convert rationals to reals in symbolic expression

[Douglas Davis]

Is there a way to convert a symbolic result from using rational expressions and constants to the corresponding expression using reals?

For example if I have a result of

-15/28*2^(2/3)*a*hh^2+32/pi*b^3

I need to convert it to

-0.85039*a*hh^2 + 10.186*b^3

Anything I try I gives  "cannot evaluate symbolic expression numerically"

I need both types of solutions for grading homework. Many expressions are more complicated and I change upstream values frequently so converting each term by hand is not practical.

I have been looking for a solution for some time.  Thank you.

[Dima Pasechnik]

On Thursday, 7 May 2015 21:04:36 UTC+1, Douglas Davis wrote:

Is there a way to convert a symbolic result from using rational expressions and constants to the corresponding expression using reals?

For example if I have a result of

-15/28*2^(2/3)*a*hh^2+32/pi*b^3

I need to convert it to

-0.85039*a*hh^2 + 10.186*b^3

Anything I try I gives  "cannot evaluate symbolic expression numerically"

I need both types of solutions for grading homework. Many expressions are more complicated and I change upstream values frequently so converting each term by hand is not practical.i

for symbolic polynomials you can use .coefficients() to extract coefficients, then you can conert them by using .n()... This is far from ideal, but still automatic.

What you could also do is substiture numbers for each variable,  and then use .n(); this way you can compute a table of numerical values...

[Michael Orlitzky]

On 05/07/2015 11:19 AM, Douglas Davis wrote:
> Is there a way to convert a symbolic result from using rational expressions
> and constants to the corresponding expression using reals?
>
> For example if I have a result of
>
> -15/28*2^(2/3)*a*hh^2+32/pi*b^3
>
> I need to convert it to
>
> -0.85039*a*hh^2 + 10.186*b^3
>

You want the (hidden!) _convert() method:

sage: a,b,hh = SR.var('a,b,hh')
sage: f = -15/28*2^(2/3)*a*hh^2+32/pi*b^3
sage: f._convert({'parent':RR})
10.1859163578813*b^3 - 0.850393420697250*a*hh^2

This is useful all the time and has been asked for before. There's a
ticket to make it public:

http://trac.sagemath.org/ticket/12577

Although now it would be nice to add some sugar on top so that e.g.
f.convert(RR) would work without the weird dict. (The dict wasn't needed
back when I created the ticket.)