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pythonlatexsympytex
Geddes
Geddes

Reputation: 1173

SymPy automatically processes expressions

I have been using SymPy to convert expressions into latex (to then be rendered by Matplotlib). e.g.

from sympy import latex, sympify
from sympy.abc import x

str = '2*x + 3*x'

TeX = latex(sympify(str))

The problem is that it automatically processes the expression, so 2*x + 3*x automatically becomes 5*x etc; which is not what I want (don't ask!).

Upvotes: 4

Views: 1717

Answers (3)

Edoot
Edoot

Reputation: 421

>>> import re

>>> re.sub("(^|[\(\[\{\+\-\*/ ,\:=])(\-?[0-9]*\.?[0-9]+)", "\\1Dummy('\\2')", '2*x + 3*x')

"Dummy('2')*x + Dummy('3')*x"

>>> eval(_)

2⋅x + 3⋅x

>>> latex(_)

'2 x + 3 x'

Upvotes: -1

gsagrawal
gsagrawal

Reputation: 3020

Actually when you call sympify(str) it tries to parse expression and convert them into the default classes. In this case Add(2*x,2*x) will be called (with default parameter evalaute=True) so this became 5*x. If you want to avoid it either you have to call Add(2*x,3*x,evaluate=False) or use some global variable and check in init method of AssocOp class in core-> operation.py

i am doing this

 try:
        import __builtin__
        evaluate_expr=__builtin__.evaluate_expr
    except AttributeError ,ex:
        pass 

    if ((not options.pop('evaluate', True)) or (evaluate_expr==False)) :

**Note - sympy use caching for function , so if you call same function (say :sympy("2*x+3*x") )two times .1st time with your gloabl variable evalute=True and 2nd time with evaluate=False. in both cases becuase of caching you will get same result. So you need to update methods like (add ,mul ) in core->expr class. something like below

def __add__(self, other):
        #simplifychange:
        evaluate_expr=self.get_evaluate()
        return Add(self, other,evaluate=evaluate_expr)

But i will suggest it would be better if you dont use evaluate=False. Behavior of sympy will change dramatically. problems like i was facing in this post

Upvotes: 1

Gary Kerr
Gary Kerr

Reputation: 14420

Sympy's Add class handles the addition of symbols. You can provide a keyword argument to stop the automatic collection of terms.

from sympy import Add
from sympy.abc import x

eq = Add(2*x, 3*x, evaluate=False)

# this will print: 2*x + 3*x
print eq

This may not be exactly what you want based on your reply to phimuemue's comment.

Upvotes: 6

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