Re: Working with the := operator

Kyle,

The re-assign operator ":=" is not intended for memory-efficeint.
The basic idea is to give use a way t re-use variable names
(or to save a 'delete' commend).

Originally, if we have code:

avar = 123 ; avar is an integer.
;do some work with avar.
delete(avar)
avar = 456.7
;do some work with avar.

With re-assign operator ":=", the above code can change to:

avar = 123 ; avar is an integer.
;do some work with avar.
avar = 456.7
;do some work with avar.

Clearly, the re-assign operator ONLY saved a "delete(agar)" line.

In the NCL source code, it did exactly the same thing above.
In the "avar := 456.7" line, what NCL did is to check
if there is anything named avar, if not, then create a variable named "avar",
here the type is float, and assign value "456.7"; if yes,
then NCL wipe-out anything related to "avar", and
then create a variable named "avar",
here the type is float, and assign value "456.7".

In your example, NCL is doing what mentioned above,
which at line: "x := x(::4)", NCL wipes anything related
to "x" first, when NCL reads in the left hand side of the
line, then when NCL tries to read "x(::4)", "x" becomes
undefined, which is what NCL tells us in the error message.

We will discuss what NCL should do for this case,
and we certainly welcome users give us their idea as well.

Here, we thank you for giving us a clear example to
demonstrate an issue.

Wei

huangwei@ucar.edu
VETS/CISL
National Center for Atmospheric Research
P.O. Box 3000 (1850 Table Mesa Dr.)
Boulder, CO 80307-3000 USA
(303) 497-8924

On May 28, 2013, at 3:28 PM, Kyle Griffin <ksgriffin2@wisc.edu> wrote:

> Hi everyone,
>
> In the most recent version of NCL, the reassignment operator (:=) was introduced. However, most of the times I've attempted to use it, I haven't been able to do so successfully. I've noticed that this happens at times where I use the operator in an attempt to re-assign itself, as I've included below.
>
> While a temporary variable would be an obvious workaround to this problem, I was wondering if this was the intended behavior of the operator and/or if there is any way to allow the type of use I've included below in a memory-efficient manor? If so, that would definitely make this operator significantly more useful in my work.
>
> Copyright (C) 1995-2013 - All Rights Reserved
> University Corporation for Atmospheric Research
> NCAR Command Language Version 6.1.2
> The use of this software is governed by a License Agreement.
> See http://www.ncl.ucar.edu/ for more details.
> ncl 0> x=ispan(0,100,1)
> ncl 1> x:=x(::4)
> fatal:["Execute.c":7270]:performASSIGN_VAR_VAR_OP: x is undefined
> fatal:["Execute.c":8126]:Execute: Error occurred at or near line 1
>
> ncl 2>
>
>
> Thanks,
>
>
> Kyle
> ----------------------------------------
> Kyle S. Griffin
> Department of Atmospheric and Oceanic Sciences
> University of Wisconsin - Madison
> 1225 W Dayton St, Madison, WI 53706
> Email: ksgriffin2@wisc.edu
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Received on Wed May 29 09:43:24 2013