奇怪如果声明 [英] Strange if statement

问题描述

我在别人的代码中找到了这个奇怪的 if -statement：

I found this strange if-statement in somebody else’s code:

if variable & 1 == 0:

我不明白。它应该有两个 == ，对吧？

I don't understand it. It should have two ==, right?

有人可以解释一下吗？

推荐答案

条件是按位运算符比较：

>>> 1 & 1
1
>>> 0 & 1
0
>>> a = 1
>>> a & 1 == 0
False
>>> b = 0
>>> b & 1 == 0
True

---

As许多评论说，对于整数，这个条件对于平均而言是真的，对于赔率是假的。写这个的流行方式是如果变量％2 == 0：或如果不是变量％2：

使用 timeit 我们可以看到性能差异不大。

Using timeit we can see that there isn't much difference in performance.

n& 1 （== 0和不是）

>>> timeit.Timer("bitwiseIsEven(1)", "def bitwiseIsEven(n): return n & 1 == 0").repeat(4, 10**6)
[0.2037370204925537, 0.20333600044250488, 0.2028651237487793, 0.20192503929138184]

>>> timeit.Timer("bitwiseIsEven(1)", "def bitwiseIsEven(n): return not n & 1").repeat(4, 10**6)
[0.18392395973205566, 0.18273091316223145, 0.1830739974975586, 0.18445897102355957]

n％2 （== 0和不是）

>>> timeit.Timer("modIsEven(1)", "def modIsEven(n): return n % 2 == 0").repeat(4, 10**6)
[0.22193098068237305, 0.22170782089233398, 0.21924591064453125, 0.21947598457336426]

>>> timeit.Timer("modIsEven(1)", "def modIsEven(n): return not n % 2").repeat(4, 10**6)
[0.20426011085510254, 0.2046220302581787, 0.2040550708770752, 0.2044820785522461]

重载运营商：

％和& 运算符都被重载。

set 。 s.intersection（t）相当于 s& t 并返回具有s和t共有元素的新集合。

The bitwise and operator is overloaded for set. s.intersection(t) is equivalent to s & t and returns a "new set with elements common to s and t".

>>> {1} & {1}
set([1])

这不会影响我们的条件：

This doesn't effect our conditional:

>>> def bitwiseIsEven(n):
...   return n & 1 == 0

>>> bitwiseIsEven('1')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in bitwiseIsEven
TypeError: unsupported operand type(s) for &: 'str' and 'int'
>>> bitwiseIsEven({1})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in bitwiseIsEven
TypeError: unsupported operand type(s) for &: 'set' and 'int'

模数运算符也将抛出 TypeError：不支持的操作数类型对于大多数非ints。

The modulo operator will also throw TypeError: unsupported operand type(s) for most non-ints.

>>> def modIsEven(n):
...   return n % 2 == 0

>>> modIsEven({1})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in modIsEven
TypeError: unsupported operand type(s) for %: 'set' and 'int'

它作为旧的 ％ - 格式化 。它抛出 TypeError：在字符串格式化期间不是所有参数都被转换如果字符串用于比较。

It is overloaded as a string interpolation operator for the old %-formatting. It throws TypeError: not all arguments converted during string formatting if a string is used for the comparison.

>>> modIsEven('1')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in modIsEven
TypeError: not all arguments converted during string formatting

如果字符串包含有效的转换说明符，则不会抛出。

This won't throw if the string includes a valid conversion specifier.

>>> modIsEven('%d')
False 

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