CODEWITHSUNDEEP
pythonsympy
user17683587
user17683587

Reputation:

Convert list of strings SymPy to symbols and use in solveset

The post "How can I convert a list of strings into sympy variables?" discusses how to generate SymPy symbols from a list of strings. My question is what steps are needed to use these symbols x, y and z in SymPy calculations?

I tried something along the lines

from sympy import symbols, solveset

var_as_strings = ['x', 'y', 'z']
var_as_symbol_objects = [sympy.symbols(v) for v  in var_as_strings]
var_as_symbol_objects

for x1, x2 in zip(var_as_symbol_objects, var_as_strings):
    x1 = symbols(x2)

soln = solveset(x-y-z, x)

but I get the error "NameError: name 'x' is not defined". Any help is greatly appreciated.

Upvotes: 0

Views: 461

Answers (3)

red-isso
red-isso

Reputation: 343

that I have a lot of items in my list that I wish to define as symbols

Just FYI, a snippet which can automatically define the symbols in an enhanced fashion

nmax = 7
names = ["phi" + str(i) for i in range(nmax)]
sympy_vars = [symbols("\\" + name.replace("phi", "phi_")) for name in names]
globals().update(dict(zip(names, sympy_vars)))
phi0

This yields a nice LaTeX type output in, e.g., jupyter notebooks.

Upvotes: 0

smichr
smichr

Reputation: 19115

You can create Python variables in the current namespace from a list of strings (or a string of space or comma separated names) with var:

>>> names = 'x y z'  # or 'x, y, z'
>>> var(names)
(x, y, z)
>>> var(list('xyz'))
[x, y, z]
>>> var(set('xyz'))
{x, y, z}

In all cases, after issuing the var command you will be able to use variables that have been assigned SymPy symbols with the same name.

Upvotes: 1

Maddy Guthridge
Maddy Guthridge

Reputation: 2001

This is answered by the first listed gotcha in the documentation.

You need to store your symbols into the required variables manually (x = symbols('x')), or import them from sympy.abc (from sympy.abc import x).

The name error is happening because you haven't defined what x, y or z are in your code.

Upvotes: 2

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