Evaluate sympy expression from an array of values

I'm experimenting with sympy and I've hit upon an issue I can't work out. Using scipy I can write an expression and evaluate it for an array of x values as follows:

import scipy xvals = scipy.arange(-100,100,0.1) f = lambda x: x**2 f(xvals)

Using sympy I can write the same expression as follows:

import sympy x = sympy.symbols('x') g = x**2

I can evaluate this expression for a single value by doing the following:

g.evalf(subs=)

However I can't work out how to evaluate it for an array of x values, like I did with scipy. How would I do this?

asked May 21, 2012 at 2:17 423 1 1 gold badge 4 4 silver badges 4 4 bronze badges

4 Answers 4

First of all, at the moment SymPy does not guarantee support for numpy arrays which is what you want in this case. Check this bug report http://code.google.com/p/sympy/issues/detail?id=537

Second, If you want to evaluate something numerically for many values SymPy is not the best choice (it is a symbolic library after all). Use numpy and scipy.

However, a valid reason to evaluate something numerically will be that deriving the expression to be evaluated was hard so you derive it in SymPy and then evaluate it in NumPy/SciPy/C/Fortran. To translate an expression to numpy just use

from sympy.utilities.lambdify import lambdify func = lambdify(x, big_expression_containing_x,'numpy') # returns a numpy-ready function numpy_array_of_results = func(numpy_array_of_arguments)

Check the docstring of lambdify for more details. Be aware that lambdify still has some issues and may need a rewrite.

And just as a side note, if you want to evaluate the expressions really many times, you can use the codegen/autowrap module from sympy in order to create fortran or C code that is wrapped and callable from python.