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From: Dan Thatcher on 4 Jul 2010 06:53
Hi everyone,

How can I process a matrix in an 'if' statement? I have a matrix of data either side of zero, for all positive values I want to apply a polynomial equation, all negative values, a different equation. For example:

g=[100 100 100 100]; %polynomial 1
n=[1 1 1 1]; %polynomial 2
d=[1 -4 5 -7 4 -3 56 87];

if d>0
y=polyval(n,d)
elseif d<0
y=polyval(g,d)
end

I can only get this to work if 'd' contains 1 value. Any suggestions?
From: us on 4 Jul 2010 07:02
"Dan Thatcher" <d.thatcher85(a)gmail.com> wrote in message <i0pp6h$bf0$1(a)fred.mathworks.com>...
> Hi everyone,
>
> How can I process a matrix in an 'if' statement? I have a matrix of data either side of zero, for all positive values I want to apply a polynomial equation, all negative values, a different equation. For example:
>
> g=[100 100 100 100]; %polynomial 1
> n=[1 1 1 1]; %polynomial 2
> d=[1 -4 5 -7 4 -3 56 87];
>
> if d>0
> y=polyval(n,d)
> elseif d<0
> y=polyval(g,d)
> end
>
> I can only get this to work if 'd' contains 1 value. Any suggestions?

one of the many solutions
- you do not need an IF statement...

d=[1 -4 5 -7 4 -3 56 87];
dpos=d(d>0);
ypos=polyval(n,dpos);
% same for neg values in D...

us
From: Dan Thatcher on 4 Jul 2010 08:39
> one of the many solutions
> - you do not need an IF statement...
>
> d=[1 -4 5 -7 4 -3 56 87];
> dpos=d(d>0);
> ypos=polyval(n,dpos);
> % same for neg values in D...
>
> us

Hi us,
Thanks for the reply, the solution is much simpler than I expected! How can I combine the answers as a matrix the same size as the input ('d')? So the answer would be a matrix:
[ypos yneg ypos. . .ypos ypos]
From: us on 4 Jul 2010 09:04
"Dan Thatcher" <d.thatcher85(a)gmail.com> wrote in message <i0pvdp$i3r$1(a)fred.mathworks.com>...
> > one of the many solutions
> > - you do not need an IF statement...
> >
> > d=[1 -4 5 -7 4 -3 56 87];
> > dpos=d(d>0);
> > ypos=polyval(n,dpos);
> > % same for neg values in D...
> >
> > us
>
> Hi us,
> Thanks for the reply, the solution is much simpler than I expected! How can I combine the answers as a matrix the same size as the input ('d')? So the answer would be a matrix:
> [ypos yneg ypos. . .ypos ypos]

one of the solutions
- note: extended use of logical indexing...

d=[1 -4 5 -7 4 -3 56 87];
ix=d>0;
d(ix)=polyval(n,d(ix)); % <- re-use D to save memory...
% now, for neg vals...
ix=d<0;

us
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