[Gretl-devel] the choose() function

Folks, while revising some old code I stumbled into a function that I'd written a while back, and I realised this could go into the extra package. Note: for large input vectors (n > 5) the recursion approach really is much slower than direct calculation, so there is quite a lot of room for optimisation. Opinions? <hansl> function matrix choose(const matrix from, scalar h) # If "from" is a vector with n elements, this function # returns a matrix whose rows are all the possible subsets # with h elements; a recursive algorithm is used. # # For example: choose({1,2,3}, 2) returns # # 1 2 # 1 3 # 2 3 # # warning: the returned matrix will have n! / (h! * (n-h)!), # rows, so handle with care. if (rows(from) > 1 && cols(from) > 1) s = argname(from) printf "%s is not a vector\n", strlen(s) ? s : "argument 1" return {NA} endif if h <= 0 ret = {} elif h == 1 matrix ret = vec(from) else ret = {} n = nelem(from) loop i = 1 .. (n - h + 1) --quiet x = from[i] matrix c = from[i+1: ] if rows(c) > 0 matrix ret |= x ~ choose(c, h-1) endif endloop endif return ret end function </hansl> ------------------------------------------------------- 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 -------------------------------------------------------