Python简单浮点除法:不准确

Question

所以我对编程很新,但我只是在研究一个简单的计算器.当我启动程序并尝试除法部分(试图将5除以2)时,输出为3.0.这两个数字是浮点数,所以我真的不明白为什么这不起作用.其次,乘法也给出了错误的答案.

from math import *

while True:

print("Options:")
print("Enter 'add' to add two numbers")
print("Enter 'subtract' or '-' to subtract two numbers")
print("Enter 'multiply' to multiply two numbers")
print("Enter 'divide' to divide two numbers")
print("Enter 'quit' to end the program")
user_input = input(": ")

if user_input == "quit":
    print ("Calculator stopped.")
    break
elif user_input == "subtract" or "-":
    num1 = float(input("num1: "))
    num2 = float(input("num1: "))
    print(num1 - num2)
elif user_input == "multiply" or "*":
    num1 = float(input("num1: "))
    num2 = float(input("num1: "))
    print(">> ", num1 * num2," <<")
elif user_input == "divide" or "/":
    num1 = float(input("num1: "))
    num2 = float(input("num1: "))
    sum = num1 / num2
    print(str(float(num1)/num2))
else:
    print("Unknown command")

顺便说一句,我使用的是Python 3.6.1.

Answer 1

这不符合你的想法:

elif user_input == "subtract" or "-":

它的工作原理就好像按如下方式分组:

elif (user_input == "subtract") or "-":

无论值如何user_input,此条件将评估为True(因为"-"非空且因此为True)并且将执行减法.

(试图将5除以2),输出为3.0

那是因为5减2是3.代码减去.

你想要更像的东西:

from math import *

while True:

    print("Options:")
    print("Enter 'subtract' or '-' to subtract two numbers")
    print("Enter 'multiply' to multiply two numbers")
    print("Enter 'divide' to divide two numbers")
    print("Enter 'quit' to end the program")
    user_input = input(": ")

    if user_input == "quit":
        print ("Calculator stopped.")
        break
    elif user_input in ( "subtract", "-"):
        num1 = float(input("num1: "))
        num2 = float(input("num1: "))
        print(num1 - num2)
    elif user_input in ("multiply", "*"):
        num1 = float(input("num1: "))
        num2 = float(input("num1: "))
        print(">> ", num1 * num2," <<")
    elif user_input in ("divide", "/"):
        num1 = float(input("num1: "))
        num2 = float(input("num1: "))
        print(num1/num2)
    else:
        print("Unknown command")