[ncl-talk] An "2d" Attribute Array Conundrum

[David Brown]

Hi Phillipe,
That's a good question. Actually the error message, as you discovered,
is a bit misleading. NCL was originally designed to only allow 1D
attributes in keeping with the NetCDF data model. Later, however, when
we needed the ability to transmit 2D coordinate information to the
graphics routines (mainly), we loosened the restriction to allow NCL
variables to have multi-d attributes as a convenience. However, as you
have found, we did not implement full support for this feature. Partly
this is because you cannot write such an attribute to any of the
output file formats we support -- so it can never be truly integrated
into the data model.
Practically speaking, if you wanted to have a routine that handles
subscripting of 2D attributes, I'm guessing you could do it by
assigning the attribute value to a local variable, subscripting the
variable, and using the 'reassign' syntax to give the attribute its
new subscripted value.
We will try to make the error message a little more informative.
Eventually maybe we can make subscripting work in this situation.
 -dave

On Wed, May 13, 2015 at 10:56 AM, Philippe Papin <ppapin at albany.edu> wrote:
> Hi NCL-talk,
>
> I came across a strange conundrum today when testing some code. I use
> attributes in my code to send 1d variables through attributes into
> subroutines without explicitly defining it in the new subroutine. This
> practice is limited to 1d arrays since attributes in theory can only be 1d.
> However, if this is the case, why can you "create" a 2d attribute array and
> then print this attribute array without error?
>
> Here is a code snippet of an example of what I am talking about. I'm using
> NCL version 6.2.1
>
> opt = True
> array2d = (/ (/1,2,3/) , (/4,5,6/) /)
> opt at array2d = array2d
> print(opt at array2d) ;;; output below seem to print out array correctly in 2d
>
> (0,0) 1
>
> (0,1) 2
>
> (0,2) 3
>
> (1,0) 4
>
> (1,1) 5
>
> (1,2) 6
>
> However if you try to actually subscript the data, you get an error message
> that attributes only have one dimension.
>
> subarray2d = opt at array2d(0,1)
>
> fatal:Attributes only have one dimension, 2 subscripts used
>
> fatal:["Execute.c":8576]:Execute: Error occurred at or near line 6
>
> This is a bit counterintuitive to me. Perhaps the 2d attribute array I
> created is really 1d, but just gives the appearance of a 2d array when
> printed. Sorry if this is a trivial question, but hopefully that means it
> won't take much to figure out what is going on here.
>
> Thanks for any additional information!
>
> -Philippe
>
>
> --
> -------------------------------------
> Philippe P. Papin
> Dept. of Atmospheric and Environmental Sciences
> University at Albany, SUNY
> 1400 Washington Ave, Albany, NY 12222
> http://www.atmos.albany.edu/student/ppapin/
>
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