse on 8 Nov 2021 at 21:22
Is "princomp" removed in Version R2021a?
I would say yes, it is removed and replaced by "pca"

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