How to evaluate a Piecewise under specific conditions?
My Name
Oscar
In [10]: x = Symbol('x')
In [11]: y = Symbol('y')
In [12]: a = Symbol('a')
In [13]: integrate(cos(x*y), (x, 0, a))
Out[13]:
⎧sin(a⋅y)
⎪──────── for y > -∞ ∧ y < ∞ ∧ y ≠ 0
⎨ y
⎪
⎩ a otherwise
In [14]: y = Symbol('y', nonzero=True)
In [15]: integrate(cos(x*y), (x, 0, a))
Out[15]:
sin(a⋅y)
────────
y
The new assumptions and refine can handle nonzero. The point about relations not being implemented is that there is not currently a way to use the assumption that e.g. x > y but that's not needed here so this should work:
In [16]: y = Symbol('y')
In [17]: integrate(cos(x*y), (x, 0, a))
Out[17]:
⎧sin(a⋅y)
⎪──────── for y > -∞ ∧ y < ∞ ∧ y ≠ 0
⎨ y
⎪
⎩ a otherwise
In [18]: refine(_, Q.nonzero(y))
Out[18]:
⎧sin(a⋅y)
⎪──────── for y > -∞ ∧ y < ∞ ∧ y ≠ 0
⎨ y
⎪
⎩ a otherwise
My Name
Aaron Meurer
>
> Thanks very much for the explanation.
>
> The assumptions issues in Sympy have deterred me from using Sympy almost completely. It's not clear to me why the old assumption system had to be replaced. It's not clear to me what's unimplemented in the new system.
>
> You say above, "The point about relations not being implemented is that there is not currently a way to use the assumption that e.g. x > y but that's not needed here so this should work". But your code shows Sympy "using" the assumptions y > -∞ ∧ y < ∞, which are relations. I you are using the word "use" in a way I don't understanding, and I'm left with this vague warning about relations being unusable in Sympy, and I go back to Maple.
with adjectives like "positive", "negative", "finite". You can
represent something like x > 0 because that's the same thing as
"positive", but you can't represent x > y because that isn't
representable by the set of things that the old assumptions can
represent. The only way to represent more advanced relations like x >
y is to use a different syntax from what the old assumptions use,
which is the idea behind the new assumptions.
Also, the old assumptions aren't going away. We originally were going
to do that, but the current plan is to keep it around as it works just
fine for those things that it can represent.
Aaron Meurer
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My Name
My Name
Oscar Benjamin
>
> Thanks trying to clear up Sympy's assumptions for me, but I get feeling the only people who know how to use it are people familiar with the source code. I may resort that, because Maple has brought me to a standstill, but I might save time by building my own CAS from scratch to do what I need done. Usually faster than deciphering other people's source code.
be quicker to learn how to use sympy for your problem than to build
your own CAS.
People here are pretty helpful if you ask your question in a
reasonable way. You could probably resolve any difficulties you have
with SymPy more efficiently by having a nice polite discussion here
rather than moaning!
Oscar
My Name
My Name
My Name
My Name
Aaron Meurer
https://github.com/sympy/sympy/blob/master/CODE_OF_CONDUCT.md. Your
most recent messages are completely inappropriate for this forum.
Aaron Meurer
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