[Gretl-users] matrix indexes question after sort

Thu Apr 24 11:51:48 EDT 2008

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

-------------- next part --------------
CODAMC	X_COORD	Y_COORD
11 0020	-63	-11
12 0020	-73	-9
12 0030	-71	-9
12 0040	-69	-10
12 0060	-72	-9
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13 0240	-66	-8
13 0260	-60	-4
13 0270	-62	-7
13 0280	-65	-3
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13 0300	-58	-1
13 0310	-59	-4
13 0340	-57	-3
13 0350	-68	-8
13 0360	-66	0
13 0380	-68	0
13 0390	-69	-4
13 0420	-66	-4
14 0010	-61	3
15 0010	-49	-2
15 0020	-49	-2
15 0030	-51	0
15 0040	-55	0
15 0050	-54	0
15 0070	-50	-1
15 0080	-48	-1
15 0090	-46	-1
15 0100	-56	-4
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15 0510	-56	0
15 0520	-50	-2
15 0530	-57	1
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15 0550	-48	-3
15 0560	-47	-1
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