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Re: A strange behaviour of parameters passing
- Subject: Re: A strange behaviour of parameters passing
- From: davidf(at)dfanning.com (David Fanning)
- Date: Fri, 4 Jun 1999 21:29:24 -0600
- Newsgroups: comp.lang.idl-pvwave
- Organization: Fanning Software Consulting
- References: <3756C16A.A7C41C10@epfl.ch> <firstname.lastname@example.org>
- Xref: news.doit.wisc.edu comp.lang.idl-pvwave:15072
William Daffer (email@example.com) writes:
> 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
> I don't think you can call this is a bug; I think it's a feature.
While we are on the subject of passing by reference (IDL variables)
and passing by value (anything else, including expressions, system
variables, structure de-references, etc.), I should just point out
that this is *exactly* why this expression does NOT work in IDL:
FOR j=0, 10 DO Readf, lun, variable[j]
Although, I have to admit, it is an expression that every one
of us has used at least once (and many times if we come to IDL
from a FORTRAN background). People are still confused about why
it produces no error message (Answer: because it actual *works*),
and still returns no useful information.
In fact, here is a riddle Bilbo could have used with Smaug:
What works perfectly in IDL, but gives you no useful
Answer: Reading into subscripted variables!
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: firstname.lastname@example.org
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155