Jaroslav Hajek wrote:
> On Tue, May 25, 2010 at 1:16 PM, Alois Schlögl <alois.schlo...@tugraz.at>
> wrote:
>> Søren Hauberg wrote:
>>>>>> - c = 1 + sum ((x-y)(:)'*z*((x-y)(:)));
>>>>>> + c = 1 + sum (vec(x-y)'*z*(vec(x-y)));
>>>>> What is the purpose of this (and similar) change?
>>>> Its again an issue with oct2mat, which can not deal well with (x-y)(:),
>>>> especially if these are used within more complicated constructs. Using
>>>> vec() avoids this problem.
>>> Can't 'oct2mat' just call 'vec' for these types of expressions? It just
>>> seems to me that changing code to fit 'oct2mat' is not the right
>>> approach (it should be the other way around; that way 'oct2mat' would be
>>> more useful). In this case it might even have an impact on performance,
>>> though I doubt it matters.
>>
>> No, its not possible to have a general purpose solution for the (...)(:)
>> issue. Lets look at the following example [F (x-y)(:)].
>> If F is a function, it should be converted to [vec(F(x-y))], but if F is
>> a variable, the correct translation is [F vec(x-y)]. oct2mat can not
>> know which is correct, because it needs to know the context of this
>> expression. In worst case, whether F is a function or variable is
>> determined only at runtime.
>>
>
> Er, no, (un?)fortunately. This is one of the design flaws of Matlab.
> [F (x-y)(:)] is always interpreted as [F, (x-y)(:)], regardless of
> whether F is a function or a variable. I think that (expr)(:) should
> be always equivalent to vec(expr), at least I can't think of a
> counter-example.
Here is a counter-example
F(expr)(:)
In this case Fvec(expr) or F(vec(expr)) won't do, only vec(F(expr)) is
the correct translation.
On the contrary, I don't want the former being
> converted to the latter, because the latter involves an extra overhead
> of a function call.
How difficult is it to have "vec" as a builtin-function, with an
overhead as small as the colon (:) operator ?
>
> I would also like to note that
>
> arglist = {args{1:narg-1}, x+a*dx, args{narg+1:end}};
> fx = feval (f,arglist{:});
>
> can be simply written as
>
> fx = feval (f, args{1:narg-1}, x+a*dx, args{narg+1:end});
>
> i.e. you can use more than one cs-lists in an expression.
>
>
Good to know.
A.
------------------------------------------------------------------------------
_______________________________________________
Octave-dev mailing list
Octave-dev@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/octave-dev
