On Fri, Apr 18, 2008 at 03:12:03PM +0200, Ondrej Certik wrote:
> On Wed, Apr 16, 2008 at 4:16 PM, Friedrich Hagedorn <[EMAIL PROTECTED]> wrote:
> >
> > On Wed, Apr 16, 2008 at 03:48:20PM +0200, Ondrej Certik wrote:
> > > That's the automatic evaluation:
> > >
> > > In [1]: L, C = symbols("LC")
> > >
> > > In [2]: sqrt(L/C)
> > > Out[2]:
> > > ⎽⎽⎽
> > > ╲╱ L
> > > ─────
> > > ⎽⎽⎽
> > > ╲╱ C
> > >
> > > In [3]: print sqrt(L/C)
> > > C**(-1/2)*L**(1/2)
> > >
> > >
> > > Basically, it's the same problem as with:
> > >
> > > In [7]: f = (x*y)**2
> > >
> > > In [8]: f
> > > Out[8]:
> > > 2 2
> > > x *y
> > >
> > > Should we do this be default? Or should we handle it like with log:
> > >
> > > In [1]: e=log(k*m)
> > >
> > > In [2]: e.expand()
> > > Out[2]: log(k) + log(m)
> >
> > I prefer the expr.expand() variant.
> >
> > I have a big expr where sqrt(L/C) is only a tedious coefficiant. Then
> > I want to derivate this big expr.diff(t) and when I read the
> > result I want recogize fastly the old coefficiants.
> >
> > But have you an other example where is the automatic eval good for?
>
> Well, so far we considered this as part of what you'd do by hand and
> thus it's useful. That's the only reason. Plus other CAS systems imho
> do the same.
And thats why sympy exists. I want to controll what is going on with my
expressions as I can do it by hand. I dont like the above because sometimes
I want to substitute these expressions:
In [1]: (x*y)**2
Out[1]:
2 2
x *y
In [2]: f=(x*y)**2
In [3]: f.subs(x*y, z)
Out[3]:
2 2
x *y
I dont need to use sympy for small calculus which I can do by hand. But I
need sympy for the calculus with _big_ expression. And I often substitute
things in thuch big expressions.
By,
Friedrich
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