如何获得pandas列的特定索引之间的最大值？

Question

我有一个数据框，其中包含日期索引和正值和负值池

values = [1,2,3,4,-1,-2,-3,10,11,12]
start_date = pd.to_datetime('2019-01-23')
dates = [start_date + datetime.timedelta(days=i) for i in range(0,len(values))]
df = pd.DataFrame(values)
df.columns = ['values'] ; df.index = dates ; df

我想要一个额外的列，其中第一组正值的最大值和第二组负值的最小值，依此类推。

输出应该是这样的

df['values_max'] = np.nan
df.loc['2019-01-26','values_max'] = 4
df.loc['2019-01-29','values_max'] = -3
df.loc['2019-02-01','values_max'] = 12
df

我将不胜感激任何帮助。

Answer 1

使用：

#map positive and negative values to 1, -1
s = np.sign(df['values'])
#create consecutive groups
g = s.ne(s.shift()).cumsum()
#create new columns with consition
df['new'] = df.groupby(g)['values'].transform(lambda x: x.max() if x.max() > 0 else x.min())
#add missing values
df.loc[df['new'] != df['values'], 'new'] = np.nan
print (df)
            values   new
2019-01-23       1   NaN
2019-01-24       2   NaN
2019-01-25       3   NaN
2019-01-26       4   4.0
2019-01-27      -1   NaN
2019-01-28      -2   NaN
2019-01-29      -3  -3.0
2019-01-30      10   NaN
2019-01-31      11   NaN
2019-02-01      12  12.0