来自变量的数学符号

Question

I have a challenge at school, but i am stuck with a small bit of my code.

Here's what I have; what I'm missing is how to finish the answer = line:

while (questions <= 9):
    randoms()
    print("What is: ", num1, symbol, num2,"?:")
    uanswer = int(input())
    answer = 
    if uanswer == answer:
        correct = correct + 1
    else:
        incorrect = incorrect + 1
    questions = questions + 1

The variables num1 , num2 and symbol are randomly generated, num1 and num2 are both integers, and symbol is either '+' , '-' , '*' or '/' I was wondering how to use these to get an answer to an equation.

For example, num1 is 50 , symbol is '-' , and num2 is 25 . How would I use the variables to get the answer 25 ?

Answer 1

The clean way to do this is to create a mapping from symbols to functions.

The only question is, what is the function behind the + operator?

The answer is in the operator module: a + b is operator.add(a, b) , and so on. So:

import operator

symbols = {
    '+': operator.add,
    '-': operator.sub,
    '*': operator.mul,
    '/': operator.floordiv
}

I'm assuming here that you want 100 / 3 to be 33 , not 33.333333333333336 . If I'm wrong, use truediv instead of floordiv above. (See Numbers in the tutorial for an introduction to the difference, with links to more details.)

Anyway, now, you can do this:

answer = symbols[symbol](num1, num2)

---

Even if you didn't have operator , you could always build a function for each operator:

symbols = {
    '+': (lambda a, b: a + b),
    '-': (lambda a, b: a - b),
    '*': (lambda a, b: a * b),
    '/': (lambda a, b: a // b)
}

This may come in handy if you want to extend your language to a symbol that doesn't have a matching operator in Python.

---

If your maths language is a strict subset of Python's expression language (which it may not be—notice that the symbol '/' maps to the Python operator // if you want integers), and you're sure that you're never going to extend it in a way that isn't (eg, using ^ for exponentiation), and you're sure that you will never be using any numbers or symbols that you didn't generate yourself, it may be simpler to just build a Python expression and eval it:

expr = '{} {} {}'.format(num1, symbol, num2)
answer = eval(expr)

However, you should always be careful about using eval ; if the clean thing to do is simple enough for you to write, and to read and understand later, it's almost always a better answer.

Answer 2

I think the best way is creating conditonals and check the symbol..

if symbol=='-':
   res = num1 - num2
elif symbol=='+':
   res = num1 + num2