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Re: A strange behaviour of parameters passing [message #15696 is a reply to message #15630]

Fri, 04 June 1999 00:00

Vapuser
Messages: 63
Registered: November 1998

Member

Daniel SAGE <daniel.sage@epfl.ch> writes:

Yeah, the answer is don't pass expressions into routines if you want
the routine to change the value of its parameters.

In the first call you make to 'one' (e.g. one, a[*]) the parameter
is passed by _value_ since it's not a variable but an expression (I
think the completely proper way to say this is that what gets passed
is a reference to a temporary variable that is destroyed when the
routine 'one' exits) The second time you call 'one' the parameter is
passed by reference, since you are passing a named variable and not a
temporary one. In that case, you can change the contents of the
parameter and have that change persist after return.

There's nothing wrong with passing expressions, unless you want to
change the value of the parameter inside a routine. It would be nice
if they gave us some way of checking whether an expression, rather
than a named variable, had been passed in. I am unaware of any such
capability.

I don't think you can call this is a bug; I think it's a feature.

William

> Hi,
> Who can explain me the strange behaviour when I passed my parameters ?
> I create a small and stupid example to see the disfunctionning of IDL.
> Why the first call of the procedure 'one' returns 1 and the second call
> return 2 ? According the documentation of IDL, the second calling is
> correct but
> not the first calling.
> Thank you
>
> pro one, b
> b[0] = 2
> end
>
> pro two
> a = intarr(1)
> a[*] = 1
> one, a[*]
> print, a
> one, a
> print, a
> end
>
> % Compiled module: ONE.
> % Compiled module: TWO.
> IDL> two
> 1
> 2
>
> ------------------------------------------------------------ ----------------
> Daniel Sage
> EPFL - Swiss Federal Institute of Technology - http://www.epfl.ch
> BIG - Biomedical Imaging Group - http://bigwww.epfl.ch
>
> Address: EPFL, DMT/IOA, BM, CH-1015 Lausanne, Switzerland
> Tel: +41 21 693 5189
> Fax: +41 21 693 3701
> Email: daniel.sage@epfl.ch
> ------------------------------------------------------------ ----------------

--
William Daffer: 818-354-0161: vapuser@catspaw.jpl.nasa.gov

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[Message index]

		A strange behaviour of parameters passing By: Daniel SAGE on Thu, 03 June 1999 00:00
		Re: A strange behaviour of parameters passing By: David Kastrup on Mon, 07 June 1999 00:00
		Re: A strange behaviour of parameters passing By: davidf on Mon, 07 June 1999 00:00
		Re: A strange behaviour of parameters passing By: rmlongfield on Mon, 07 June 1999 00:00
		Re: A strange behaviour of parameters passing By: davidf on Fri, 04 June 1999 00:00
		Re: A strange behaviour of parameters passing By: Vapuser on Fri, 04 June 1999 00:00
		Re: A strange behaviour of parameters passing By: steinhh on Fri, 04 June 1999 00:00
		Re: A strange behaviour of parameters passing By: Liam Gumley on Thu, 03 June 1999 00:00

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