sympy sub( )

adaped from  https://vimsky.com/zh-tw/examples/usage/python-sympy-subs-method.html

method  ==> sympy.subs(source, destination)

返回：Return the same expression by changing the variable.

 
from sympy import *
x,y=symbols("x,y")
formu=2*x+3*y
print(formu)
reformu=formu.subs({x:2,y:3})
print(reformu)

結果

2*x + 3*y 

13 

==================================

 
from sympy import *
x,y,z=symbols("a,b,c")
print(z.name)
formu=3*x+2*y+z
print(formu)
result=formu.subs({x:3,y:2,z:1})
result2=formu.subs({a:3,b:2,c:1})
print(result)
print(result2)

結果是

c

3*a + 2*b + c

14

14

=====將字串轉為數學表達式

from sympy import *
x=Symbol("x")
formula=input("請輸入公式\n")
eq=sympify(formula)          #將字串轉為數學表達式
value=int(input("請輸入值"))
print(eq)
result=eq.subs({x:value})   #將數值代入公式
print(result)