Re: About diagonal matrices

[Jaroslav Hajek]

On Fri, Feb 20, 2009 at 5:51 PM, Jaroslav Hajek <address@hidden> wrote:
> On Fri, Feb 20, 2009 at 5:45 PM, John W. Eaton <address@hidden> wrote:
>> On 20-Feb-2009, José Luis García Pallero wrote:
>>
>> | Playing with the new diag() function in octave 3.1.52 I can see some
>> | extrange behaviour (or I don't understand some particular cases). For
>> | example, for the matrix
>> | a = diag([2 3 4])
>> |
>> | This is a diagonal matrix: typeinfo(a) -> diagonal matrix
>> | But if I add a scalar: typeinfo(4+a) -> matrix
>> | If I multiply by a scalar: typeinfo(4*a) -> diagonal matrix
>> | But if I multiply by a scalar in element-by-element form: typeinfo(4.*a) ->
>> | matrix
>> | If I divide by a scalar in element-by-element form: typeinfo(4./a) -> 
>> matrix
>> | For a power operation: typeinfo(a^2) -> diagonal matrix
>> | But element-by-element: typeinfo(a.^2) -> matrix
>> | For functions: typeinfo(sqrt(a)) -> matrix (the same for sin(), cos(), 
>> etc.)
>> |
>> | Has any reason for this behaviour?
>>
>> A definition is needed in each case for oprators that receive special
>> treatment.  So I guess some have just not been implemented yet.
>>
>> But I think there are also some bugs.  Shouldn't the following return
>> full matrices with the zero elements replaced by NaN (or -0, in the
>> case of dividing by -Inf)?
>>
>>  diag ([1,2,3]) / 0
>>  diag ([1,2,3]) / NaN
>>  diag ([1,2,3]) / -Inf
>>  diag ([1,2,3]) * NaN
>>  diag ([1,2,3]) * Inf
>>
>
> I think it's better in the current manner. I don't like the idea that
> the memory can suddenly explode just because the multiplier happened
> to be Inf. Right now, scalar * diag gives invariantly diag. This is
> somewhat analogous to how sparse matrices behave.
>

Maybe I could write a short chapter in the manual documenting the use
of sparse and permutation matrices
and the subtle differences when they meet NaNs or Infs.

-- 
RNDr. Jaroslav Hajek
computing expert
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz