Re: fortran indexing? (was Re: oct file: multiply a matrix by a vector)

[Paul Kienzle]

On Jul 2, 2005, at 3:31 PM, Brian Blais wrote:

> David Bateman wrote:
> > To reduce the number of cache misses you should traverse the matrix 
> > in the way it is stored in memory. Octave uses Fortran conventions 
> > for the storage and so you should reverse the for loops
>
> this is the first time that I have heard that Octave uses Fortran
> conventions for storage.  Is there a reason for this?  It seems like it
> would be more difficult, given that both the scripting language, and
> C++, indexes arrays down the column, row by row.

In practice, C's builtin array reference syntax is rarely used.  That's 
because C doesn't let you pass the dimensions of the array as 
arguments.  Instead you pass a simple vector and translate i,j into a 
single index.  This technique doesn't favour one array layout over 
another.

Fortran does allow you to pass the array dimensions so the array layout 
does matter.  IIRC, the original Matlab was coded in Fortran and they 
kept that convention when they recoded it in C.

Octave/Matlab let you treat a matrix as a simple vector by only using 
one index argument and it is easier to get this consistent if they use 
the same internal format.

- Paul



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