如何接受像'2.0'这样的整数，但例如'2.1'作为浮点数？ Python

Question

我试图让用户在输入字母或非整数数字时收到一条错误消息，并且当输入整数时，程序将继续显示该整数的平方和立方。我的老师不希望代码中有任何“中断”或任何 ValueErrors。

print("Squaring and cubing integer program has started") #Introduction of program for user
UserNumber=input("Enter an integer to be raised to the power of 2 and 3: ") #Asks user to input an integer, that will then be raised to the power of 2 and 3
while '.' in UserNumber or UserNumber.isalpha(): #Error-trap to test if input contained letters of the alphabet or contains decimal point
  print("You have not entered an integer. Try again and enter an integer!") #Alert for user to inform that their entry was invalid (not an integer)
  UserNumber=input("Enter an integer to be raised to the power of 2 and 3: ")  #Asks user again to enter an integer, if they did not previously.
print(int(UserNumber), " is the integer you entered.") #Displays to user their inital integer
print(int(UserNumber), " squared is ", int(UserNumber)**2) #Displays to user their input integer squared
print(int(UserNumber), " cubed is ", int(UserNumber) **3 ) #Displays to user their input integer cubed
print("Calculations are now finished.") #Output to show program has ended

Answer 1

2.0 不是整数；它是一个浮动。 Python 中的float 类似于精度有限的标准 IEEE 754 浮点（如果平台支持 IEEE 754 浮点，那么 Python 可能会使用它）。 Python 中的整数具有无限精度（直到内存不足）：

>>> i = 123456789012345678901234567890
>>> i ==  123456789012345678901234567890
True
>>> f = i * 1.0 # make it a float
>>> f.is_integer() # check whether it is a whole number
True
>>> f ==  123456789012345678901234567890
False

切勿混合使用 float 和 int ，除非您知道它不会改变您的情况下的结果，例如 2 == 2.0 但如示例所示，对于较大的数字它可能是错误的。

您的老师可能希望您找到.is_integer() 方法：

>>> 2.0 .is_integer()
True
>>> 2.1 .is_integer()
False

另请参阅，How to check if a float value is a whole number。

要将'2.0' 转换为浮点数，您可以调用float() 函数：

#!/usr/bin/env python3
while True:
    try:
         f = float(input('Enter a number: '))
    except ValueError:
         print('Try again')
    else:
         break # got a number

这是在 Python 中从 stdin 获取数字的正确方法。如果您的老师禁止使用ValueError 和break，那么您可以使用正则表达式：
if re.match("^[+-]? *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?$", input_string):，它接受20e-1 == 2.0 == 2。见Extract float/double value。

要测试输入是否不为空且仅包含十进制数字，即检查它是否为非负整数，如果调用int(input_string) 并捕获@987654339，则可以使用input_string.isdecimal() 方法@ 禁止。见How do I check if a string is a number in Python?。

Answer 2

A) 我不想删除任何数字 B) 我想检查用户输入的不是整数、字母和小数（不是像 2.0、1.0 这样的小数，因为它们分别等于 2,1）

根据你的描述，你可以使用re模块来实现你的工作：

import re


def is_integer(num):
    s = str(num)
    return re.match('^(-?\d+)(\.[0]*)?$', s) is not None


print is_integer(1123123)  # True
print is_integer(1123123.)  # True
print is_integer(1123123.00000)  # True
print is_integer(1123123.00100)  # False

Answer 3

OP 的问题基于课堂作业。这个答案假设解决方案必须符合老师决定的规则。

只需去掉尾随的零，然后去掉最后的小数点：

UserNumber = UserNumber.rstrip('0').rstrip('.')

将此行放在input 语句之后但在执行测试之前：

print("Squaring and cubing integer program has started") #Introduction of program for user
UserNumber=input("Enter an integer to be raised to the power of 2 and 3: ") #Asks user to input an integer, that will then be raised to the power of 2 and 3
UserNumber = UserNumber.rstrip('0').rstrip('.')
while '.' in UserNumber or UserNumber.isalpha(): #Error-trap to test if input contained letters of the alphabet or contains decimal point
    print("You have not entered an integer. Try again and enter an integer!") #Alert for user to inform that their entry was invalid (not an integer)
    UserNumber=input("Enter an integer to be raised to the power of 2 and 3: ")  #Asks user again to enter an integer, if they did not previously.
    UserNumber = UserNumber.rstrip('0').rstrip('.')
print(int(UserNumber), " is the integer you entered.") #Displays to user their inital integer
print(int(UserNumber), " squared is ", int(UserNumber)**2) #Displays to user their input integer squared
print(int(UserNumber), " cubed is ", int(UserNumber) **3 ) #Displays to user their input integer cubed
print("Calculations are now finished.") #Output to show program has ended

Answer 4

x = 2.0
try:
    x = int(x)
except ValueError:
    pass
return x

这是 Python 中唯一可接受的方式，如果你的老师不理解异常，他就不应该教 Python 编程（老实说，他根本不应该教编程）