| Re: User selectable lower array bound? [message #26117 is a reply to message #25978] |
Fri, 03 August 2001 07:36   |
Paul van Delst
Messages: 364
Registered: March 1997
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Senior Member |
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bennetsc@NOSPAMucs.orst.edu wrote:
>
> In article <3B69CA57.FD3B1D8D@noaa.gov>,
> Paul van Delst <paul.vandelst@noaa.gov> wrote:
>> Hey there,
>>
>> 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.?
>
> Yes, that would make a lot of code much more understandable
> and less prone to errors during development.
Tell me about it! :o)
>>
>> I know this can be done now with judicious use of proxy indices, e.g.
>>
>> FOR i = -5, 5 DO BEGIN
>> ix = i + 5
>> PRINT, x[ ix ]
>> ....do other stuff with negative i's....
>> ENDFOR
>>
>> but sometimes this makes code hard to follow (or explain to
>> someone who's never used the
>> code before) in direct correspondence with a physical process.
>>
>> It seems like such a simple thing to be able to do (with default
>> action being start at
>> index 0) although I'm sure the amount of work required to
>> implement this would be
>> horrendous. Still, it shur would be nice.....
>>
> That depends upon how IDL already keeps track of arrays
> internally. In PL/1, for example, one declared an array with the
> boundaries for each dimension in the form lowerbound:upperbound,
> where specification of the lower bound and the colon were optional.
> If only the upper bound were specified, then the lower bound defaulted
> to 1. In its internal representation of arrays, IIRC, PL/1 kept
> the lower and upper boundaries of each dimension as part of a control
> block preceding the actual array memory. If a language implementation
> doesn't already store both boundaries, or equivalently, the lower
> boundary and number of elements, for each dimension, then yes, adding
> such support might well be a major headache.
One big problem that occurred to me was how one would implicitly or explicitly specify the
array bounds over a procedure or function call in IDL.
Consider the following Fortran 90 code:
program test_bounds
integer, parameter :: n = 20
real, dimension( 0:n ) :: x
integer :: i
! -- Fill the array (like FINDGEN)
x = (/ (real(i),i=0,n) /)
print *, 'In Main'
print *, 'LBOUND(x)=',LBOUND( x )
print *, 'UBOUND(x)=',UBOUND( x )
print *, 'SIZE(x) =',SIZE( x )
call sub( x )
contains
subroutine sub( sx )
! -- Asummed shape dummy argument
real, dimension( : ) :: sx
print *, 'In Sub'
print *, 'LBOUND(sx)=',LBOUND( sx )
print *, 'UBOUND(sx)=',UBOUND( sx )
print *, 'SIZE(sx) =',SIZE( sx )
end subroutine sub
end program test_bounds
The results of which are:
In Main
LBOUND(x)= 0
UBOUND(x)= 20
SIZE(x) = 21
In Sub
LBOUND(sx)= 1
UBOUND(sx)= 21
SIZE(sx) = 21
So the upper and lower bounds as declared in the "Main" program are by default not
preserved when passing arrays unless your subroutine declaration of "sx" is
real, dimension( 0: ) :: sx
i.e. from index 0->however-big-the-array-is minus 1.
So you can specify whether you wanted the lower bound of sx in Sub to be 0 or 1 (or
anything else for that matter). This seems like a simple thing but it can be a
tremendously useful feature. I don't know how you would replicate that in IDL since you
don't declare stuff in procedures/functions.
Hmmm.
paulv
--
Paul van Delst A little learning is a dangerous thing;
CIMSS @ NOAA/NCEP Drink deep, or taste not the Pierian spring;
Ph: (301)763-8000 x7274 There shallow draughts intoxicate the brain,
Fax:(301)763-8545 And drinking largely sobers us again.
Alexander Pope.
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