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Re: User selectable lower array bound?

On Thu, 2 Aug 2001, Paul van Delst wrote:

> Is is just me, or would anyone else find useful the ability to define
> arrays in IDL such that the lower bound is *not* always zero? Sorta
> like:
>   x = FINDGEN( 11, LOWER = -5 )
> or
>   y = DBLARR( 100, LOWER = 1 )
> so that accessing elements such as x[ -4 ] or y[ 100 ] are o.k.?  [...]

   Here, here!!  This was #1 on my (13-item) contribution to last summer's
"Top 10 IDL Requests" discussion.  As I pointed out then, Fortran's had
this capability for decades.  (And IDL is expressly a data-analysis
language, like Fortran, not a systems-programming language like C.)  The
biggest problem I see is that certain IDL intrinsics, like WHERE(), return
-1 to indicate an invalid index.  Perhaps WHERE could return
(lowerbound-1) instead, on the presumption that existing programs would be
using 0-based arrays?  Of course it's much better to check the COUNT=
keyword anyway.  (This would also be a good application for some sort of
"undefined value" type.)

   (IMHO, the two worst features IDL picked up from (presumably) C are
starting arrays at 0 (which makes some sense in C, due to the tight
coupling of arrays and pointers, but this isn't the case in IDL), and
prefix syntax for the array dereferencing operator.  Concerning the
latter, I've since learned that even Dennis Ritchie apparently now thinks
it was a mistake; from his "The Development of the C Language"
(http://cm.bell-labs.com/who/dmr/chist.pdf), page 12:

       An accident of syntax contributed to the perceived complexity of
       the language.  The indirection operator, spelled * in C, is
       syntactically a unary prefix operator, just as in BCPL and B [C's
       predecessor languages]. ...  Sethi [Sethi 81] observed that many of
       the nested declarations and expressions would become simpler if the
       indirection operator had been taken as a postfix operator instead
       of prefix, but by then it was too late to change.

   Oh, make that THREE worst features: Also the use of integers for
Boolean data, instead of having a true logical or Boolean data type.
VERY confusing.)

    Of course, these opinions are my own and don't reflect those of
Raytheon or NASA.

                                             Jeff Guerber
                                             Raytheon ITSS
                                             NASA Goddard Space Flt Ctr
                                             Oceans & Ice Branch (971)