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how do i pull all the elements from the lower half of a matrix?

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Contributor
Posts: 23

how do i pull all the elements from the lower half of a matrix?

I'm trying to pull the lower half of a matrix in proc iml. It seems like the VECH function would work perfectly, but I'm running SAS 9.2 and VECH appears to be limited to 9.3. Any suggestions for alternatives? It's all inside a loop and the matrix size changes (with each loop), but these are fairly large matrices (e.g., 3000 X 3000). Thanks!

Frequent Contributor
Posts: 133

Re: how do i pull all the elements from the lower half of a matrix?

This can be recreated using a short function module,  but it is complicated slightly by the fact that the LOC function operates in row major order while the VECH function operates in column major order, and so the need to transpose the matrix.  The code below should do the trick, but I have no idea how efficient or inefficient this might be with your large matrices.

proc iml;

  start vechalf(x);
    a=nrow(x);
    b=a#a-1;
    return(t(x)[loc(mod(0:b,a)>=int((0:b)/a))]);
  finish;

  x=shape(10:25,4,4);
  print x;
  y=vechalf(x);
  print y;

quit;

Contributor
Posts: 23

Re: how do i pull all the elements from the lower half of a matrix?

Actually it appears the symsqr function did what i needed.

thanks,

Rick

Frequent Contributor
Posts: 133

Re: how do i pull all the elements from the lower half of a matrix?

Thanks for the info, I had read this article and come to the conclusion that VECH was something completely new, but it appears that SYMSQR does the same thing, only it reads the lower diagonal elements in row-major order, so the two do give different results.  It would be nice if  the 9.3 documentation for VECH could  be changed to include a link to SYMSQR (and vice versa).

Ian.

SAS Super FREQ
Posts: 3,415

Re: how do i pull all the elements from the lower half of a matrix?

Thanks for the suggestion. The 9.3 documentation can't be changed, but the next release of the documentation will include links between SYMSQR <--> VECH and between SQRSYM <--> SQRVECH.

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