python - 在第 n 行使用 enumerate 做一些事情

Question

我有两列，我想比较每 n 行。如果遇到第 n 行，它将比较它们并将 if 语句的结果放入新列中。

当我尝试枚举函数时，它总是以 if 语句的 true 部分结束。不知怎的，这段代码总是有效的:

如果(计数% 3)== 0:

for count, factors in enumerate(df.index):
    if (count % 3)== 0: #every 3th row
        df['Signal']=np.where(df['Wind Ch']>=df['Rain Ch'],'1', '-1')
    else:
        df['Signal']=0

在“信号”列中，我预计每第三行有一个“1”或“-1”，所有其他行上有​​一个“0”。但是我在每行上得到“1”或“-1”

现在我得到:

            Date  Wind CH Rain CH  Signal
    0   5/10/2005  -1.85%  -3.79%       1
    1   5/11/2005   1.51%  -1.66%       1
    2   5/12/2005   0.37%   0.88%      -1
    3   5/13/2005  -0.81%   3.83%      -1
    4   5/14/2005  -0.28%   4.05%      -1
    5   5/15/2005   3.93%   1.79%       1
    6   5/16/2005   6.23%   0.94%       1
    7   5/17/2005  -0.08%   4.43%      -1
    8   5/18/2005  -2.69%   4.02%      -1
    9   5/19/2005   6.40%   1.33%       1
    10  5/20/2005  -3.41%   2.38%      -1
    11  5/21/2005   3.27%   5.46%      -1
    12  5/22/2005  -4.40%  -4.15%      -1
    13  5/23/2005   3.27%   4.48%      -1

但我想得到:

              Date  Wind CH Rain CH  Signal
     0   5/10/2005  -1.85%  -3.79%     0.0
     1   5/11/2005   1.51%  -1.66%     0.0
     2   5/12/2005   0.37%   0.88%    -1.0
     3   5/13/2005  -0.81%   3.83%     0.0
     4   5/14/2005  -0.28%   4.05%     0.0
     5   5/15/2005   3.93%   1.79%     1.0
     6   5/16/2005   6.23%   0.94%     0.0
     7   5/17/2005  -0.08%   4.43%     0.0
     8   5/18/2005  -2.69%   4.02%    -1.0
     9   5/19/2005   6.40%   1.33%     0.0
     10  5/20/2005  -3.41%   2.38%     0.0
     11  5/21/2005   3.27%   5.46%    -1.0
     12  5/22/2005  -4.40%  -4.15%     0.0
     13  5/23/2005   3.27%   4.48%     0.0

我在这里缺少什么？

Answer 1

你可以像这样使用np.vectorize避免循环:

import numpy as np

def calcSignal(x, y, i):
    return 0 if (i + 1) % 3 != 0 else 1 if x >= y else -1

func = np.vectorize(calcSignal)

df['Signal'] = func(df['Wind CH'], df['Rain CH'], df.index)

df

         Date  Wind CH Rain CH  Signal
0   5/10/2005  -1.85%  -3.79%       0
1   5/11/2005   1.51%  -1.66%       0
2   5/12/2005   0.37%   0.88%      -1
3   5/13/2005  -0.81%   3.83%       0
4   5/14/2005  -0.28%   4.05%       0
5   5/15/2005   3.93%   1.79%       1
6   5/16/2005   6.23%   0.94%       0
7   5/17/2005  -0.08%   4.43%       0
8   5/18/2005  -2.69%   4.02%      -1
9   5/19/2005   6.40%   1.33%       0
10  5/20/2005  -3.41%   2.38%       0
11  5/21/2005   3.27%   5.46%      -1
12  5/22/2005  -4.40%  -4.15%       0
13  5/23/2005   3.27%   4.48%       0