Re: Scalars and Matrices:

[Johan Kullstam]

(Ted Harding) <address@hidden> writes:

> On 13-Oct-99 A. Scottedward Hodel wrote:
> > 
> > Sorry; I forwarded the article without fixing mangled spacing.  The
> > values are:
> > A = [1,  1 ];     # 1 x 2 row vector
> > B = [1 ; 1];      # 2 x 1 column vector
> > C = [1 ; 2 ; 3];  # 3 x 1 column vector; could be any length, as long
> > as its
> >                   # a column
> > this_works    = (A*B)*C;
> > this_does_too = A*B*C;
> > this_fails    = A*(B*C);
> > 
> > With scalar = 1x1 matrix, (A*B)*C is defined, but A*(B*C) is not.  It's
> > an annoying detail that the order of operations not only changes flop
> > counts, but even whether or not a result is defined.
> 
> Sorry, I don't think I can sympathise with this. As a general convention
> for associativity,
> 
>   A*B*C = (A*B)*C

no. associativity says, by definition, that

  (A*B)*C = A*(B*C)             (A)

and one draws the conclusion that since (A) holds, we can write

  A*B*C

with no risk of confusion.  *order doesn't matter* if star (*)
is associative.

what you are promoting is implicit grouping.  this is *not*
associativity.  it is, in fact, nearly the *opposite* of
associativity, since order clearly *does* matter.  you even go out of
your way to define a default order convention.

one could also take the point of view that the octave interpreter
should *require* parenthesis in cases of ambiguity.  

we could still allow automatic rearrangement of the operations to
whichever uses fewer flops whenever we are in a non-ambiguous case.

-- 
J o h a n  K u l l s t a m
address@hidden
Don't Fear the Penguin!



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