编程模拟一个简单的计算器，从键盘按照a+或减乘除b的格式输入一个表达式，其中a和b是实数。

你可以通过编写一个简单的命令行程序来模拟一个基本的计算器。下面是一个使用Python编写的示例：

def addsubtract(num1, operator, num2):
    if operator == '+':
        return num1 + num2
    elif operator == '-':
        return num1 - num2
    else:
        raise ValueError("Invalid operator. Only '+' and '-' are supported.")

def multiplydivide(num1, operator, num2):
    if operator == '*':
        return num1 * num2
    elif operator == '/':
        if num2 != 0:
            return num1 / num2
        else:
            raise ZeroDivisionError("Cannot divide by zero.")
    else:
        raise ValueError("Invalid operator. Only '*' and '/' are supported.")

try:
    expression = input("请输入表达式 (格式: a+/-*/*b): ")
    parts = expression.split()
    a = float(parts[0])
    operator = parts[1]
    b = float(parts[2])

    result = None
    if len(parts) == 3:
        result = addsubtract(a, operator, b)
    elif len(parts) == 4:
        result = multiplydivide(a, operator, b)

    print(f"计算结果: {result}")

except ValueError as ve:
    print("错误:", str(ve))
except ZeroDivisionError as zde:
    print("错误:", str(zde))