`lambdify` is intended for numeric evaluation (but can be made to evaluate
symbolically). By default, the functions `sin`, `cos`, etc... are pulled
from `math` or `numpy`, which expect floats (and if not given a float, they
attempt to convert to float).
What's happening here, is you're passing in a number for `x`, but `y` is
still a symbol, and is enclosed in the function as a local. `lambdify`
doesn't know this, and uses `math.cos` by default, which causes the error.
Using the `modules` kwarg, you can tell lambdify to use `sympy.cos` instead.
>>> f = lambdify(x, x*cos(y), modules='sympy')
>>> f(1)
cos(y)
You can also pass a dictionary of {function_name_as_string: function} to
modules, if you want finer control:
>>> f = lambdify(x, x*cos(y), modules={'cos': sympy.cos})
>>> f(1)
cos(y)
On Saturday, January 10, 2015 at 11:50:29 AM UTC-6, Quirine wrote:
>
> Dear Sympy community,
>
>
> The following line of code gives:
>
>
> >>> from sympy import *
>
> >>> x,y=symbols('x y')
>
> >>> f=lambdify(x,y*cos(x))
>
> >>> f(1)
>
> 0.54030230586814*y
>
>
> works fine, but interchanging x and y:
>
>
> >>> f=lambdify(x,x*cos(y))
>
> >>> f(1)
>
> Traceback (most recent call last):
>
> File "<stdin>", line 1, in <module>
>
> File "<string>", line 1, in <lambda>
>
> File
> "/Users/krol/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/sympy/core/expr.py",
>
> line 225, in __float__
>
> raise TypeError("can't convert expression to float")
>
> TypeError: can't convert expression to float
>
>
> gives me a headache,
>
> Does someone know a solution to this?
>
>
> kind Regards,
>
> Quirine
>
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