On Sun, Nov 18, 2012 at 10:03 PM, Chris Smith <smi...@gmail.com> wrote:
>>>> _.replace(f, lambda x: x**2)
No I mean: I have individual values for lamb0, lamb1, lamb2, lamb3. So
what I did was:
from sympy import *
var('i j n')
lamb=Function('lamb')
f=Sum(lamb(i)*(-1)**(n-j)*binomial(i,n-j),(i,n-j,n))
mylamb=list(var('lamb:4'))
nval=3
for jval in range(nval+1):
print(f.subs(((j,jval),(n,nval))).doit().replace(lamb,lambda
x:mylamb[x]))
which gave the "desired" output of:
-lamb3
lamb2 + 3*lamb3
-lamb1 - 2*lamb2 - 3*lamb3
lamb0 + lamb1 + lamb2 + lamb3
Obviously I'll have to have a proper mylamb list (containing floats)
for my application, but this is just experimentation...
But what's with so many ((( with the subs method?
> I'm not sure what you mean. Using summation is ding it with sympy semantics.
> Perhaps you can post the way you think is un-sympy-ish.
What I considered *unsympyish* is to do instead of summation(f,(i,a,b)):
s=0
for ival in range(a,b+1): s+=f.subs(i,ival)
BTW can you please clarify the difference between Sum and summation?
The doc says "represents unevaluated summation". So is it just that
until I say doit() it doesn't give me the output but remains a
symbolic function or such?
Another curious thing which I came across in the process of the above:
In [1]: from sympy import *
In [2]: lamb=list(var('lamb:4'))
In [3]: lamb
Out[3]: [lamb0, lamb1, lamb2, lamb3]
In [4]: lamb0=4
In [5]: lamb
Out[5]: [lamb0, lamb1, lamb2, lamb3]
In [6]: lamb[0]
Out[6]: lamb0
In [7]: type(lamb[0])
Out[7]: sympy.core.symbol.Symbol
In [8]: type(lamb0)
Out[8]: builtins.int
I don't get it -- there are *two* objects with name lamb0 in the
current namespace? How can that be?
Thanks as ever!
--
Shriramana Sharma
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