CODEWITHSUNDEEP
pythonsympy
Alderven
Alderven

Reputation: 8260

Python: interpret expression as a text and not calculate it

After a working with SyMpy library I receive an expression (in yprime variable).

from sympy import *
x = Symbol('x')
y = 1 - (0.1 * coeff1) / (x + 2) - sin(x) * (2 * x + coeff1)
yprime = y.diff(x)

Then I try to use yprime in calculations, but Python interpret it as a text: -(2*x + 1.0)*cos(x) - 2*sin(x) + 0.1/(x + 2)**2

How may I calculate -(2*x + 1.0)*cos(x) - 2*sin(x) + 0.1/(x + 2)**2 ?

After some manipulations according to mtadd:

from sympy import *
x, coeff1 = symbols('x coeff1')
y = 1 - (0.1 * coeff1) / (x + 2) - sin(x) * (2 * x + coeff1)
yprime = y.diff(x).evalf(subs={x:0,coeff1:1})

I receive a digital result, but still it's unable to operate with further logic. It says:

TypeError: can't convert expression to float

Upvotes: 1

Views: 594

Answers (2)

mtadd
mtadd

Reputation: 2555

Call the method subs on your symbolic expression to calculate an analytic expression. You can call evalf on the analytic expression to a numeric solution in a Python float. Below is some example output from isympy after entering your input from above (coeff1 = 1.0).

In [18]: yprime
Out[18]:
                                   0.1
-(2⋅x + 1.0)⋅cos(x) - 2⋅sin(x) + ────────
                                        2
                                 (x + 2)

In [19]: yprime.subs(x,2*pi)
Out[19]:
                0.1
-4⋅π - 1.0 + ──────────
                      2
             (2 + 2⋅π)

In [20]: yprime.subs(x,2*pi).evalf()
Out[20]: -13.5649131254944

The following program:

from sympy import *
x, C1 = symbols('x C1')
y = 1-(0.1*C1)/(x+2)-sin(x)*(2*x+C1)
print y.diff(x).evalf(subs={x:pi/2,C1:1})

prints -1.99215722345589 with sympy version 0.7.2 and python 2.7.3.

Upvotes: 2

Jake Schievink
Jake Schievink

Reputation: 409

I think this might help Evaluating a mathematical expression in a string I think yprime is being interpreted as a string and this first answer of the post describes how to evaluate a string as an arithmetic expression.

Upvotes: 0

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