Re: scope of variable in NCL

Dear David, Jonathan and others,

Thanks for the explanations, although you are out of town.
(Also Jonathan, thanks for the sample code.
I am sorry I did not reply, as I had caught a bad cold and has been out of
action for a while).

Firstly, it is my mistake that I did not read the documentation
carefully and made assumptions (the most common package used by most of
us, Grads, passes arguments by value and some other languages I
use such as C or ruby also allows for function parameters to be
local if needed).
It is NCL's rule and I should play by it. I hope it is a good
experience to others too.

> The rationale for passing arguments by reference in NCL is that
> arguments might be large data arrays and it would be expensive to copy
> them. Unlike C, NCL (in the interest of keeping things simple) does not
> have a pointer type, so the only way to avoid copying is to pass
> arguments by reference.

I am not a computer scientist and I may be completely wrong and
I apologize beforehand in this case. In
my understanding although we can pass arguments by value in C,
the exception is for arrays (since
a string is an array of characters, same for strings too), in that
arrays passed in as arguments are considered as pointers by the recieving
function. In this case, if one modifies a subset of the array(string),
the changes will be seen outside the function. I was wondering
if the same paradigm may be applied with benefit to
the NCL language (simple arguments
are local and arrays are global)? I imagine that most people may not want
'side-effects' and may prefer this way. And if somebody needs changes
in his/her variable made inside the function to be available
during the next call, maybe there could be a 'static' keyword as in C.

Best regards,

saji

* David Brown <dbrown@ucar.edu> [2009-10-21 14:37:30 -0600]:

> Hi Saji,
>
> I'd like to return to this question, because I may have been somewhat
> misleading in my response of Oct 16:
>
> In particular, I need to note the difference between what in Fortran is
> called the 'actual' argument versus the 'dummy' argument. In the second
> of your examples the function
> 'testme' has 3 dummy arguments a, b, and c. These are used in the
> definition of the function. When called it is passed the actual
> arguments a, 1, and 2. Thus in the example the symbol 'a' is both an
> actual argument and a dummy argument, somewhat obscuring the true
> behavior of NCL. Here is an alternate version of your example:
>
> a=5
> foo = 42
> function testme(foo,b,c)
> begin
> foo=2
> return(1)
> end
> print(""+a)
> print(""+testme(a,1,2))
> print(""+a)
> print(""+foo)
>
> The output is as follows:
>
> (0) 5
> (0) 1
> (0) 2
> (0) 42
>
> So yes, an actual argument can be modified within a function, but a
> global variable that just happens to have the same name as a dummy
> argument is not affected by calling the function. So in that sense, the
> dummy arguments are inherently 'local'. Presumably if you call a
> function using a variable that has been defined globally you know you
> are doing so. Any function or procedure that potentially changes an
> input argument should definitely be documented as doing so.
>
> The rationale for passing arguments by reference in NCL is that
> arguments might be large data arrays and it would be expensive to copy
> them. Unlike C, NCL (in the interest of keeping things simple) does not
> have a pointer type, so the only way to avoid copying is to pass
> arguments by reference.
>
> The only difference between a function and a procedure in NCL is that a
> function returns a value.
> -dave
>
>
> On Oct 17, 2009, at 6:42 PM, Saji wrote:
>
>> Hi Dave and Wei and others,
>>
>> Thanks for the feedback and clarification. I understand it
>> , but doesn't it illustrate a situation, where some unsuspecting
>> user accidentally uses the same name somewhere in his script
>> as the function argument and lands up a nasty surprise. If the
>> library developer cannot ensure the 'local' scope of the arguments
>> within the function, it is a bit inconvenient, as the developer
>> has to be careful not to change the arguments that are passed on
>> to the function. I still wonder why this behaviour is implemented
>> in NCL functions, as a function is supposed to return a single data
>> object.
>> ...why should it change other variables or arguments that it does not
>> return. Also this means that there is no real difference between a
>> so called 'procedure' and 'function'. If I am a paranoid user, I would
>> have
>> to check all function definitions, before I feel confident to use
>> them.
>>
>> I wonder if it will be possible to rethink this design issue in NCL at
>> least for functions --
>>
>> Best regards,
>> saji
>>
>>
>> * David Brown <dbrown@ucar.edu> [2009-10-16 09:29:30 -0600]:
>>
>>> Hi Saji,
>>> Wei is quite correct about this case. Arguments are passed by
>>> reference and the variables they reference can be modified inside
>>> the function. The 'local' statement has no meaning for arguments.
>>> You can ignore the last message I sent. I apologize that I did not
>>> notice that 'a' was being used as an argument to the function.
>>> -dave
>>>
>>> On Oct 16, 2009, at 8:09 AM, Wei Huang wrote:
>>>
>>>> Saji,
>>>>
>>>> That is because you use "a" as an argument element, and local.
>>>>
>>>> If you do "print("" + test(0,1,2))", then you can not change "a"
>>>> out side your function.
>>>>
>>>> Wei Huang
>>>> 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 Oct 15, 2009, at 11:03 PM, Saji N Hameed wrote:
>>>>
>>>>> Even declaring "a" as local does not seem to work :(
>>>>> ncl 1> function test(a,b,c)
>>>>> ncl 2> local a,b,c
>>>>> ncl 3> begin
>>>>> ncl 4> a=-999
>>>>> ncl 5> return(1)
>>>>> ncl 6> end
>>>>> ncl 7> print(""+test(a,1,2))
>>>>> (0) 1
>>>>> ncl 8> print(""+a)
>>>>> (0) -999
>>>>>
>>>>> * Saji N Hameed <saji@apcc21.net> [2009-10-16 13:59:15 +0900]:
>>>>>
>>>>>> Dear NCL Developers,
>>>>>>
>>>>>> I suppose that the following should not happen. Isn't it a bug?
>>>>>> We are told that variables inside a function have local scope, and
>>>>>> this unusual feature can lead to a lot of headaches. Any
>>>>>> solutions,
>>>>>> other than "local a" or using another variable name than "a"
>>>>>>
>>>>>> Copyright (C) 1995-2009 - All Rights Reserved
>>>>>> University Corporation for Atmospheric Research
>>>>>> NCAR Command Language Version 5.1.1
>>>>>> The use of this software is governed by a License Agreement.
>>>>>> See http://www.ncl.ucar.edu/ for more details.
>>>>>> ncl 0> a=5
>>>>>> ncl 1> function testme(a,b,c)
>>>>>> ncl 2> begin
>>>>>> ncl 3> a=2
>>>>>> ncl 4> return(1)
>>>>>> ncl 5> end
>>>>>> ncl 6> print(""+a)
>>>>>> (0) 5
>>>>>> ncl 7> print(""+testme(a,1,2))
>>>>>> (0) 1
>>>>>> ncl 8> print(""+a)
>>>>>> (0) 2
>>>>>>
>>>>>> Thanks,
>>>>>> saji
>>>>>> --
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> ncl-talk mailing list
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>>>>>>
>>>>>
>>>>> -- Saji N. Hameed
>>>>>
>>>>> APEC Climate Center
>>>>> 1463 U-dong, Haeundae-gu, +82 51 745
>>>>> 3951
>>>>> BUSAN 612-020, KOREA saji@apcc21.net
>>>>> Fax: +82-51-745-3999
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> ncl-talk mailing list
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>>>>
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>>>
>>
>> --
>> Saji N. Hameed
>>
>> APEC Climate Center
>> 1463 U-dong, Haeundae-gu, +82 51 745
>> 3951
>> BUSAN 612-020, KOREA saji@apcc21.net
>> Fax: +82-51-745-3999
>>
>>
>>
>> _______________________________________________
>> ncl-talk mailing list
>> List instructions, subscriber options, unsubscribe:
>> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>
>

-- 
Saji N. Hameed
APEC Climate Center          				
1463 U-dong, Haeundae-gu,                               +82 51 745 3951
BUSAN 612-020, KOREA                    		saji@apcc21.net
Fax: +82-51-745-3999
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