The SymPy package (http://sympy.org/) is a Python library for symbolic mathematics.
With the excellent PyCall package of julia, one has access to the
many features of SymPy from a julia session.
This SymPy package provides a light interface for some of the
features of SymPy that makes working with SymPy objects a bit
easier.
The tutorial provides an overview. It is
viewable as an IJulia notebook
here.
To use this package, both Python and its SymPy library must be
installed on your system. If PyCall is installed using Conda
(which is the default if no system python is found), then the
underlying SymPy library will be installed via Conda when the
package is first loaded. Otherwise, installing both Python and
SymPy (which also requires mpmath) can be done by other means.
The Anaconda distribution is suggested, as it provides a single
installation of Python that includes SymPy and many other
scientifice libraries that can be profitably accessed within Julia
via PyCall. (Otherwise, install Python then download the sympy
library from https://github.com/sympy/sympy/releases and install.)
The only point to this package is that using PyCall to access
SymPy is somewhat cumbersome. The following is how one would define
a symbolic value x, take its sine, then evaluate at pi, say:
using PyCall
@pyimport sympy
x = sympy.Symbol("x")
y = sympy.sin(x)
y[:subs](x, sympy.pi) |> float
The Symbol and sin function of SymPy are found within the
imported sympy object. They may be referenced with Python's dot
notation. However, the subs method of the y object is accessed
differently, using indexing notation with a symbol. The call above
substitutes a value of sympy.pi for x. This leaves the object as a
PyObject storing a number which can be brought back into julia
through conversion, in this case with the float function.
The above isn't so awkward, but even more cumbersome is the similarly
simple task of finding sin(pi*x). As this multiplication is done at
the python level and is not a method of sympy or the x object, we
need to evaluate python code. Here is one solution:
x = sympy.Symbol("x")
y = pyeval("k*x", k=sympy.pi, x=x)
z = sympy.sin(y)
z[:subs](x, 1) |> float
This gets replaced by a more julian syntax:
using SymPy
x = symbols("x") # or @syms x, Sym("x"), or Sym(:x)
y = sin(pi*x)
y(1) # Does subs(y, x, 1). Use y(x=>1) to be specific as to which symbol to substitute
The object x we create is of type Sym, a simple proxy for the
underlying PyObject. We then overload the familiar math functions so
that working with symbolic expressions can use natural julia
idioms. The final result is here is a symbolic value of 0, which
prints as 0 and not PyObject 0. To convert it into a numeric value
within Julia, the N function may be used, which acts like the
float call, only attempts to preserve the variable type.
However, for some tasks the PyCall interface is still needed, as
only a portion of the SymPy interface is exposed. To call an
underlying SymPy method, the getindex method is overloaded for
symbol indices so that ex[:meth_name](...) dispatches to either to
SymPy's ex.meth_name(...) or meth_name(ex, ...), as possible. Any
Sym objects are projected down onto the underlying PyObject for
use within PyCall. Otherwise, to dig the PyObject out of a Sym
object, you access its property x, as in y.x. To find a SymPy
method, a call like sympy.meth_name(...) is possible,
e.g. sympy.harmonic(10). Any Sym-type arguments must be projected
into PyObjects or an error will be thrown.
Some aspects of SymPy require more modern versions of sympy to be
installed. For example, the matrix functions rely on features of
sympy that are not exposed in the sympy installed with Ubuntu LTS
12.04.
In that particular instance, calls such as
x = symbols("x")
a = [x 1; 1 x]
det(a)
Can be replaced with
a[:det]()
Similarly for :trace, :eigenvects, ... . Note these are sympy
methods, not Julia methods that have been ported. (Hence,
:eigenvects and not eigvecs.)
- Try
@docfor documentation links back to SymPy's documentation.