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Re: Assignment Time for a 3d Variable [message #46459]

Wed, 23 November 2005 12:17

Mark Hadfield
Messages: 783
Registered: May 1995

Senior Member

Fï¿½ldy Lajos wrote:
> Hi,
>
> Fortran is column major, IDL and C are row major. In IDL (unlike C or
> Fortran) the first subscript is column, not row.
>
> It is best not to refer to Fortran or C, IDL is like IDL :-)

Dragging in "rows" and "columns" here is a distraction. They relate
(presumably) to how you print an array or possibly to how you interpret
the array dimensions in matrix operations. I am not really a
matrix-oriented guy and that is why I find Matlab so painful. It also
may be why when I hear people use the terms row-major and column-major I
put my fingers in my ears and go "la la la" until the noise stops.

IDL is like Fortran in that the left-most index refers to the dimension
that varies fastest in memory. Ie. if we have a = fltarr(3,2) then the
order of the elements in memory is...

a[0,0] a[1,0] a[2,0] a[0,1] a[1,1] a[2,1]

So it generally makes sense to loop over the left-most index (inner
dimension)

a = fltarr(m, n)
for j=0,n-1 do begin
for i=0,m-1 do begin
; Do something with a[i,j]
endfor
endfor

This way you're stepping thru memory from right to left.

Previous messages have suggested that because IDL is implemented in C it
must use C's convention. Not so.

--
Mark Hadfield "Kei puwaha te tai nei, Hoea tahi tatou"
m.hadfield@niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)

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[Message index]

		Re: Assignment Time for a 3d Variable By: Mark Hadfield on Wed, 23 November 2005 12:17
		Re: Assignment Time for a 3d Variable By: Foldy Lajos on Wed, 23 November 2005 11:13
		Re: Assignment Time for a 3d Variable By: David Fanning on Wed, 23 November 2005 11:09
		Re: Assignment Time for a 3d Variable By: James Kuyper on Wed, 23 November 2005 10:35
		Re: Assignment Time for a 3d Variable By: K. Bowman on Wed, 23 November 2005 10:16
		Re: Assignment Time for a 3d Variable By: Antonio Santiago on Wed, 23 November 2005 09:51
		Re: Assignment Time for a 3d Variable By: David Streutker on Wed, 23 November 2005 08:52
		Re: Assignment Time for a 3d Variable By: David Fanning on Wed, 23 November 2005 07:57
		Re: Assignment Time for a 3d Variable By: Antonio Santiago on Wed, 23 November 2005 04:46

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