Thread Subject: convert symbolic function

Maybe an obvious question for some of you:

Say I define the function f as:

syms a b x
f=a*x+b

Later on in my code I would like to evaluate this function
for given values of a,b and x. How can I do this?

Ture

In article <fvvnmm$ad9$1@fred.mathworks.com>, Ture VP <turevp@gmail.com> wrote:
>Maybe an obvious question for some of you:

>Say I define the function f as:

>syms a b x
>f=a*x+b

>Later on in my code I would like to evaluate this function
>for given values of a,b and x. How can I do this?

I haven't used the symbolic toolbox, but by scanning past postings,
it appears to me that there are two strategies:

1) use subs() to substitute the specific values in for the symbols; or

2) assign values to the variable names (which might remove the name's
status as being a symbol), and then use vpa() to evaluate the expression.

vpa() is the Variable Precision Arithmetic command and it tries to
resolve things as far as possible to numeric values with the
number of digits you have set up via digits(). This can result in
long numbers where you only want simple numbers -- e.g., if you
wanted to preserve sqrt(2) as a factor in the expression, then
you would not use vpa() because it would evaluate to 1.414<whatever>;
similarily, 2/3 would become 0.6666<etc> . vpa() is suitable for
-numeric- evaluation of a function, but not for -symbolic- evaluation.
For example if your expression had tan(x) - 2*sin(x)/cos(x)
and you wanted that resolved symbolically to -tan(x) then you would not
use vpa, which would go ahead and calculate tan(x) and sin(x) and cos(x)
numerically and do the division and so on.
--
   "Any sufficiently advanced bug is indistinguishable from a feature."
   -- Rich Kulawiec

"Ture VP" <turevp@gmail.com> wrote in message
news:fvvnmm$ad9$1@fred.mathworks.com...
> Maybe an obvious question for some of you:
>
> Say I define the function f as:
>
> syms a b x
> f=a*x+b
>
> Later on in my code I would like to evaluate this function
> for given values of a,b and x. How can I do this?

Use the SUBS function.

--
Steve Lord
slord@mathworks.com

"Steven Lord" <slord@mathworks.com> wrote in message
<fvvqgk$qje$1@fred.mathworks.com>...
>
> "Ture VP" <turevp@gmail.com> wrote in message
> news:fvvnmm$ad9$1@fred.mathworks.com...
> > Maybe an obvious question for some of you:
> >
> > Say I define the function f as:
> >
> > syms a b x
> > f=a*x+b
> >
> > Later on in my code I would like to evaluate this function
> > for given values of a,b and x. How can I do this?
>
> Use the SUBS function.
>
> --
> Steve Lord
> slord@mathworks.com
>
>

Exactly what I was looking for.
Not sure why I missed that one when going through the
Function Reference Alphabetical List for the symbolic math
toolbox.

Thanks!

Ture