Re: Correct behavior of common_size()?

[Robert T. Short]

On 03/14/2012 10:20 AM, Jordi Gutiérrez Hermoso wrote:
> On 14 March 2012 12:59, Robert T. Short<address@hidden>  wrote:
> > When I include an empty vector in common_size() I get, for example,
> >
> > octave>  [ec,x,y,z]=common_size([],1,[1 2 3])
> > ec =  1
> > x = [](0x0)
> > y =  1
> > z =
> >
> >    1   2   3
> >
> > In other words, I get an error because the empty matrix can't be sized to
> > match the other elements.  I wonder if this is the correct behavior, though.
> It seems to me that common_size should be a special case of
> broadcasting arrays to a common shape. Accordingly, the broadcasting
> rules do not apply to dimensions of size zero, only size one.
>
> Ben was asking a while ago for a function like common_size that
> applied all of the broadcasting rules. I was hesitant to encourage
> that development, because one of the objectives of broadcasting is to
> not actually fill your RAM with useless copies of the same data (not
> to mention the time it takes to do so). Perhaps it's nevertheless
> useful to extend common_size to do all of the broadcasting.
>
> - Jordi G. H.
Hmmm.  It seems to me that common_size is something that one should use 
when broadcasting really doesn't apply.  An example is my current 
fanaticism - Bessel functions.  The bessel{j,y,i,k} routine will take a 
column vector of orders and a row vector of arguments and create a 
table.  Each element of the table needs to be computed individually 
though since convergence properties and the algorithm itself is likely 
to vary considerably from element to element.  All of my current 
problems fall into this situation.

I guess, more succinctly, using common_size is a convenient way of 
allowing non-vectorizable functions to return vectors and/or matrices.

BTW, attached is a function that might be what Ben was asking for.  It 
is a generalization of common_size in that it will do what the bessel 
function routines do - create tables from vectors of compatible 
dimensions in very much the same way as broadcasting.  If there is any 
community interest you are welcome to it.  The only thing I ask in 
return is for someone to think of a better name for the function.

Bob

tablify.m
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