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IanWakeling
Barite | Level 11

I developed my idea above into a small helper function that others might find useful.

 

proc iml;
  /* The TRI_IDX function returns the index numbers of the triangular elements in a
     square matrix of side n.

       The parameter UPPER is a Boolean that gives the upper triangle when true,
       the lower triangle when false. 

       The parameter MAINDIAG is a Boolean that indicates whether or not the
       elements from the main diagonal should be included.
  */
  start tri_idx(n, upper, maindiag);
	idx = cusum( vech( diag((1-maindiag):(n-1)) + 1));
	if upper then return(idx); else return((n#n+1) - idx[nrow(idx):1] );
  finish;

  /* demonstrate function for a 4x4 matrix */
  print (shape(1:16,4)) [format=2.0];
  print (tri_idx(4,0,0)) [l='lower triangle no diagonal'];
  print (tri_idx(4,0,1)) [l='lower triangle including diagonal'];
  print (tri_idx(4,1,0)) [l='upper triangle no diagonal'];
  print (tri_idx(4,1,1)) [l='upper triangle including diagonal'];
quit;
Rick_SAS
SAS Super FREQ

For additional thoughts on @IanWakeling's function, see "Extract the lower triangular elements of a matrix", which uses the built-in VECH function when the diagonal elements are included.

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