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pythonparsingexpressionsympyequation
shafay
shafay

Reputation: 41

Parsing Symbolic Equations to Sympy functions

I am trying to pass image1 in my code when i copy and paste this as text it looks like this ∑(i=1,n)(a+m*x*(i)-y*(i))^2.

but it does not work.

following is my code which is working with a different syntax:

from sympy import symbols,sympify,solve,Eq,Symbol

from sympy import symbols,Eq,sympify,summation
expr = **('summation((m * x*(i) + a - y*(i))^2, (i, 0, n))')**
Eq1 = Eq(sympify(expr))
print(Eq1)
values = {2001,10,2,3,5}
arr_symbols = list(Eq1.free_symbols)
print(arr_symbols)
Method1(arr_symbols,values,expr)

def Method1(arr_symbols,Values,expr):
    from sympy import symbols, Eq, solve, pprint, integrate, sympify
    z = symbols('z')
    Formula = Eq(sympify(expr),z)
    print(Formula)
    index = 0
    for i in Values:
        Formula = Formula.subs(arr_symbols[index],i)
        index+=1
    print(solve(Formula))

but what i want to do is to use ∑(i=1,n)(a+m*x*(i)-y*(i))^2 and ask sympy to convert it for me.

Upvotes: 4

Views: 1079

Answers (3)

shafay
shafay

Reputation: 41

it cannot be done. we have to pass latex code.

Upvotes: 0

asmeurer
asmeurer

Reputation: 91450

SymPy can represent this equation, but it can only parse Python. You might be able to write extensions to its parser to handle this sort of thing (see https://docs.sympy.org/latest/modules/parsing.html). It should be possible in principle, although it might not be straightforward. I would only recommend doing this the syntax of your expressions is already very close to Python. If it isn't (and it looks like it isn't), it would be better to a real parsing library like ANTLR to build up a grammar for your expressions. You can then use that to parse into SymPy (see for example how the sympy.parsing.latex module is written).

I don't know if there is pre-existing library in Python that handles your types of expressions. I'm not aware of any. At best you might be able to find a grammar that someone has already written, so you don't have to write it yourself.

Upvotes: 1

smichr
smichr

Reputation: 18939

If you are, given a, m and values for x and y and are trying to compute the sum of the squares of the residuals, then it would be better to write a function that does that or do something like this:

>>> x = [1, 2, 3]
>>> y = [4, 7, 11]
>>> a = 2
>>> m = 5
>>> sum((a + m*xi - yi)**2 for xi, yi in zip(x, y))
70

Upvotes: 0

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