[Gretl-users] new function proposal

From time to time, when writing hansl code, I find myself having to select parts of a matrix based on some logical condition. In many cases, selifr/selifc do the job very well, but it's often useful to keep the indices of the relevant rows/columns. In this cases, it'd be nice to have a function vaguely like R's which(), possibly adapted to the matrix case. For example: <hansl> set verbose off set seed 1234 function matrix which(const matrix cond) r = rows(cond) c = cols(cond) scalar n = r*c ret = selifr(seq(1,n)', vec(cond)) if min(r, c) > 1 # matrix case: return a 2-column matrix # with row and columns of the elements # that satisfy the condition rcoord = 1 + ((ret-1)%r) ccoord = ceil(ret/r) ret = rcoord ~ ccoord endif return ret end function # vector case, return a vector x = mnormal(2048, 1) outliers = which(abs(x) .> 3) print outliers eval x[outliers] # matrix case, return a 2-column matrix A = muniform(5,3) small = which(A .< 0.2) print A small </hansl> returns <output> outliers (6 x 1) 506 553 565 1036 1181 1710 3.0376 3.6543 -4.1870 3.3249 -3.1812 3.1397 A (5 x 3) 0.66637 0.022258 0.35193 0.69651 0.57633 0.11178 0.96863 0.71386 0.96540 0.81544 0.62609 0.71537 0.14030 0.46443 0.63596 small (3 x 2) 5 1 1 2 2 3 </output> Of course, as the example above shows, it's quite easy to code this in hansl, but perhaps this is one of those cases when we want to have this done in C for maximum efficiency. Thoughts? ------------------------------------------------------- Riccardo (Jack) Lucchetti Dipartimento di Scienze Economiche e Sociali (DiSES) Università Politecnica delle Marche (formerly known as Università di Ancona) r.lucchetti(a)univpm.it http://www2.econ.univpm.it/servizi/hpp/lucchetti -------------------------------------------------------