| Re: HDF SDS array access in IDL [message #13261 is a reply to message #13178] |
Wed, 28 October 1998 00:00   |
William Clodius
Messages: 30
Registered: December 1996
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Member |
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Dr. G. Scott Lett wrote:
> <snip>
> IDL uses a storage scheme like C, not Fortran. The question of
> column-majority and row-majority was a source of confusion at RSI for some
> time, and this confusion was evident in some of their documentation. Recent
> editions should be better.
> <snip>
Technically what we are talking about is an access scheme not a storeage
scheme. Storeage per se is typically defined by the OS/processor, and
the language only defines a mapping to that storeage. For
multidimensional arrays, Fortran promises that access is most efficient
if the left most index varies most rapidly. C promises that access is
most efficient if the right most index varies most rapidly. In two
dimensional array the first index is commonly termed the column index,
the second index the row index. In Fortran, and IDL, varying the column
index and keeping the row constant is most efficient, in C varying the
row index and keeping the column constant is most efficient. Fortran's
access mode is commonly refered to as column major, I have never hear
the term row order.
IDL's (5.1) Help is confusing on the issue. For Arrays and Matrices it
states
"In a computer, a multidimensional data set ... can be indexed in
column-major format, which means that the linear order of the data
elements proceeds from the first element in the first column through the
last element in the first column before beginning on the second column,
and so on. This is the format used by FORTRAN to index data."
so far correct
"Alternatively, data can be indexed in row-major format, meaning that
the linear order of the data elements proceeds from the first element of
the first row through the last element of the first row before beginning
on the second row, and so on. This is the format used by the C and
Pascal computer languages, and is traditionally associated with image
processing, because it keeps all the elements of a single image scan
line together. Because IDL's origins are in image processing, it
indexes data in row-major format."
Almost all languages keep scan lines together and this naming convention
has no impact on image processing. They just access them together in
different indices, C groups lines by the leftmost index Fortran by the
rightmost. C may be more commonly used for image processing for a
variety of reasons (more often taught at universities, easier access to
hardware, structs have more flexibility than what is available in F77,
wide availability of cheap compilers), but this is not one of them.
"NoteMany computer languages�e.g. C, Pascal, Visual Basic, and
Fortan�index 2-dimensional arrays in (row, column) order. IDL indexes
arrays in (column, row) order."
Note the above is Humpty Dumpty. The C and Fortran standards do not
define which index is associated with the row and which with the column
so the definition should follow other conventions. Historically, the
convention in column major versus row major is that in column major the
first dimension elements are contiguous and in row major the last index
elements are contiguous. Under that convention, Fortran and IDL are both
column major, C and Pascal are row major. IDL is trying to make up a
confusing convention to make itself sound as though it is more like C
than Fortran. See for example
http://member.aol.com/CORLISS100/chapter5.html
http://webster.ucr.edu/Page_asm/ArtofAssembly/CH05/CH05-2.ht ml
http://www.cs.uregina.ca/dept/manuals/Manuals/3_0NumRep/3_Nu m_Rep.html
(Note because of the current limitations of C arrays I suspect that
IDL's arrays are actually implemented as C pointers with indexing
similar to F2C's.)
--
William B. Clodius Phone: (505)-665-9370
Los Alamos Nat. Lab., NIS-2 FAX: (505)-667-3815
PO Box 1663, MS-C323 Group office: (505)-667-5776
Los Alamos, NM 87545 Email: wclodius@lanl.gov
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