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From |
Hind Sbihi <sbihi@interchange.ubc.ca> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: A Question on Looping |

Date |
Tue, 18 Sep 2007 14:38:00 -0700 (PDT) |

quick refresher : A symmetric matrix is a square matrix that is equal to its transpose. The entries with respect to the main diagonal are symmetric so that a_ij =a_ji where a_ij are the entries of a square matrix A. HTH. Hind Sbihi School of Occupational and Environmental Hygiene University of British Columbia -----Original Message----- > Date: Tue Sep 18 14:22:28 PDT 2007 > From: "Asgar Khademvatani" <akhademv@ucalgary.ca> > Subject: Re: st: A Question on Looping > To: statalist@hsphsun2.harvard.edu > > Hello Nick, > > First of many thanks for your invaluable comments and clues on the loop > codes. On using " matrix symeigen" command you are correct. I had a typo > mistake and i meant what you meant. I need only check with you on the > basic definition of a symmetric matrix. A symmetric matrix ,by > definition, is a quadratic matrix that its off-diagonal element are the > same but its diagonal elements can be equal or not. Is not it? > > Thank you very much again for the help. > > Regards, > Asgar Khademvatani > University of Calgary > n j cox wrote: > > > I see problems on various levels. > > > > 1. You are confusing the syntax of -foreach- and -forval-. Look > > again at the syntax diagrams for these commands. > > > > 2. You are using subscript notation for two purposes, (a) to > > refer to individual observations and (b) to tag different matrices. > > The first is fine; the second is not correct syntax at all. > > Stata does not have any concept of an array of matrices. > > Your -while- loop is falling over because of your syntax > > errors under this heading. > > > > 3. -matrix list- will only list one matrix at a time. > > > > 4. -matrix symeigen- will only play if your matrices are > > symmetric. > > > > This code should be more nearly legal, but it is not tested: > > > > forval i = 1/43 { > > matrix K = (A[`i'], B[`i'] \ B[`i'], A[`i']) > > matrix symeigen X L = K > > di "`i'" > > matrix list L > > } > > > > However, I get the impression that you think -matrix symeigen- > > will oblige you if your matrices are asymmetric. It will not. > > In Stata 8.2, however, you could write your own code to > > extract your eigenvalues as the roots of each characteristic > > equation, which in this case is always a quadratic. In general, > > some eigenvalues may be complex. > > > > Alternatively, Stata 9 will happily oblige, or there are > > other programs that will do this. > > > > Nick > > n.j.cox@durham.ac.uk > > > > Asgar Khademvatani [edited] > > > > I have generated four variables A, B, C, and D that > > each contain 43 observations, in Stata 8.2. I want to make 43 square > > matrices (2 by 2) using each of the observations. Then I want to > > calculate the real eigenvalues of each matrix and list the 43 matrices > > and their eigenvalues. I need to write a loop to do this job. > > > > I have already written the following loop using -foreach- command but > > I am getting the following error as follows; > > > > foreach i = 1/43 { > > matrix K[`i'] = (A[`i'], B[`i'] \ C[`i'], D[`i']) > > matrix symeigen X[`i'] V[`i'] = K[`i'] > > matrix list K[`i'] V[`i'] > > } > > invalid syntax > > r(198); > > > > end of do-file > > r(198); > > > > > > Also, alternatively, I have written another alternative do file > > with a -while- loop but it is also subject to error as follows; > > > > 1.local i 1 > > 2. while `i' < 43 { > > 3. matrix K[`i'] = (lpl_m[`i'], lpm_m[`i'] \ le_m[`i'],lk_m[`i']) > > 4. matrix symeigen X[`i'] V[`i'] = K[`i'] > > 5. matrix list V[`i'] > > 6. local i = `i' + 1 > > 7. } > > too many ')' or ']' > > r(132); > > > > end of do-file > > r(132); > > > > Could you help me out on this matter and say what is wrong with any of > > these alternative loops? > > > > * > > * For searches and help try: > > * http://www.stata.com/support/faqs/res/findit.html > > * http://www.stata.com/support/statalist/faq > > * http://www.ats.ucla.edu/stat/stata/ > > > > > > * > * For searches and help try: > * http://www.stata.com/support/faqs/res/findit.html > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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