| Re: [Q] readf into an array error [message #5714] |
Mon, 05 February 1996 00:00 |
Ken Knighton
Messages: 44
Registered: May 1995
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peter@hpl.hp.com (Peter Webb) wrote:
> This is, at best, a feeble attempt to describe pass by reference. The
> parameters are not copied, they are referenced (try passing an array so
> large that copying would use up all memory -- you can do it, ergo no
> copying happened).
>
> Time for a rewrite, RSI!
I tried it and Peter seems to be correct. IDL pass by reference seems
to work the way it should and not the way it is documented.
While we are on the subject of documentation, my personal favorite is
the way in which the /NO_COPY keyword is documented for the WIDGET_*
routines. The paragraph given leaves much to the imagination. On the
other hand, the documentation for the HANDLE_* routines makes the
operation of this keyword very clear.
IMHO, the documentation is much better than it used to be, but there
is room for improvement.
Ken Knighton knighton@gav.gat.com knighton@cts.com
General Atomics
San Diego CA
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| Re: [Q] readf into an array error [message #5717 is a reply to message #5714] |
Mon, 05 February 1996 00:00  |
peter
Messages: 80
Registered: February 1994
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Member |
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Ken Knighton (knighton@gav.gat.com) wrote:
: peter@hpl.hp.com (Peter Webb) wrote:
: >This is due to the argument passing mechanism used in IDL. If an array
: >is passed to a function, it is passed by reference. If a subarray is
: >passed, it is passed by value, and thus cannot be overwritten.
: By the way, I found the documentation for this on page 10-7 of the
: user's guide. This documents everything except that structure
: members are passed by value. This tidbit is documented on page
: 7-5 of the user's guide.
As Ken mentions, this behavior is documented. However, the whole
section on argument passing is very badly written. From the PV-Wave
manual, which I believe is a direct copy of the IDL manual,
"When a procedure...is called, the actual parameters are copiend ito the
formal parameters... On exit,...formal are copied back to actual if they
are not expressions or constants."
This is, at best, a feeble attempt to describe pass by reference. The
parameters are not copied, they are referenced (try passing an array so
large that copying would use up all memory -- you can do it, ergo no
copying happened).
Time for a rewrite, RSI!
Peter
--------------------------------
Peter Webb, HP Labs Medical Dept
E-Mail: peter_webb@hpl.hp.com
Phone: (415) 813-3756
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| Re: [Q] readf into an array error [message #5723 is a reply to message #5714] |
Sat, 03 February 1996 00:00 |
Ken Knighton
Messages: 44
Registered: May 1995
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Member |
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peter@hpl.hp.com (Peter Webb) wrote:
> Iarla Kilbane-Dawe (iarla@atm.ch.cam.ac.uk) wrote:
>
> : Hello,
>
> : why does IDL give me an error when I try to do?
>
> : a=fltarr(10)
> : i = 0
> : readf,unit,a(i)
>
> This is due to the argument passing mechanism used in IDL. If an array
> is passed to a function, it is passed by reference. If a subarray is
> passed, it is passed by value, and thus cannot be overwritten.
Thanks for the much better description than I gave earlier. One can
also create this problem by using a structure member (a.b) as the
argument. It has been awhile since I thought about it much, so I was a
little bit hazy with my answer.
This little gotcha can really ruin your day if you write or use a
routine that passes data back to the calling routine through arguments
and then use a subarray or a structure member for the argument.
Thinking about this piece of trivia again will probably inoculate me
against this bug for another year.
By the way, I found the documentation for this on page 10-7 of the
user's guide. This documents everything except that structure
members are passed by value. This tidbit is documented on page
7-5 of the user's guide.
Ken Knighton knighton@gav.gat.com knighton@cts.com
General Atomics
San Diego CA
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