Eval in R

[Vincent MAILLE]

Hello,

I want to eval an expression in R, e.g for E=sqrt(x-1)*sqrt(x-3), E.sub(x,0).is_real is true :( 
Is there a way ton force calculus in R sqrt(-1)=nan for exemple ?

Thanks,
Vincent

[Chris Smith]

Only by doing the substitution by hand, I guess. You write your own
function to pull out the sqrt's:

>>> def presub(expr, reps):
... sr = [p for p in expr.atoms(Pow) if p.exp is S.Half]
... for s in sr:
... ssub = s.subs(reps)
... if not ssub.is_real:
... return S.NaN
... return expr.subs(reps)
...
>>> presub(x,{x:0})
0
>>> presub(sqrt(x-1)*sqrt(x-3),{x:0})
nan

[Aaron Meurer]

So what you really want is sqrt(-1*-3), not sqrt(-1)*sqrt(-3). The
powsimp() function with the force argument will let you combine square
roots:

In [20]: powsimp(E, force=True)
Out[20]:
_________________
╲╱ (x - 3)⋅(x - 1)

In [21]: powsimp(E, force=True).subs(x, 0)
Out[21]:
___
╲╱ 3

Making it return nan instead is more difficult. Something like
Chris's solution should work (depending on what kinds of expressions
you want it to work on). You could probably hack the core to make I
give nan, but that would (obviously) break a lot of stuff.

Aaron Meurer

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[Vincent MAILLE]

Thanks, yes, I think doing the substitution by hand is the best way for my problem.

Thanks,

Vincent