如何对多个值测试一个变量？ [英] How do I test one variable against multiple values?

问题描述

我试图创建一个函数，将多个变量比较一个整数，并输出一个三个字母的字符串。我想知道是否有一种方法来将它翻译成Python。所以说：

  x = 0 
y = 1 
z = 3 
 Mylist = [] 
 
如果x或y或z == 0：
 Mylist.append（c）
 elif x或y或z == 1：
 Mylist。 append（d）
 elif x或y或z == 2：
 Mylist.append（e）
 elif x或y或z == 3：
 Mylist.append（f）
  

会返回

  [c，d，f] 
  

$ b

解决方案

你误解了布尔表达式是如何工作的？他们不像英语句子那样工作，并猜测你在这里对所有名字都进行了相同的比较。您正在寻找：

 如果x == 1或y == 1或z == 1：
  

x 和 y 否则自己评估（ False 如果 0 ， True

 <$ c $ <$ p> 
 c> if（x，y，z）中的1：
  

 如果在{x，y，z}中为1：
  

使用设置以利用常量成本测试（ in

当使用或时，python将操作符的每一侧视为独立的表达式。表达式 x或y == 1 被视为首先对 x 的布尔检验，如果是False，

这是由于 docs.python.org/2/reference/expressions.html#operator-precedence\">运算符优先顺序。 或运算符的优先级比 == 测试的优先级低，因此，

但是，即使这不是的情况，表达式 x或y或z = = 1 实际上被解释为（x或y或z）== 1 ，但这仍然不会做你期望的做。

x或y或z 会计算第一个参数为truey不是 False ，数字0或空（请参阅布尔表达式，以了解Python在布尔上下文中认为是什么的细节。）

所以对于 x = 2; y = 1; z = 0 ， x或y或z 会解析为 2 参数中的第一个真实值。那么 2 == 1 将是 False ，即使 y == 1 将 True 。

I'm trying to make a function that will compare multiple variables to an integer and output a string of three letters. I was wondering if there was a way to translate this into Python. So say:

x = 0
y = 1
z = 3
Mylist = []

if x or y or z == 0 :
    Mylist.append("c")
elif x or y or z == 1 :
    Mylist.append("d")
elif x or y or z == 2 :
    Mylist.append("e")
elif x or y or z == 3 : 
    Mylist.append("f")

which would return a list of

["c", "d", "f"]

Is something like this possible?

解决方案

You misunderstand how boolean expressions work; they don't work like an English sentence and guess that you are talking about the same comparison for all names here. You are looking for:

if x == 1 or y == 1 or z == 1:

x and y are otherwise evaluated on their own (False if 0, True otherwise).

You can shorten that to:

if 1 in (x, y, z):

or better still:

if 1 in {x, y, z}:

using a set to take advantage of the constant-cost membership test (in takes a fixed amount of time whatever the left-hand operand is).

When you use or, python sees each side of the operator as separate expressions. The expression x or y == 1 is treated as first a boolean test for x, then if that is False, the expression y == 1 is tested.

This is due to operator precedence. The or operator has a lower precedence than the == test, so the latter is evaluated first.

However, even if this were not the case, and the expression x or y or z == 1 was actually interpreted as (x or y or z) == 1 instead, this would still not do what you expect it to do.

x or y or z would evaluate to the first argument that is 'truthy', e.g. not False, numeric 0 or empty (see boolean expressions for details on what Python considers false in a boolean context).

So for the values x = 2; y = 1; z = 0, x or y or z would resolve to 2, because that is the first true-like value in the arguments. Then 2 == 1 would be False, even though y == 1 would be True.

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