Nested piecewise functions

[Vincent Noel]

Hi,

I'm trying to represent a weird case using piecewise functions :

z = sympy.Piecewise((True, x>0), (False, True))

A piecewise function returning a boolean value... so far so good. The problems comes when I want to use that as a condition inside another Piecewise :

z2 = sympy.Piecewise((1,z), (0, True), evaluate=False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/sympy/functions/elementary/piecewise.py", line 104, in __new__
    " Boolean, or a built-in bool." % (cond, type(cond)))
TypeError: Cond Piecewise((True, x > 0), (False, True)) is of type Piecewise, but must be a Relational, Boolean, or a built-in bool.

z being obviously a boolean, is there no way for sympy to recognize that and accept it as a condition ?

Vincent

[Aaron Meurer]

I would use ITE for this case. It's the boolean version of Piecewise.

Aaron Meurer

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[Aaron Meurer]

Of course, for Piecewise((True, x>0), (False, True)), you could just
replace it with x > 0. It seems ITE doesn't do this automatically but
it does if you call to_nnf() on it.

In [12]: ITE(x > 0, True, False).to_nnf()
Out[12]: x > 0

Aaron Meurer