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Re: [ferret_users] arithmetics not working on a variable (broken netCDF file?)

Hi Steven,

Thank you for your diagnosis!  I now understand in what way my
netCDF file is "interesting".

| The netCDF file has a few "interesting" features that are confusing
| ferret, as you can see from the warning messages that it generates
| during your session:
|  *** NOTE: Ignored modulo length exceeding axis length: lont
|  *** NOTE: Ignored modulo length exceeding axis length: lonv
|  *** NOTE: Evenly spaced axis has edges definition: dept - ignored
| In particular, you are specifying modulo lengths for the axes lont and
| lonv, even though these are only of length one.

Actually I'd like to have the modulo behavior even for the x-axis of
length one.  My current model is a two-dimensional (y-z) ocean,
assuming uniformity (symmetry) in the x-direction, and
I'd like to plot the flow field for an arbitrary longitudinal
width.   The modulo index i' is defined as

   i' = mod(i-1,IMAX) + 1

and the IMAX = 1 case is naturally included in the
definition (i' = 1 always).

Another practical reason is that I don't want to special-case the
IMAX=1 situation in my code to generate netCDF files.  The netCDF format
doesn't allow for easy change in dimensionality, so the special-casing
would complicate the code.

| You are then trying to shade a 1D region. The variable tdmb has
| dimensions 1x100x40x1, so tdmb[k=1,l=1] has dimensions 1x100. When you
| enter the command
| shade/k=1/l=1 tdmb
| the modulo attribute on the axis lont appears to be confusing ferret
| into thinking that this is really a 2D region, so the shade command
| succeeds.

That's what I expected.  Because it's 2D even though IMAX = 1
(See below).

| However, when you enter the command
| shade/k=1/l=1 tdmb*10
| ferret realises that it's a 1D region, and so the shade command fails.
| I tried generating a netCDF file that's identical to yours, but
| without the modulo attribute on the axis lont. Now both shade commands
| fail, as they should:
| yes? shade/k=1/l=1 tdmb
|  **ERROR: dimensions improperly specified: must be a 2D region
|           shade/k=1/l=1 tdmb
| yes? shade/k=1/l=1 10*tdmb
|  **ERROR: dimensions improperly specified: must be a 2D region
|           shade/k=1/l=1 10*tdmb

Hmm.  I think it's more systematic to distinguish
size one from no-dimension.  In Fortran notation,

  real:: a      !! scalar (zero-dimensional)
  real:: b(1)   !! one-dimensional array of size 1
  real:: c(3)   !! one-dimensional array of size 3
  real:: d(1,3) !! two-dimensional array

Here array d is two-dimensional even though its first
dimension is of size one.

I thought Ferret allows for a command like

  shade temp

when temp is 50x100x1 (dimension(50,100,1) in Fortran notation),
not because "temp" is two-dimensional but because it supplies
"k=1" when the dimension the user fails to specify is of size

In my interpretation above, "modulo-ness" wouldn't change
the dimensionality of the variable even when IMAX = 1
because the size doesn't affect the dimensionality.

Maybe such an interpretation would be hard to implement or it
would break some backward compatibility?


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