Re: Has anyone tried netCDF for Java?

Hi Glenn,

Thanks for the explanation of some of the netcdf/multiarray package
details. I'm now a lot clearer on some of the package design issues.

 > The MultiArray framework actually allows you to avoid intermediate array 
 > copies
 > altogether. The pattern of use is as follows. There is a MultiArray
 > that contains some numbers of interest that you are going to do something 
 > with.
 > You isolate the numbers of interest (clipping, subsampling, and slicing)
 > using a MultiArrayProxy. Use the proxy's lengths to construct an 
 > IndexIterator
 > to visit the numbers of interest, using the primitive MultiArray
 > get() or set() operations.

This is pretty much what I'm doing at the moment (but without using
the IndexIterator). Basically, I'm picking out 2d sections from a 3d
Variable, using MultiArrayProxy, then constructing a 2d plot by
looping over the 2d section and doing a get() call at each
index. Something along the lines of:

for ( int j = 0; j < height; j++ ) {
  for ( int i = 0; i < width; i++ ) {
     index[0] = j; index[1] = i;
     pixels[count++] = *** Some expression involving 
                       *** ma.getDouble(index)
  }
}

Essentially, my question is 'Can I avoid doing a getDouble call at
every index, and instead do something like a single getAllDoubles()
call which gets me all the primitive data in a single chunk?'
Something like:

double[] x = *** Somehow grab the contents of the full 2d array 
             *** from ma in one go
for ( int j = 0; j < height; j++ ) {
  for ( int i = 0; i < width; i++ ) {
     pixels[count++] = *** Some expression involving x[count] ***
  }
}

 > Another aggregate copy is the MultiArrayImpl(MultiArray) constructor.
 > MultiArrayImpl exposesits internal storage, so java.lang.System.arraycopy()
 > can be used for aggregate copy into and out of a MultiArrayImpl.

Aha. I think this is what I'm looking for!! Will try some tests on
MultiArrayImpl. Basically I'm happy with the MultiArray framework, I
think it expresses array manipulations in a very clean way, but I'm
talking about that final step where we've now done all our various
slicing/clipping/.. operations and we actually need to get out a piece
of PRIMITIVE DATA to do some operation on, and the granularity of that
data extraction process. Rather than just get the value at a single
index out (and iterating over index values), I'd like to be able to
grab with a single method call, say the first 10000 doubles into a
double[], or ALL doubles in the MultiArray into a double[].

Anyway, thanks again for the info. I think that quite a few of the
'problems' that I've come across are actually just lack of familiarity
with the packages on my part. Maybe some of this stuff could usefully
go in a FAQ?

Cheers
Pete

+--------------------------------------------------------+
| Pete Lockey,                 | Email: address@hidden    |
| Supercomputing Support,      |                         |
| Proudman Oceanographic Lab,  | Tel: +44 151 653 8633   |
| Bidston Observatory,         | Fax: +44 151 653 6269   |
| Birkenhead, L43 7RA, UK.     |                         |
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