python - Pandas:如何创建一个列，该列指示值何时出现在另一列中预先设定的行数？

Question

我试图确定如何创建一个列，该列预先指示(X 行)何时在另一列中下一次出现值时，pandas 本质上执行以下功能(在本例中 X = 3):

df

rowid  event   indicator
1      True    1 # Event occurs
2      False   0
3      False   0
4      False   1 # Starts indicator
5      False   1
6      True    1 # Event occurs
7      False   0

除了对每一行进行迭代/递归循环之外:

i = df.index[df['event']==True]
dfx = [df.index[z-X:z] for z in i]
df['indicator'][dfx]=1
df['indicator'].fillna(0)

然而，这似乎效率低下，是否有更简洁的方法来实现上述示例？谢谢

Answer 1

一个 pandas和 numpy解决方案:

# Make a variable shift:
def var_shift(series, X):
    return [series] + [series.shift(i) for i in range(-X + 1, 0, 1)]

X = 3
# Set indicator to default to 1
df["indicator"] = 1

# Use pd.Series.where and np.logical_or with the 
#  var_shift function to get a bool array, setting
#  0 when False
df["indicator"] = df["indicator"].where(
    np.logical_or.reduce(var_shift(df["event"], X)),
    0,
)

#    rowid  event  indicator
# 0      1   True          1
# 1      2  False          0
# 2      3  False          0
# 3      4  False          1
# 4      5  False          1
# 5      6   True          1
# 6      7  False          0

In [77]: np.logical_or.reduce(var_shift(df["event"], 3))
Out[77]: array([True, False, False, True, True, True, nan], dtype=object)