python - 枚举函数

Question

谁能给我解释一下。

所以，我有一个列表 s，其中的数字从 -1 到 1，我想提取列表中某些数字的位置。

例子:

s= [-1, 0.5, 0.2, -0.9]

z = enumerate(s)
y1 = []
for i,j in z:
    if j<=-0.8 and j>=-1:
        k = i
        y1.append(k)

y2 = []
for i,j in z:
    if j<=0.8 and j>=-0.8:
        k = i
        y2.append(k)

我得到 y1 = [0, 3] 和 y2 = []

但是如果我定义第二个 enumerate:

z1 = enumerate(s)
y1 = []
for i,j in z1:
    if j<=-0.8 and j>=-1:
        k = i
        y1.append(k)

z2 = enumerate(s)
y2 = []
for i,j in z2:
    if j<=0.8 and j>=-0.8:
        k = i
        y2.append(k)

我得到结果 y1 = [0, 3] 和 y2 = [1, 2]

为什么我需要第二个enumerate？

Answer 1

enumerate 返回序列的迭代器。一旦遍历，就不能再次使用:

In [1]: l = [1, 2, 3, 4, 5]

In [2]: e = enumerate(l)

In [3]: e
Out[3]: <enumerate at 0x7fad7aea25e8>

In [4]: list(e)
Out[4]: [(0, 1), (1, 2), (2, 3), (3, 4), (4, 5)]

In [5]: list(e)
Out[5]: []

你可以用一个列表来包装它:

In [6]: e = list(enumerate(l))

In [7]: e
Out[7]: [(0, 1), (1, 2), (2, 3), (3, 4), (4, 5)]

In [8]: list(e)
Out[8]: [(0, 1), (1, 2), (2, 3), (3, 4), (4, 5)]

In [9]: list(e)
Out[9]: [(0, 1), (1, 2), (2, 3), (3, 4), (4, 5)]

您还可以使用列表理解来简化您的解决方案:

y1 = [i for i, x in enumerate(s) if -1 <= x <= -0.8]
y2 = [i for i, x in enumerate(s) if -0.8 <= x <= 0.8]