Hey all:
I'm back with a follow up question on the topic of substitution in
Sage. How can i work the following example? I have an expression
that involves the derivative of a function, and into that expression i
want to substitute the value of the derivative. Simple, eh? I tried
Mike's ** trick and variations thereof, but that doesn't seem to work:
sage: var('x,y')
(x, y)
sage: f= function('y',x)
sage: g= diff(f,x)
sage: h= diff(f^2,x)
sage: print h
d
2 y(x) (-- (y(x)))
dx
sage: print h.substitute(**{str(g):cos(x)})
---------------------------------------------------------------------------
ValueError Traceback (most recent call
last)
/Users/arai021/<ipython console> in <module>()
/Applications/sage/local/lib/python2.5/site-packages/sage/calculus/
calculus.py in substitute(self, in_dict, **kwds)
3605 X = self.simplify()
3606 kwds = self.__parse_in_dict(in_dict, kwds)
-> 3607 kwds = self.__varify_kwds(kwds)
3608 return X._recursive_sub(kwds)
3609
/Applications/sage/local/lib/python2.5/site-packages/sage/calculus/
calculus.py in __varify_kwds(self, kwds)
3660
3661 """
-> 3662 return dict([(var(k) if isinstance(k,str) else k, w)
for k,w in kwds.iteritems()])
3663
3664 def substitute_over_ring(self, in_dict=None, ring=None,
**kwds):
/Applications/sage/local/lib/python2.5/site-packages/sage/calculus/
calculus.py in var(s, create)
5297 return tuple([var(x.strip()) for x in s.split(',')])
5298 elif ' ' in s:
-> 5299 return tuple([var(x.strip()) for x in s.split()])
5300 try:
5301 v = _vars[s]
/Applications/sage/local/lib/python2.5/site-packages/sage/calculus/
calculus.py in var(s, create)
5306 raise ValueError, "the variable '%s' has not been
defined"%var
5307 pass
-> 5308 v = SymbolicVariable(s)
5309 _vars[s] = v
5310 _syms[s] = v
/Applications/sage/local/lib/python2.5/site-packages/sage/calculus/
calculus.py in __init__(self, name)
5140 raise ValueError, "variable name must be nonempty"
5141 elif not is_python_identifier.match(name):
-> 5142 raise ValueError, "variable name is not a valid
Python identifier"
5143
5144 def __hash__(self):
ValueError: variable name is not a valid Python identifier
Alex
On Sep 19, 1:18 pm, Alex Raichev <[EMAIL PROTECTED]> wrote:
> Sweet! Thanks, Mike.
>
> On Sep 19, 11:56 am, "Mike Hansen" <[EMAIL PROTECTED]> wrote:
>
> > On Thu, Sep 18, 2008 at 4:54 PM, Alex Raichev <[EMAIL PROTECTED]> wrote:
>
> > > Now, with the above in mind, how do you write a function to evaluate
> > > that sine limit given a variable from the user? We know the following
> > > does not work.
>
> > > sage: var('x')
> > > x
> > > sage: def limmy(w):
> > > ....: return limit(sin(w)/w,w=0)
> > > sage: limmy(x)
> > > sin(x)/x
>
> > > More generally, how do you execute functions requiring keyword
> > > arguments, such as limit(), when you don't know the symbolic variable
> > > names involved (but have references to them)?
>
> > Here's one way to do it:
>
> > sage: def limmy(w):
> > ....: return limit(sin(w)/w,**{str(w):0})
> > ....:
> > sage: limmy(x)
> > 1
> > sage: limmy(var('w'))
> > 1
>
> > Not the most elegant, but it works.
>
> > --Mike
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