CODEWITHSUNDEEP
pythonpython-3.xsympy
homocomputeris
homocomputeris

Reputation: 509

How to collect fractions in SymPy?

In SymPy 1.3 I have some linear combinations like

N1*(-sqrt(15)/36 + 5/36) - 5*N2/18 + N3*(sqrt(15)/36 + 5/36)

How do I force the fractions to be (5-sqrt(15))/36?

Upvotes: 1

Views: 2118

Answers (2)

user6655984
user6655984

Reputation:

Apply together, which can be called either as a function together(expr) or a method expr.together(). If it's applied to the entire expression, the result may be too much collecting: 

N1, N2, N3 = symbols('N1 N2 N3')
expr = N1*(-sqrt(15)/36 + S(5)/36) - 5*N2/18 + N3*(sqrt(15)/36 + S(5)/36)
print(together(expr))

prints (N1*(-sqrt(15) + 5) - 10*N2 + N3*(sqrt(15) + 5))/36.

Here is a version that selectively applies together to the coefficient of each symbol.

for sym in expr.free_symbols:
    expr = expr.xreplace({expr.coeff(sym): together(expr.coeff(sym))})
print(expr)

prints N1*(-sqrt(15) + 5)/36 - 5*N2/18 + N3*(sqrt(15) + 5)/36.

Upvotes: 3

Bill Bell
Bill Bell

Reputation: 21643

This might be what you want.

Use sympy's Rational operator to force it to retain 5/36 in fractional form.

>>> from sympy import *
>>> N1 = 2
>>> N2 = 5
>>> N3 = 7
>>> (N1*(-sqrt(15)/36 + Rational(5, 36)) - 5*N2/18 + N3*(sqrt(15)/36 + Rational(5,36)))
-0.138888888888889 + 5*sqrt(15)/36
>>> import sympy
>>> sympy.__version__
'1.3'

Upvotes: 0

Related Questions