| Re: Shortcut needed [message #10356] |
Mon, 17 November 1997 00:00  |
Martin Schultz
Messages: 515
Registered: August 1997
|
Senior Member |
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Daniel Williams wrote:
>
> Hi fellow IDL-users,
>
> I have a program which runs slower than I like, and wonder if someone
> could help me speed it up a bit.
>
> My data is an array of structures, call it 'a'. One element of the
> structure, call it 'n' is an integer, another is a quantity which I
> want to add up, call it 'x'.
>
> There are about 100000 elements in this array. The integer n, marks
> data as being of a certain type. I want to make a new array, call it
> 'y', which has as its nth element the sum of all the x's which have a
> n-value of n. I currently do this as follows,
>
> for n=0, n_max do begin
> y[n] = total( a[where(a.n eq n)].x )
> endfor
>
> The trouble is that the loop is slow, as for loops always are. Does
> anyone out there know a better way to do this kind of sorting?
>
> Thanks,
> Daniel Williams
> --
> +---------------------------------------------------------+
> | Daniel L. Williams | Email: williams@srl.caltech.edu |
> | Space Radiation Lab | Tel: 626/395-6634 |
> | Caltech 220-47 | Fax: 626/449-8676 |
> | Pasadena, Ca 91125 | |
> | WWW: www.srl.caltech.edu/personnel/williams.html |
> +---------------------------------------------------------+
another option may be to sort the data and use the uniq function:
a=a(sort(a.n))
ua=uniq(a.n)
This would give you an array of indices of the first occurence of
all n values and you could then loop like
for i=0,n_elements(ua)-2 do tot(i)=total(a.value(ua(i):ua(i+1)))
Haven't tried it, but should work - don't know how fast though.
Martin.
--
------------------------------------------------------------ -------
Dr. Martin Schultz
Department for Earth&Planetary Sciences, Harvard University
186 Pierce Hall, 29 Oxford St., Cambridge, MA-02138, USA
phone: (617)-496-8318
fax : (617)-495-4551
e-mail: mgs@io.harvard.edu
IDL-homepage: http://www-as.harvard.edu/people/staff/mgs/idl/
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