| Re: Assigning structure variables [message #67956 is a reply to message #67955] |
Thu, 03 September 2009 05:09   |
Bernhard Reinhardt
Messages: 26
Registered: October 2008
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Junior Member |
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pp wrote:
> On Sep 3, 5:22 am, Bernhard Reinhardt
> <wirdseltengele...@freisingnet.de> wrote:
>> Hi,
>>
>> I just ran into a problem when I tried to assign the minimum-index
>> returned by the min()-function to an array-element I_LON[i].
>>
>> a=min(ABS(GLON - PLON[i]),I_LON[i])
>> Attempt to store into an expression: <LONG ( 0)>.
>>
>> IDL> help, I_LON
>> I_LON LONG = Array[126236]
>>
>> a=min(ABS(GLON - PLON[i]),b)
>> I_LON[i]=b
>> works
>>
>> Well, I read Davids tips on precedence and Assigning Structure Values
>> but they are all about pointers and nested structures. To me it seems
>> I_LON is a plain array. I guess I haven't yet understood how variables
>> are exchanged with functions/procedures.
>>
>> Anyone can shed a light on this?
>>
>> Regards
>>
>> Bernhard
>
> a=min(ABS(GLON - PLON[i]),I_LON[i]) does not work because subscripted
> arrays are passed to routines by value, instead of by reference. So an
> argument that is a subscripted array works to pass values to the
> routine, but the routine cannot pass anything back to it, which is
> what min is trying to do, and causes the error.
>
> To say it another way, the error happens because that line is
> essentially equivalent to
> a=min(ABS(GLON - PLON[i]),0L), which obviously makes no sense, and
> that is why IDL complains you are trying to store a value into a
> constant (<LONG ( 0)>).
>
> It is easy to forget it because usually arguments are passed by value
> in IDL, which is what happens in a=min(ABS(GLON - PLON[i]),b), and is
> why it works. But subscripted arrays and structure members are passed
> by value.
Thanks for your explanation. So would you say that
a=min(ABS(GLON - PLON[i]),b)
I_LON[i]=b
is an appropriate workaround?
Regards
Bernhard
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