| How to get numbers into passed structure elements (pass-by-value/reference problem). [message #58956] |
Wed, 27 February 2008 07:02  |
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Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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Hello,
This subject comes around every now and again, but I can't find anything useful searching.
I'm calling a function, CRTM_Read_Atmosphere_Record(), like so:
Atm = PTRARR(n_Profiles)
FOR m = 0, n_Profiles-1 DO BEGIN
Atm[m] = PTR_NEW({CRTM_Atmosphere})
result = CRTM_Read_Atmosphere_Record( FileID, *Atm[m], DEBUG=Debug )
...check result....
ENDFOR
That function in turns calls another function, CRTM_Read_Cloud_Record(), like so:
FUNCTION CRTM_Read_Atmosphere_Record, FileID , $ ; Input
Atm , $ ; Output
DEBUG=Debug ; Optional input
.....
FOR n = 0, Atm.n_Clouds-1 DO BEGIN
result = CRTM_Read_Cloud_Record( FileID, (*Atm.Cloud)[n], DEBUG=Debug )
...check result....
ENDFOR
And inside the CRTM_Read_Cloud_Record() function I do the following:
FUNCTION CRTM_Read_Cloud_Record, FileID , $ ; Input
Cloud , $ ; Output
DEBUG=Debug ; Optional input
.....
Type = Cloud.Type
READU, FileID, Type, $
*Cloud.Effective_Radius, $
*Cloud.Effective_Variance, $
*Cloud.Water_Content
Cloud.Type = Type
(and similar for an Aerosol structure and I/O function. Also, assume the pointer component
allocation has been done to the correct array size))
As you have probably guessed, by the time I inspect all the data that is returned in the
original Atm pointer array, everything is fine *except* the cloud type flag. It is zero.
So, I realise this is one of those pass-by-reference or pass-by-value things, but how does
one get around it? Do I:
a) make the Type component of the Cloud structure a pointer? (Yuk!)
b) change the way I pass the Cloud structure into the CRTM_Read_Cloud_Record() fn?
i.e. not reference the cloud structure array via the index [n].
I would much prefer (b), but will that entail copying entire structures? The number of
"Clouds" associated with any particular "Atm[m]" profile is variable.
Thanks for any info.
cheers,
paulv
p.s. FWIW, the structure definitions are:
PRO CRTM_Atmosphere__Define
void = { CRTM_Atmosphere, $
n_Layers : 0L, $
n_Absorbers : 0L, $
n_Clouds : 0L, $
n_Aerosols : 0L, $
Climatology : 0L, $
Absorber_ID : PTR_NEW(), $
Absorber_Units : PTR_NEW(), $
Level_Pressure : PTR_NEW(), $
Pressure : PTR_NEW(), $
Temperature : PTR_NEW(), $
Absorber : PTR_NEW(), $
Cloud : PTR_NEW(), $
Aerosol : PTR_NEW() }
END
and
PRO CRTM_Cloud__Define
void = { CRTM_Cloud, $
n_Layers : 0L, $
Type : 0L, $
Effective_Radius : PTR_NEW(), $
Effective_Variance : PTR_NEW(), $
Water_Content : PTR_NEW() }
END
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| Re: How to get numbers into passed structure elements (pass-by-value/reference problem). [message #59014 is a reply to message #58956] |
Thu, 28 February 2008 07:19  |
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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David Fanning wrote:
> Paul van Delst writes:
>
>> I'll repeat my comment in my reply to David Fanning's post in this thread:
>> I find it a fundamental flaw of IDL that the argument passing mechanism (reference
>> or value) is so exposed to the user that they have to tailor their code to prevent
>> data loss.
>
> I find this equivalent to dynamic tying and structuring:
> powerful, but dangerous. Like keeping a loaded 357 Magnum
> in the drawer of the night stand. In the right hands ... etc.
Hmm. As a colleague pointed out to me just yesterday, if you don't have a 357 Magnum
(loaded or otherwise) in the drawer of your nightstand there is no associated danger.
:o)
However, I do think you have a point with regard to the dynamics of IDL. Maybe we could
employ a slightly less aggressive analogy? Such as the old omelette and breaking eggs
chestnut? :o)
cheers,
paulv
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