Re: how to find polynomial roots in OCTAVE?help...

[Søren Hauberg]

In your example x is a vector (containing 1000 elements). The 
multiplications is matrix multiplications. To perform pointwise 
multiplications use the .* operator. So, you function f should be 
defined with:

function [y] = f(x)
  y = x.^(4/3)-21.3.*x.^(1/3)-4.53.^(4/3);
endfunction

Søren

John B. Thoo skrev:
> Hi, Henry.  When I tried your example, the "plot" choked on me (see  
> below).  Any ideas?  I'm using 2.1.73 on Mac OS X 10.4.9.
>
> octave:1> function [y] = f(x)
>  > y=x^(4/3)-21.3*x^(1/3)-4.53^(4/3);
>  > end
> octave:2> f(0)
> ans = -7.4954
> octave:3> x = linspace (-100, 100, 1000);
> octave:4> plot (x, f(x))
> error: for A^b, A must be square
> error: evaluating binary operator `^' near line 2, column 4
> error: evaluating binary operator `-' near line 2, column 10
> error: evaluating binary operator `-' near line 2, column 23
> error: evaluating assignment expression near line 2, column 2
> error: called from `f'
> error: evaluating argument list element number 2
> octave:4> [X, INFO, MSG] = fsolve ('f', 10)
> X = 23.902
> INFO = 1
> MSG = solution converged within specified tolerance
>
> Thanks.
> ---John.
>
>
> On Jun 15, 2007, at 6:39 PM, Henry F. Mollet wrote:
>
> > fsolve was suggested and it works.
> > octave:10> function [y]=f(x)
> > > y=x^(4/3)-21.3*x^(1/3)-4.53^(4/3);
> > > end
> > octave:11> f(0)
> > ans = -7.4954
> >
> > octave:20> x=linspace(-100,100,1000);
> > octave:21> plot (x,f(x))
> > Shows that good initial value is important. x=0 won't do.
> >
> > octave:22> [X, INFO, MSG] = fsolve ('f', 10)
> > X = 23.902
> > INFO = 1
> > MSG = solution converged within specified tolerance
> > Henry
> >
> >
> > on 6/14/07 7:57 PM, jub at address@hidden wrote:
> >
> > > James Sherman Jr.-2 wrote:
> > > >  then just have y^3 = x;
> > > you mean y^(1/3)=x is it? yeah the 'h' is the 'x', sorry :-P
> _______________________________________________
> Help-octave mailing list
> address@hidden
> https://www.cae.wisc.edu/mailman/listinfo/help-octave