Re: Retained Value

[Thomas D. Dean]

On 07/13/2016 09:19 AM, Przemek Klosowski wrote:
> On 07/13/2016 11:44 AM, Thomas D. Dean wrote:
> > I have some code, that unless I clear all, does not change value when
> > changed.
> >
> > X=[0,38000]
> > Y=[0,3000]
> > poly_deg = 4
> > global P = polyfit(X,Y,poly_deg)
> > P
> >
> > X=[0,38000]
> > Y=[0,3000]
> > poly_deg = 8
> > global P = polyfit(X,Y,poly_deg)
> > P
> >
> > P has the same value as before.
> >
> > What am I doing wrong?
> Well, you are fitting polynomials of fourth and eight degree to two
> points---no good can come out of that.
> But your problem has nothing to do with the poly* math: it's just that
> globals are weird:
>
> P
>    undefined
>
> global P=1
>
> P
>    1
>
> global P=2
>
> P
>     1                                   That's strange---the value
> didn't change...
>
> clear P
>
> P
>    undefined                        OK, we got rid of P.......
>
> global P=2
>
> P
>    1                                   .....  except that somehow
> previous value was retained.
>
> Wha?????
>
> I think the explanation is that 'global' is a statement about the
> variable, and the assignment to this variable should be a separate
> statement. Apparently  multiple in-line assignments don't work that
> well. If you write it as
>
> global P
> P=1
> P=2
>
> it works as expected.
>
> By the way, global only matters when you want some variables to be
> available within functions, so the usage pattern is
>
> global P
> function fun(); global P; ... P... ; end
>
> and it has to be said that the concept of global is a crutch, and should
> be avoided if possible.
I simplified the problem to something small that still showed my 
mis-understanding of what I was doing!  I have a large script file I 
used for 'proof of principle' so it was very messy.  I changed most of 
this file into a function file that works better.

I now use persistent inside a function.  This is better than global, as 
I believe, it makes better code.  The function is defined as

  function [r] = xx(v, newP=[])
    persistent P = [];
    if ( numel(newP) > 0 )
        P = polyder(newP);
    endif
    r = polyval(P,v);
  endfunction;

Tom Dean