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Re: Specification for a new array slicing function
- Subject: Re: Specification for a new array slicing function
- From: steinhh(at)ulrik.uio.no (Stein Vidar Hagfors Haugan)
- Date: 20 May 1999 08:52:29 GMT
- In-reply-to: Liam Gumley's message of Wed, 19 May 1999 14:58:04 -0500
- Newsgroups: comp.lang.idl-pvwave
- Organization: University of Oslo, Norway
- References: <374317CC.E1AC89EA@ssec.wisc.edu>
- Xref: news.doit.wisc.edu comp.lang.idl-pvwave:14828
In article <374317CC.E1AC89EA@ssec.wisc.edu> Liam Gumley
> Please find below a suggested specification for a new array slicing
> function, formatted as a standard IDL prolog. The intention here is to
> provide a means to extract n-dimensional array 'slices' from an existing
> array in memory. The caller can choose to skip elements along any or all
> dimensions if desired.
> Comments are invited. There's no code yet, so now is the time.
> ; INPUT KEYWORD PARAMETERS:
> ; START Set this keyword to a vector containing the start
> ; STRIDE Set this keyword to a vector containing the
> ; COUNT Set this keyword to a vector containing the
> ; EXAMPLE:
> ; ;Extract every other element along each dimension
> ; array = findgen( 1, 10, 5, 6, 7 )
> ; ndims = size( array, /n_dimensions )
> ; stride = replicate( 2L, ndims )
> ; result = array_slice( array, stride=stride )
> ; help, result
> ; ;RESULT FLOAT = Array[1, 5, 2, 3, 3]
IMO, the use of keyword parameters for START, STRIDE and
COUNT is a bit "wordy" for my liking. And these items
are really essential to the routine as such. So why not
use positional parameters?
For something that really ought to be a part of the IDL
syntax, I would also like a shorter name (despite the
possibility for name conflicts), like "arex", short
My suggestion would be something a bit more like the native
Fortran 9X syntax (not that I actually *know* exactly how that
syntax works!) , e.g.:
a(0:5:2,:,5:9) would be translated into
I.e., each positional parameter signifies the extraction
operation for one array dimension. There are some issues
that I would like to clear up, though: What exactly does
the 0:5:2 sequence mean? Does it mean elements 0:5, sampled
with a stride of 2? Or does it mean 5 elements sampled with
a stride of 2, starting from 0? Or is it START:STRIDE:COUNT,
meaning 2 elements, sampled with a stride of 5?
Just curious.... And I would strongly recommend following
Fortran conventions, whatever they are....
Anyway, the three elements in each parameter appear in
"optionality" order: start [, stride [, count]] (if that's
what the syntax is supposed to be).
Looking at the example above, you may wonder what the "-1" is
doing there... Well, the idea is that one could use a
nonnegative *scalar* parameter to signify extraction of a
slice at a given position, whilst -1 really means "*", in IDL
Since we now have one extra "degree of freedom" in that a
start position (and nothing else) may be specified in two
similar ways, as e.g. 0 or ... I have great use for
this (since we're at now at liberty to rewrite the rules.. :-)
I've always disliked the way this works:
a = fltarr(5,5,5)
% SURFACE: Array must have 2 dimensions: <FLOAT Array[5, 1, 5]>.
I mean - if I'm extracting an "image" out of a "cube", why
would I want the last dimension to stick around...???
So, I would like to be able to say
with no error messages! On the other hand, if I do want the
dangling dimension, I could specify it:
(and get the error message :-)
Or the other way around, if people feel strongly about leaving
this dimension in.....
I would also like to see a corresponding index function,
returning the one-dimensional indices to the extracted
elements instead of the elements themselves. This could
be used for assignments. I.e.:
a(arexi(a,-1,,[0,2])) = data_block
Just some thoughts...