| Re: Where vs Histogram vs ?? [message #32655 is a reply to message #32526] |
Wed, 23 October 2002 15:31  |
Andrew Cool
Messages: 219
Registered: January 1996
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Senior Member |
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"Pavel A. Romashkin" wrote:
>
> Hi Andrew,
> Sorry for delaying the answer.
> No, no, no. No. It needs to be just what it is. It will be an array of
> just 9 pointers. Each of them points to a vector (well, except for the
> last one which is a matrix), and as such is searchable quite quickly
> using WHERE.
> You may notice that for an array of structures:
>
> a = {a: 0, b: 0.0, c: fltarr(5)}
> a = replicate(a, 1000)
> help, a.(0)
> ;<Expression> INT = Array[1000]
> help, a.(2)
> ;<Expression> FLOAT = Array[5, 1000]
>
> Therefore, when you loop over just *fields* of a structure array, you
> get the contents of the entire array. In your case, this is perfect for
> indexing the data. I use this a lot - it allows to shift arrays
> throughout the entire structure array just as if it were a plain matrix
> or vector, and is just as fast.
> As I said, you can basically do away with the sreucture array becasue
> now your 9-element pointer array contains everything the old structure
> array contained. In fact, yopu can dump the old array do free up some
> RAM, but that is not critical. Also, in a general case, you want only to
> include those fields in the ptr array that you use for searching, and
> then use the resulting index to extract the data from the original
> structure array.
> Regarding memory use:
>
> ; Here, A is an array of structures of exactly your type of size 16 mln.
> ; I have nothing else in the IDL session.
> IDL> help, /mem
> heap memory used: 512482366, max: 512483544, gets: 1719, frees:
> 1167
> IDL> ind = ptrarr(n_tags(a)
> IDL> for i = 0, n_tags(a)-1 do ind[i] = ptr_new(a.(i))
> ; The above takes less than a minute
> IDL> help, /mem
> heap memory used: 1024484012, max: 1024484732, gets: 3656, frees:
> 3093
>
> As expected, the memory use doubles; if that's a problem, discard the
> original array.
>
> Hope this helps.
> Pavel
Hi Pavel,
I doubt that I'd be able to hold both the structure and ptrarr in
memory
at any one time - our VMS SYSMAN1 would have conniptions if I asked to
increase my user quotas anymore - as it is I totally hog one Alpha
server
when this code runs...
But you seem pretty sure of your onions on this. I'll give it a whirl
and
to see if Pavel > Stein Vidar!
Thanks,
Andrew
------------------------------------------------------------ -----------------
Andrew D. Cool .->-.
Electromagnetics & Propagation Group '-<-'
Intelligence, Surveillance & Reconnaissance Division Transmitted on
Defence Science & Technology Organisation 100% recycled
PO Box 1500, Edinburgh electrons
South Australia 5111
Phone : 061 8 8259 5740 Fax : 061 8 8259 6673
Email : andrew.cool@dsto.defence.gov.au
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