| Re: Q: Array of Structures [message #12023] |
Wed, 17 June 1998 00:00  |
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Messages: 31
Registered: April 1998
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korpela@islay.ssl.berkeley.edu (Eric J. Korpela) writes:
One may also use anonymous structures, as Dave did in after 'but
now, I try this'. I concatenate anonymous structures frequently, so
when I saw Dave' assertion that you can't do it, I shook my head in
wonder. However, I found out, only too quickly, that all of you are
right. So, how do I do it? The secret is that at the beginning of
whatever program I'm doing the concatenation I define the anonymous
structure and a blank copy of it. I keep filling the blank copy with
data and then concatenating it to create the larger array of
structures. Like this.
Start program
a={some anonymous structure definition} & b=a
start your loop construct here
do your processing
fill in b structure
if first iteration
output_array_of_structures=b
else
output_array_of_structures = [output_array_of_structures,b]
end loop
end program
No conflicts that I can see.
You could use named structures, with the additional hassle, perhaps
small in your case, of having to exit IDL everytime you make a change
to the structure.
I suspect it has to do with an underlying 'naming' convention for
anonymous structures, (do a help,/structure. See the '** Structure
<long hex number>' The long hex number is the 'type' I believe) so
that deep down in IDLs code there is some comparison of the structure
'types' going on, not comparison of the actual contents of the
structure.
I would point out that if you know how big your output will be, you
should just go ahead and create the array in the beginning of the
processing. Concatenation is a *slow* process. Alternately, you can
create the largest array you think you'll need then enlarge it if your
processing produces more output than expected.
>
> In article <MPG.feb2857db08f77a989681@news.frii.com>,
> David Fanning <davidf@dfanning.com> wrote:
>> But, look. I just tried this:
>>
>> a = { data:FltArr(10), name:'' }
>> b = { data:FltArr(10), name:'' }
>> c = [a, b]
>> % Conflicting data structures: B,concatenation.
>> % Execution halted at: $MAIN$
>>
>> But now, I try this:
>>
>> a = { data:FltArr(10), name:'' }
>> b = a
>> c = [a, b]
>>
>> No problem!
>
> I think that the problems it that the first example creates anonymous
> structures for each variable. Because the structures won't have the
> same tag (anonymous structures are still tagged invisibly), they are
> treated as different structure types. The problem goes away for tagged
> structures. The following works just fine.
>
> a = {StruTag, data:FltArr(10), name:'' }
> b = {StruTag, data:FltArr(10), name:'' }
> c = [a, b]
>
> In the second example, the assignment gives b the same structure tag
> as a, so the concatenation works just fine.
>
> Personally, I'd like to see IDL check for identical anonymous structures
> and set the structure tags appropriately.
>
> Eric
> --
> Eric Korpela | An object at rest can never be
> korpela@ssl.berkeley.edu | stopped.
> <a href="http://sag-www.ssl.berkeley.edu/~korpela">Click for home page.</a>
--
I don't speak for JPL, it doesn't speak for me.
Well, not all the time, at least.
William Daffer <vapuser@haifung.jpl.nasa.gov>
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