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Re: Order of argument evaluation (Was: Re: making a checkerboard array?) [message #55990]

Wed, 26 September 2007 09:51

Allan Whiteford
Messages: 117
Registered: June 2006

Senior Member

Fï¿½LDY Lajos wrote:
>

<snip>

>
> In C/C++/Fortran/... there is a rule that a memory cell can be changed
> at most once in a statement, otherwise the result is undefined (and so
> different implementations can create different results without violating
> the standard). I guess a similar rule is in effect in IDL, too.
>

That's part of the question... does such a rule exist for IDL? You can
change the same variable twice in Perl (the implementation is the
standard etc. etc.) even with strict and warnings on it doesn't complain
about something like:

$c=3;
$d[$c++]=$c++;

I've never found anything in the IDL manual which explictly says either
way what is allowed in IDL. Maybe I should write to the nice support
people and ask if they have an opinion.

> These examples give syntax error prior IDL 6.0:
>
> IDL> print, !version
> { sparc sunos unix 5.3 Nov 11 1999}
> IDL> ((a=fltarr(3)))[1]=10 & print, a
> % Expression must be named variable in this context: <FLOAT Array[3]>.
> % Execution halted at: $MAIN$
> IDL>
>

In IDL 6 they changed it so that setting a variable returns the variable
set rather than the value of the variable set (i.e. an expression):

IDL6> help,(a=fltarr(3))
A FLOAT = Array[3]

IDL5> help,(a=fltarr(3))
<Expression> FLOAT = Array[3]

I wonder if this means that they think it's ok for things like
((a=fltarr(3)))[1]=10 to be written or if they changed it for some other
purpose. Again, maybe a question for the support people.

Thanks,

Allan

>
> regards,
> lajos
>
>
>

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[Message index]

		Re: Order of argument evaluation (Was: Re: making a checkerboard array?) By: Allan Whiteford on Wed, 26 September 2007 09:51
		Re: Order of argument evaluation (Was: Re: making a checkerboard array?) By: Foldy Lajos on Wed, 26 September 2007 07:39
		Re: Order of argument evaluation (Was: Re: making a checkerboard array?) By: pgrigis on Wed, 26 September 2007 07:18
		Re: Order of argument evaluation (Was: Re: making a checkerboard array?) By: Allan Whiteford on Wed, 26 September 2007 06:37
		Re: Order of argument evaluation (Was: Re: making a checkerboard array?) By: David Fanning on Wed, 26 September 2007 06:17

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