| Re: Bug/feature in matrix multiply [message #14657 is a reply to message #14582] |
Mon, 15 March 1999 00:00   |
Martin Schultz
Messages: 515
Registered: August 1997
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Senior Member |
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Mark Fardal wrote:
> [...]
> So my question becomes, why does this happen? If IDL is going to
> treat an 3x1 array differently than a 3-element vector, it shouldn't
> just cavalierly remove the trailing dimension in my opinion. The
> behavior is probably documented somewhere but I couldn't find it in
> the hyperhelp. There is this one sentence in "Combining Array
> Subscripts with Others": "As with other subscript operations, trailing
> degenerate dimensions (those with a size of 1) are eliminated."
>
> I also notice that the behavior is somewhat inconsistent, in that
> converting an expression to one of the same type does _not_ remove
> the trailing dimension:
> [...]
I agree (somewhat). Generally, I found that IDL is quite "smart" in
removing trailing dimensions so that one doesn't have to worry too much
about them. But when you do encounter a case where you need to (and
this is frequent for any type of matrix manipulation), IDL is just
too smart and you have to think twice to outsmart it ;-)
Anyway: you can always make sure you get what you want with
a = transpose(reform(a))
These statements are not very costly in terms of execution time, because
it's only messing around with the array descriptor (at least I believe
so).
Regards,
Martin.
--
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Dr. Martin Schultz
Department for Engineering&Applied Sciences, Harvard University
109 Pierce Hall, 29 Oxford St., Cambridge, MA-02138, USA
phone: (617)-496-8318
fax : (617)-495-4551
e-mail: mgs@io.harvard.edu
Internet-homepage: http://www-as.harvard.edu/people/staff/mgs/
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