[ncl-talk] using attributes to define 2D arrays

[Dennis Shea]

A strength of NCL is that, the variable data structure adheres to the
classic netCDF variable data structure. ie: NCL variables 'look like'
netCDF variables. This includes following the variable rules established by
netCDF. One netCDF variable 'rule' is that a variable attribute can not
have an attribute. In short, there *can not* be an *'attribute cascade'*
[ie: attributes having attributes that have attributes .... etc)

*By default*, when the NCL *new*
<http://www.ncl.ucar.edu/Document/Functions/Built-in/new.shtml>  statement
is used as follows:

      xxx = *new*
<http://www.ncl.ucar.edu/Document/Functions/Built-in/new.shtml>((/nest,ndays/),"string")
*; no _FillValue specified *

it will *automatically create *a _FillValue appropriate to the variable
type.
To avoid this, as documented, you can specify *"No_FillValue" *and no
attribute will be created.


ncl 0> DATE = 0
ncl 1> nest = 2
ncl 2> ndays =3
ncl 3> DATE*@date* = new((/nest,ndays/),"string"*,"No_FillValue"*)

This *works *because the attribute '*date*' will *not itself have any
attribute*. ie: no attribute cascade.
-----
netCDF does **NOT* *support multi-dimensional attributes

*NCL allows the creation of multi-dimensional attributes within a script.*
Again .. no attributes can be associated with an attribute.

However, NCL does *NOT support directly accessing the multi-dimensional
attribute*.
The script must explicitly extract the attribute and make it a distinct
variable..

ncl 4> *date_att* = DATE at date             ; extract the multidimensional
attribute to a variable
ncl 5> printVarSummary(date_att)       ; two-dimensions

Variable: *date_att*
Type: string
Total Size: 48 bytes
            6 values
Number of Dimensions: 2
Dimensions and sizes:    *[2] x [3]*
Coordinates:

ncl 6> qqq = date_att(1,2)                   ; access an element of the
variable

ncl 7> QQQ = DATE at date(1,2)          ; NCL does *not *support direct access
to an element
                                                             ; within a
multidimensional attribute

*fatal:*Attributes only have one dimension, 2 subscripts used
*fatal:*["Execute.c":8638]:Execute: Error occurred at or near line 7

=============

A bit more:

As noted: Attributes can not  have other attributes associated with them.
Also, attributes can not have named dimensions or coordinate variables.

=============

Hope this helps!
Cheers
D

On Sun, Aug 19, 2018 at 11:19 AM, Francesco Trotta <
francesco.trotta4 at unibo.it> wrote:

> Dear ncl-support,
>
>     I'm using attributes to define 2D array but I get this error:
>
>
> fatal:Attributes only have one dimension, 2 subscripts used
> fatal:["Execute.c":8640]:Execute: Error occurred at or near line 416 in
> file ./procPREPRO_surf.ncl
>
>
> Usually I define 1d array with attribute and it is all right but with 2d
> array the program stop.
>
> Here some lines of the code where the program stop:
>
>
>    DATE at date        = new((/nnest,ndays_max+15/),string,"")
>    DATE at year        = new((/nnest,ndays_max+15/),string,"")
>    DATE at mounth   = new((/nnest,ndays_max+15/),string,"")
>    DATE at day          = new((/nnest,ndays_max+15/),string,"")
>
>    do inest=0,nnest-1
>
>       ;compute date yyyymmdd of the simulation
>       iday  = 0
>       DATE at date(inest,iday) = runMan_date000(inest)
>
>       .......
>
>
>
>    Can I define 2d array via attribute in ncl?
>
>    If not, is there a way to define a data structure with heterogeneous
> multiple arrays 2d, 1d or scalar?
>
>
>    Thanks
>
> Francesco
>
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