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Re: Structure of arrays or arrays of structures? [message #64845 is a reply to message #64841] Thu, 22 January 2009 16:15 Go to previous messageGo to previous message
Craig Markwardt is currently offline  Craig Markwardt
Messages: 1869
Registered: November 1996
Senior Member
On Jan 22, 3:52 pm, Brian Larsen <balar...@gmail.com> wrote:
> All,
>
> this may be philosophy but I have fought with both today and I am
> wondering if there are pros and cons to the different
> implementations.
>
> My example is that when I read in text files I build a structure of
> arrays:
> IDL> help, data
> DATA            STRUCT    = -> <Anonymous> Array[1]
> IDL> help, data, /str
> ** Structure <1a07a08>, 5 tags, length=212480, data length=212480,
> refs=1:
>    JD              DOUBLE    Array[5312]
>    PAS0            DOUBLE    Array[5312]
>    PAS90           DOUBLE    Array[5312]
>    MEP0E3          DOUBLE    Array[5312]
>    MEP90E3         DOUBLE    Array[5312]
>
> and when I read cdf data (at least from ACE) I get arrays of
> structures
> IDL> help, data
> DATA            STRUCT    = -> <Anonymous> Array[36451]
> IDL> help, data, /str
> ** Structure <19cf008>, 45 tags, length=200, data length=196, refs=1:
>    DNUM            DOUBLE           0.0000000
>    YEAR            LONG              2007
>    DAY             LONG               165
>    HR              LONG                 0
>    MIN             LONG                 0
>    SEC             FLOAT           8.98560
>
> Are there memory issues with one way or the other? Other things I
> haven't thought about?
>
> I prefer the feel of the structure of arrays since I like typing
>    tmp = data.jd[0:10]
> more than I like typing
>    tmp = data[0:10].jd

My thoughts:

Both methods will be slower than pure arrays.

I believe the array-of-structures approach will be the slowest,
because every time you need to fetch a field (as your second example),
IDL has to assemble the array from structure parts. That's been my
experience anyway.

However, the array-of-structures approach is natural if you can think
of your data as a database, and you can add or remove more entries at
a later time. You can do this easily by filtering the array of
structures and/or appending, without destroying the existing entries.
For a structure of arrays you basically need to tear apart the
structure and re-build it if you want to change any of the contents.

The structure-of-arrays approach is natural when you will be passing
it as an _EXTRA argument, since for that you need all of your data
bundled into a single structure.

Craig

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