Re: [ferret_users] undesirable missing_value

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi everyone,
What Martin says is absolutely correct, but there is a way to specify 
the region of interest without writing intermediate files.  If you want 
to average over some region, specify that region within the square brackets

  temp[x=138:290@ave,y=-23.5:23.5@ave]

This is one number (for each depth and time that the variable may have)

The expression,

  temp[x=@ave,y=@ave]

is not evaluated until an "action" command is issued -- a plotting 
command, a LIST command to list data to the screen or a file, or a LOAD 
command to load a variable containing the expression into memory.  
Whatever contxt is in effect for the action will determine the range of 
X and Y over which the average is applied.

Ansley

Martin Schmidt wrote:

> Hi David,
>
> the operation
>
> list/x=140w/y=0 temp, newtemp
>
> limits the region of temp and newtemp. Hence temp[x=@ave,y=@ave]
> gets a different meaning. This is often not taken into account.
> Please note also, that temp is defined at x=140.5 and x=139.5. So some
> interpolation is done here.
>
> What helps.
>
> - save newtemp to a new file. 
> save/clobber/file=newtemp.nc newtemp
>
> - delete the variable newtemp
>
> cancel var newtemp
>
> - get the complete set newtemp from newtemp.nc
>
> use newtemp.nc
> list/x=140w/y=0 temp[d=1], newtemp   ! testing
>
> It should be now, what you expect. 
> My experience with ferret is the following: Doing things with
> averaging, filling or shift operation and applying some
> regional limits later gives unexpected results. These are not
> errors but is a "misunderstanding" between user and ferret. 
> Ferret executes commands, when output is required. When you
> define newtemp about nothing happens. In matlab the field would
> be generated, ferret says "yessir - understood" and waits what 
> really is to do and is so much faster in many cases. If you list
> newtemp at x=140w and y=0 ferret executes the commands defined
> previously only for a small region around x=140w. If you
> really want the average over all x and y you have to tell ferret
> to execute this. Saving to a file does exactly this. Making
> a figure with "shade" also. However, issuing the list command later,
> ferret gets "oops, the same again, but now only for x=140w and y=0."
>
> One could try to "think like ferret". However, a saver way is to save complete
> intermediate variables to intermediate files, remove the variable definition
> and to read data from file. This "stores" the ferret results.
> This seems to  be slower and expensive in compute power but is save and there is no need
> for error search. Finally one gets the desired result much faster.
>
> Regards,
> Martin Schmidt
>
>