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Re: HDF SDS array access in IDL [message #13258 is a reply to message #13178]

Thu, 29 October 1998 00:00

Dr. G. Scott Lett
Messages: 14
Registered: February 1998

Junior Member

Ok, ok. Let me try this a different way.

Standard linear algebra packages written in FORTRAN, such as Linpack,
followed the _convention_ that general matrices were stored as two
dimensional arrays, accessed as (row, column). Because there were no matrix
operations built into old FORTRAN, this was just a convention.

Fortran now has a number of matrix operations built into the standard
language. An example is the MATMUL intrinsic function, which follows the
(row, column) convention. So, now the convention is part of the standard.
You can multiply an array A of shape (3,4) by an array B of shape (4,3).
MATMUL(A,B) returns an array of shape (3,3).

IDL has a number of matrix operations and linear algebra functions. They
follow the convention of storing matrices as two dimensional arrays in
(column, row) order. The matrix multiplication operator in IDL can multiply
an array A of size
[3,4] and an array B of size [4,3]. A ## B returns an array of size [4,4].

This whole question can be academic, or at most cosmetic, unless one does
things such as linking Fortran linear algebra codes into IDL. The
difference in storage/access conventions can also have a profound impact on
the efficiency of certain algorithms, as David Fanning pointed out. Is an
LU decomposition of a matrix faster when the matrix is stored by rows or by
columns? Try it and see.

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[Message index]

		HDF SDS array access in IDL By: William Clodius on Mon, 26 October 1998 00:00
		Re: HDF SDS array access in IDL By: thompson on Thu, 29 October 1998 00:00
		Re: HDF SDS array access in IDL By: davidf on Thu, 29 October 1998 00:00
		Re: HDF SDS array access in IDL By: Liam Gumley on Thu, 29 October 1998 00:00
		Re: HDF SDS array access in IDL By: Dr. G. Scott Lett on Thu, 29 October 1998 00:00
		Re: HDF SDS array access in IDL By: davidf on Wed, 28 October 1998 00:00
		Re: HDF SDS array access in IDL By: William Clodius on Wed, 28 October 1998 00:00
		Re: HDF SDS array access in IDL By: Dr. G. Scott Lett on Wed, 28 October 1998 00:00
		Re: HDF SDS array access in IDL By: davidf on Wed, 28 October 1998 00:00
		Re: HDF SDS array access in IDL By: thompson on Wed, 28 October 1998 00:00
		Re: HDF SDS array access in IDL By: Dr. G. Scott Lett on Wed, 28 October 1998 00:00
		Re: HDF SDS array access in IDL By: davidf on Tue, 27 October 1998 00:00

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