[Gretl-users] matrix indexes question after sort

Hi all, greetings from Paris ! I am still developping my spatial tools under gretl, but I have a difficulty with some matrix functions. My code is an adaptation of the "brute force" find_neighbor of James LeSage. Let me explain, I am given a list of coordinates for my data (spatial X and Y location coordinates). The function is to find the k nearest neighbors of each of my N data points. The function computes for each of them a distance to all the others, then i call a sort command to order the distances from smallest to largest to finally extract the k smallest distances i am interested in (starting from line 2 to k+1 because the lowest distance is zero, ie: the shortest distance to data i is itself !). Well, my problem is that I do not see how to grab not the values of sorted distances but the indexes of data points. Here is the code : matrix xc = X_COORD matrix yc = Y_COORD scalar n = rows(xc) ### i fix the number of near neighbors to 4, a typical value (rook neighborhood) scalar m = 4 ### I create a list of indexes of each of the 4 nearest neighbors set to zero matrix nnlist = zeros(n,m) ### I added this dirty trick to keep track of the indexes I want to sort with ### the distances: matrix idx = transp(seq(1,n)) ### the main (brute force) loop loop i = 1..n --quiet matrix xi = xc[$i,1] matrix yi = yc[$i,1] matrix dist = sqrt((xc-xi*ones(n,1)).^2 + (yc - yi*ones(n,1)).^2)~idx matrix xind = sort(dist[,1]) matrix nnlist[i,1:m] = transp(xind[2:m+1,1]) end loop ### I display the results to see the results dist xind nnlist As you can see, I compute a matrix dist holding distances concatenated with indexes. Then i sort the distances and store them into the matrix xind. The problem is that, I am not interested by the distances, but the indexes of the units but xind only returns a vector of sorted distances and not the indexes. In fact i would be interested in just extracting the values of indexes in the sorted dist matrix. I do not understand how to use the command sort for matrices. any advice ? thanx & cheers Franck PS: a text file with my (X,Y) data -- Franck Nadaud Economiste CIRED UMR 8568 CNRS - EHESS 45 bis avenue de la Belle Gabrielle 94736 Nogent-sur-Marne Cedex TEL 33-1-43-94-73-94 FAX: 33-1-43-94-73-70 MOB: 06-07-39-92-75 France