How do you evaluate only one part of an equation in python?

Question

import math

term = ['math.sqrt', 'math.pi', 'math.sin', 'math.cos', 'math.tan', 
'math.log', 'math.log', 'math.pow', 'math.cosh', 'math.sinh', 
'math.tanh', 'math.sqrt', 'math.pi', 'math.radians', 
'math.e','math.radians']
replace = ['√', 'π', 'sin', 'cos', 'tan', 'log', 'ln', 'pow', 'cosh', 
'sinh', 'tanh', 'sqrt', 'pi', 'radians', 'e', 'rad']

equation = input('')

for word in replace:
    equation = equation.replace(word, term[replace.index(word)])

I am trying to evaluate only the calculations in 'equation' that are also found in the 'term' and afterwards replace the values that we found in 'equation'

Ex. input: x + 5 - sqrt(4) = 9

then my program will replace sqrt(4) with math.sqrt(4)

then it should calculate math.sqrt(4); so math.sqrt(4) = 2

and lastly replace the 2 with sqrt(4) in 'equation'; so, equation = x + 5 - 2 = 9

score 0 · Accepted Answer · answered Jun 01 '20 at 10:30

I can help you, but with using an eval statement, which is generally considered somewhat of bad practice

The solution, in one line, is:

' '.join([str(eval(s)) if ('(' in s and ')' in s) else s for s in equation.split(' ')])

Let's break it down:

equation.split(' ') splits the equation string into a list of strings separated by an empty space: ['x', '+', '5', '-', 'math.sqrt(4)', '=', '9']
[str(eval(s)) if ('(' in s and ')' in s) else s for s in equation.split(' ')] runs str(eval(s)) on any element of the list that satisfies the condition ('(' in s and ')' in s) (I'm assuming that you only open and close parentheses on the function itself).
str(eval(s)) evaluates the function as it is written, and then casts it back to string.
' '.join([...]) joins the list elements, putting the spaces back in place.

The results comes out as:

'x + 5 - 2.0 = 9'

Just remember putting your spaces in the right places in the input.

How do you evaluate only one part of an equation in python?

1 Answers1