Manish - 3 months ago 7

Python Question

I want to be able to define an expression which takes all the values of variable where it is defined and evaluates the expression as **0** when it is not defined.

Similar to this: -

`import numpy as np`

import sympy as sp

def expr(k1, k2):

x, y =sp.symbols('x y')

if x == k1 :

fn = 0

else:

fn = np.divide(1,(x-k1)*(y-k2))

return fn, x, y

f,x, y = expr(1,2)

print(f)

fx = f.subs({x:1,y:4})

print(fx)

So how is the equality or conditionality going to be checked once the function has been defined?

fn = 1/ (x-1)(y-2); How to set it as 0 for x=1 or y=2?

Answer

You should define a function inside your function and then return it. Like this:

```
import numpy as np
import sympy as sp
def expr(k1, k2):
x, y =sp.symbols('x y')
def fn(x, y):
if x==k1:
return 0
else:
return np.divide(1, (x-k1)*(y-k2))
return fn, x, y
f, x, y = expr(1, 2)
print(f(x, y))
print(f(1, 4))
```

**EDIT:**

Here is one way to use `sp.lambdify`

as asked in the comments:

```
x_dot = 1 / ((x - 1) * (y - 2))
f = lambda a, b : 0 if a==1 or b==2 else sp.lambdify((x,y), xdot, "numpy")(a,b)
```

Another option is to use `sp.subs`

```
f = lambda a, b: 0 if a==1 or b==2 else float(x_dot.subs({x:a, y:b}))
```

Source (Stackoverflow)

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