numpy array eval representation formula

Question

I would like to evaluate symbolic expression with np array

example:

import numpy as np
a = np.array([1]*4)
b = np.array([2]*4)
res = repr(a) + ' + ' + repr(b)
value = eval(res)

error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
NameError: name 'array' is not defined

I have a workaround but I will know if I can solve my initial problem

workaround found on stackoverflow Python eval function with numpy arrays via string input with dictionaries

formula = 'x+y'
res = eval(formula,{'x':a, 'y':b})

Edit:

in order to solve the problem add the array definition in the import module

from numpy import array

When doing a copy-n-paste from a question I sometimes need to add a array=np.array to the interactive session. — hpaulj
– hpaulj, Commented Oct 10, 2017 at 10:09
@EricDuminil: i have a symbolic expression and I want a method evaluate which will apply np.array on each variable of my expression — parisjohn
– parisjohn, Commented Oct 10, 2017 at 16:32

JuniorCompressor · Accepted Answer · 2017-10-10 08:53:13Z

2

Representations are of the form: array([1, 1, 1, 1]). So we need an array definition imported. So the following should work:

from numpy import array
a = array([1] * 4)
b = array([2] * 4)
res = repr(a) + ' + ' + repr(b)
eval(res)

Result:

array([3, 3, 3, 3])

answered Oct 10, 2017 at 8:53

JuniorCompressor

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3 Comments

parisjohn Over a year ago

thanks a lot for your prompt answer, it was the same kind of answer in this link : stackoverflow.com/questions/35750639/… but I had not understood the problem with the array definition

Eric Duminil Over a year ago

eval is dangerous. I wonder if there's not a better way, e.g. with sympy.

parisjohn Over a year ago

I know sympy but I need the symbolic expression graph to make other more complicated stuff like Theano feature (GPU, OpenMP etc...), so I prefer to not use a whole module just to construct an expression tree

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