st: Re: RE: Re: manipulating matrix elements

["Michael Blasnik" <michael.blasnik@verizon.net>]

One more twist/caveat I discovered is that you can get unexpected results 
from the code I posted because:

scalar x=x+p*ln(p)

will refer to any variable beginning with the letter p before it goes to 
look at the fact that scalar p exists.   I tried my code on the auto dataset 
and found that it used price in the calculation, not the scalar I intended. 
Instead, the code should read:

scalar x=x+scalar(p)*ln(scalar(p))

and Nick's corrected code should also use the scalar() notation.  In an ado 
file, I would have used tempname to name the scalar and this problem 
wouldn't occur.

Michael Blasnik

----- Original Message ----- 
"Nick Cox" <n.j.cox@durham.ac.uk> wrote:

> A twist to this problem is that it is customary
> in entropy calculations to define 0 ln 0 as 0.
>
> However, Stata doesn't know that and will
> return missing instead. So, it is necessary
> to trap 0 within some expression as
>
> scalar x = x  + cond(p = 0, 0, p * ln(p))
>
> Nick
> n.j.cox@durham.ac.uk
>
> Michael Blasnik
>
> > -findit matmap- will lead you to the matmap package which can perform
> > arbitrary elementwise operations on matrices, but since you
> > want the sum of
> > the calculation across all elements, I think you should just
> > do the looping
> > yourself.  Here's one approach (untested):
> >
> > scalar x=0
> > tab var1 var2, matcell(Cell)
> > matrix P = Cell/r(N)
> > local rows=rowsof(P)
> > local cols=colsof(P)
> > forvalues i=1(1)`rows' {
> >     forvalues j=1(1)`cols' {
> >         scalar p=P[`i',`j']
> >         scalar x=x+p*ln(p)
> >   }
> > }
> > scalar x=x+ln(`rows'*`cols')
> > di x
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