| Astronomys` Sixth Neighbour Needs Help [message #35884] |
Thu, 24 July 2003 13:40  |
touser2001
Messages: 8
Registered: July 2003
|
Junior Member |
|
|
I have to first say hello to everyone!! I am new to this (and all)
newsgroup(s).
I had a small IDL class and several months of on-off IDL work in
making astronomy-related programs.
My latest program needs some expert help and thus I am writing this
post in hope of a small discussion to enlighten me. I sincerely
apologize if the post is too long!!
Basically, the program reads a 2-column n-line file (n is number of
stars in field, the columns are the position of the stars, Right
Ascencion and Declination which are similar to latitude and
longitude). My objective is to obtain, for each star, the distance to
its 6th neighbour. Simply, Right ascencsion is x and Declination is y
on a xy grid.
What I am doing is:
k=0
while k lt u do begin ; I know the number of stars
(lines) which is equal to "u"
i=0 ;
while i lt u do begin ;
dxki = (x(k) - x(i)) ;this variable is the RA distance between
point i
;and the kth point in the array
dyki = (y(k) - y(i)) ;this variable is the Dec distance between
point i
;and the kth point in the array
dki = sqrt (dxki^2 + dyki^2) ;Now I define the distance, in 2-d
space, between the two objects i and k
D2(k,i) = dki ;This array contains all the distance data for all
the stars
;The reference star is the column number (k)
i = i+1 ;for all the values of i
endwhile ;
k=k+1
endwhile
The above loop calculates the distance from star 1 to 2, 1 to 3 etc
etc and then 2 to 1, 2 to 2. I note that distance 1 to 2 is the same
as 2 to 1 so a first improvement would be to say that once 1 to 2 is
done it need to do 2 to 1 (and this for 3 to 1 etc) ... can I
implement that in the loop? Or any ideas as to improve this distance
calculation?
The loop gives me an array whose dimension is u x u. This is a huge
array when the star file is 10,000 stars... Which brings me to the
second critical stage: calculating the sixth neighbour by first
sorting, in ascending order, the distances in each column.
I did:
n=0
while n lt u do begin
D2n = D2(n,*) ; this is so that I am sorting column by
column
for j=0, n_elements(D2n)-2 do begin ;
for l=j+1, n_elements(D2n)-1 do begin ;
if D2n[l] lt D2n[j] then begin ;This just arranges distances in
increasing order.
temp=D2n[j] ;
D2n[j]=D2n[l] ;
D2n[l]=temp ;So the array that I get is now the array of
distances
endif ;to the nearest neighbours, in order of the neighbour
number!
endfor ;
endfor
D2(n,*)=D2n
n=n+1 ;
endwhile
Now that all columns are sorted I just go to line number six to get
the sixth neighbour of each star!
I really don`t know if this is the best (quickest) method for
organizing a column in ascending order... the second improvement
would be to implement a different sorting algorithm. Note: the
program does this for each of the 10,000 columns...
My problem is that for a field with 10,000 stars the program takes
around 3 days (!) to run and it slows down all computer programs (unix
system) (last time I ran the program it was killed by the system
administrator since it took up too much memory).
I wish the department would hire some professional programmers!! Not
that I don`t like IDL but I wouldn`t mind doing some astronomy too ;-)
Greatly appreciate and and all help!!!!!!!!!!!!
Bruno
PhD Student
University of Florida
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
