Re: scope of variable in NCL

Hi Saji,
    If you're paranoid/worried about this issue, a simple workaround is
to simply assign the values to different variables once they've been
read into a function. Since you are only concerned about the function
output, this will *guarantee* that variables of the same name as
arguments passed to the function won't get accidentally affected.

Example:

function safe(arg1,arg2,arg3)
begin

  local_arg1 = arg1
  local_arg2 = arg2
  local_arg3 = arg3

. . . do some stuff with local_args . . .

return(stuff)
end

If you're writing libraries, it might be worth the extra little trouble
- and this wouldn't require a redesign of NCL :) If you're worried
about this issue with other people's libraries, you could probably write
a wrapper function to check the argument values, call the function, then
check the arguments again. Not ideal, but it might not be that hard.

Have a good weekend,
Jonathan

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
>>>>> 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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>>
>
>

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Received on Sat Oct 17 19:22:32 2009