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Re: How to get numbers into passed structure elements (pass-by-value/reference problem). [message #58951]

Wed, 27 February 2008 07:56

Paul Van Delst[1]
Messages: 1157
Registered: April 2002

Senior Member

David Fanning wrote:
> Paul van Delst writes:
>
>> 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.
>
> Too much misdirection for me to follow this early in the morning,
> but if you pass a structure array in by reference, what are you
> copying?

Well I meant if I do this:

FOR n = 0, Atm.n_Clouds-1 DO BEGIN
; Need to store data into (*Atm.Cloud)[n]
result = CRTM_Read_Cloud_Record( FileID, Cloud, DEBUG=Debug )
...Copy Cloud into (*Atm.Cloud)[n]
ENDFOR

> I think you are only "copying" if you pass something in
> by value. I would go with (b).

I just completed an experiment where I passed the entire pointer array and looped over
clouds in my reader,

result = CRTM_Read_Cloud_Record( FileID, *Atm.Cloud, DEBUG=Debug )

since it was passing (*Atm.Cloud)[n] that was buggering me up.

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 understand why it's there, but even Fortran has moved beyond it (for about 18(!) years now).

cheers,

paulv

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[Message index]

		Re: How to get numbers into passed structure elements (pass-by-value/reference problem). By: Paul Van Delst[1] on Wed, 27 February 2008 07:56
		Re: How to get numbers into passed structure elements (pass-by-value/reference problem). By: David Fanning on Wed, 27 February 2008 07:25
		Re: How to get numbers into passed structure elements (pass-by-value/reference problem). By: lecacheux.alain on Thu, 28 February 2008 02:23

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