如何让程序理解输入

Question

我正在制作一个可以加减乘除代数项的计算器。我已经制作了一个可以“制作”代数项的 class，现在我希望计算机向我询问代数表达式，读取它，然后将其注册为一个。（我真的不确定正确的措辞，请原谅，我是编码新手。）

# Make a calculator that adds, subtracts, multiplies, and divides algebraic expressions.
# An algebraic expression has: a coefficient, a variable, and a power

# Class that makes the algebraic expression
class ExpressionMaker:

    # Defines the coefficient, variable, and power of the algebraic term.
    def __init__(self, coefficient, variable, power):
        self.coefficient = coefficient
        self.variable = variable
        self.power = power

    # Types down/returns/defines? the whole algebraic expression as one term.
    def term(self):
        return "{}{}^{}".format(self.coefficient, self.variable, self.power)


# Examples of algebraic terms of varying coefficients, variables, and powers.
expression_1 = ExpressionMaker(7, "x", 1)
expression_2 = ExpressionMaker(4, "y", 1)
expression_3 = ExpressionMaker(-3, "a", 2)

# Make the program understand what the user inputs and convert it into an algebraic expression.
# Make the program add the algebraic expressions from the user.
# An algebraic term has: a coefficient, a variable, and a power.
# Let the program check if the input has those 3 and if they are in the right order.

expression_holder1 = input("What is your first algebraic expression?: ")
print("You first algebraic expression is: " + expression_holder1)

我真的不知道这之后该怎么办。我能想到的最多的是使用“If 语句”来检查 expression_holders 是否有整数（对于系数）、字符串（对于变量），我不知道该怎么做检查电源。我也不知道如何检查顺序是否正确。例如，正确的输入应该是 7x^3，但是如果他们输入 x7^3 会怎样呢？如果输入错误，我如何让用户知道并让他们重新输入？

Answer 1

此解决方案使用正则表达式将系数和变量分开。使用 parition 检索电源。如果给定输入的格式不正确，它会打印错误。

import re #adds regular expression module

#Code for class can go here

while True:
    try:  
        expression_holder1 = input("What is your first algebraic expression?: ")
        
        expression = expression_holder1.partition("^") #Paritions the expression using the ^, the power if stored in the 2nd index
        coefficient_variable = expression[0] #THe coefficient and varible is stored in the first index
        res = re.split('(\d+)', coefficient_variable) #Uses regular expression to split the character and number
        power = expression[2] # Stores the value of given power
        coefficient = res[1] # store the value of coefficient 
        variable = res[2] # Stores the varible given
                  
        if power.isdigit() == False or coefficient.isdigit() == False or variable.isalpha() == False:
            print("Wrong input") #prints error if the incorrect format is given
        else:
            break

    except IndexError: #prints error if a coefficient, variable or power is missing
        print("Index Error -> Wrong input")
        
expression1 = ExpressionMaker(coefficient, variable, power)
print(expression1.term())

如何让程序理解输入

How do I let the program understand the input

python

math

calculator

algebra

python-3.x