Hi, On Thu, Dec 30, 2010 at 04:48:14PM -0700, Aaron S. Meurer wrote: > You want to use tuple unpacking. If you pass a list or tuple to a function > in Python and put a '*' before it, it "unpacks" the elements of the tuple > into arguments of the function. For example: > > In [4]: a = [1, 2, 3] > > In [5]: def f(a, b, c): > ...: return a + b + c > ...: > > In [6]: f(*a) > Out[6]: 6 > > You may also need to use something like > > In [7]: a[1:] > Out[7]: [2, 3] > > to only get the arguments and not the function. > > Now, one thing you should be aware of is that operator.add only supports two > arguments. However, SymPy's Add() supports multiple arguments. Actually, > don't know why you wouldn't use that anyway if you are going to create SymPy > expressions: > > In [8]: Add(x, y, z, 2) > Out[8]: 2 + x + y + z > > Unfortunately, there is no such thing as Sub() in SymPy, but you could > probably write some simple function to do it: > > In [9]: def sub(*items): > ...: return Add(*[-i for i in items]) > ...: > > In [10]: sub(x, y, z, 2) > Out[10]: -2 - x - y - z >
Maybe we should have something like Sub()?
btw. The previous example can be easily extended to the N-ary case, e.g.:
In [1]: mapping = {'add': operator.add, 'subtract': operator.sub}
In [2]: def convert(tree):
....: op, args = tree[0], tree[1:]
....: result = None
....: for arg in args:
....: if isinstance(arg, (tuple, list)):
....: value = convert(arg)
....: else:
....: value = sympify(arg)
....: if result is not None:
....: result = mapping[op](result, value)
....: else:
....: result = value
....: return result
....:
In [3]: tree = ('add', 'x', ('subtract', 'x', 'y'), 'z', ('add', 't', 117))
In [4]: convert(tree)
Out[4]: 117 + t + z - y + 2⋅x
But as pointed out by Aaron, it's better to use Add(), Mul(), etc.
> Aaron Meurer
>
> On Dec 30, 2010, at 4:38 PM, Jeff wrote:
>
> > Mateusz,
> >
> > thanks for your quick reply.
> >
> > Your suggestion handles this case well. If I have a function f has N
> > children in the tree,
> >
> > e.g. tree = ('add', 'x', ('f', 'a1', 'a2', 'a3', 'a4', ..., 'a100'))
> >
> > how can I express mapping[f](a1, a2, ...an)? N here could be
> > arbitrary and I don't know its value before hand.
> >
> >
> > thanks a lot
> >
> > Jeff
> >
> >
> > On Dec 30, 5:35 pm, Mateusz Paprocki <matt...@gmail.com> wrote:
> >> Hi,
> >>
> >> On Thu, Dec 30, 2010 at 01:34:07PM -0800, Jeff Cen wrote:
> >>
> >>> Hi,
> >>
> >>> Suppose I have already had an expression tree in a tree structure
> >>> (like a nested list, say ['add', 'x', ['subtract', 'x', 'y']]), I
> >>> would like to convert it to sympy expression and simplify it in sympy.
> >>
> >>> When I read my tree structure, I first create an Add function. Then
> >>> how can I append x and the rest?
> >>
> >> A simple helper function will do the job, e.g.:
> >>
> >> In [1]: from sympy import *
> >>
> >> In [2]: import operator
> >>
> >> In [3]: mapping = {'add': operator.add, 'subtract': operator.sub}
> >>
> >> In [4]: tree = ('add', 'x', ('subtract', 'x', 'y'))
> >>
> >> In [5]: def convert(tree):
> >> ....: op, a, b = tree
> >> ....: if isinstance(a, tuple): a = convert(a)
> >> ....: else: a = sympify(a)
> >> ....: if isinstance(b, tuple): b = convert(b)
> >> ....: else: b = sympify(b)
> >> ....: return mapping[op](a, b)
> >> ....:
> >>
> >> In [6]: convert(tree)
> >> Out[6]: -y + 2⋅x
> >>
> >>> thanks
> >>
> >>> Jeff
> >>
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> >>
> >> --
> >> Mateusz
> >>
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