Boolean Indexing / Fortran Indexing

[Christoph Mecklenbraeuker]

Hello Francesco, 

You wrote:
> octave:2> a(a<0) = 4711
> error: single index only valid for row or column vector
> error: evaluating assignment expression near line 2, column 8

Sorry, i forgot to tell:
There is an internal-octave-flag called ``do_fortran_indexing'',
which (if it is set to "true") allows you to perform single-index-indexing
on matrices:

Try:
> octave:1> do_fortran_indexing = "true"
> do_fortran_indexing = true
> octave:2> a = [ -5 2; 4 7]
> a =
>   -5   2
>    4   7
>
> octave:3> a(1:4)
> ans =
>   -5   4   2   7

This feature is *very* helpful in the context of boolean expressions:
(Special Thanx to John W.E.!)

> octave:4> a(a<0) = 4711
> a =
> 
>  4711     2
>     4     7

Cheers, and all the best,
Christoph

PS: i like this do_fortran_indexing so much, that i have set this flag
to "true" in the system-wide file 
/usr/local/lib/octave/1.1.0/m/startup/octaverc.
This is, why I forgot to tell you.