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Re: HDF SDS array access in IDL [message #13243 is a reply to message #13178]

Thu, 29 October 1998 00:00

thompson
Messages: 584
Registered: August 1991

Senior Member

"Dr. G. Scott Lett" <slett@holisticmath.com> writes:

> 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].

Ah, but there are two different kinds of matrix operations in IDL. You can
also use A # B to return an array of size [3,3]. If you use the # operator,
then IDL behaves as having matrices in (row,column) order, and the rows and
columns are stored as in Fortran. If you use the ## operator, then it behaves
as having matrices in (column,row) order, and the rows and columns are stored
as in C.

I don't believe the ## operator was even introduced until IDL/v4. An old
IDL/v3 manual that I have laying around states that # is the matrix
multiplication operator, and there is no mention of ## as an operator. I
suspect that ## was added to IDL to make it more C-like. Originally, IDL was
written to more closely emulate Fortran.

All the IDL procedures in use here that I'm aware of uses the # operator, and
thus follow a (row,column) convention. However, since those matrices are used
internally, there's no confusion to anyone who prefers to use the ## operator
instead.

Actually, even if one does use ## instead of #, isn't there still a difference
in the way IDL indexes elements of a matrix? If you follow the ## convention,
then MATRIX(3,5) would be the third column, fifth row. However, in C, wouldn't
you write this MATRIX[5][3]?

> This whole question can be academic, or at most cosmetic, unless one does
> things such as linking Fortran linear algebra codes into IDL. ...

It's also vitally important if one is passing data arrays back and forth
between IDL and Fortran or C routines. If one has a 1000x300 array in IDL,
it's also a 1000x300 array in Fortran, but one had better treat it as a
300x1000 array in C.

William Thompson

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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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