如何将列中的值作为索引去取值

Question

我将符合条件的索引值赋值给了A，现在要根据A的值作为索引取得price的值，然后赋值给expected列，但我不知道该怎么调用

源数据

import pandas as pd
import numpy as np
d={'trade_date':['2021-08-10','2021-08-11','2021-08-12','2021-08-13','2021-08-14','2021-08-15','2021-08-16','2021-08-17','2021-08-18','2021-08-19','2021-08-20',],'price':[2,12,8,10,11,18,7,19,9,8,12],'cond':[True,False,True,False,True,False,True,False,True,True,True]}
df = pd.DataFrame(d)
df.index=pd.to_datetime(df.trade_date)
df['A']=df.index.where(df['cond'])
df['A']=df['A'].ffill()
df.to_clipboard()
df

如何计算expected的值

trade_date price   cond    A   expected
2021/8/10   2   TRUE    2021/8/10   2 
2021/8/11   12  FALSE   2021/8/10   2 
2021/8/12   8   TRUE    2021/8/12   8 
2021/8/13   10  FALSE   2021/8/12   8 
2021/8/14   11  TRUE    2021/8/14   11 
2021/8/15   18  FALSE   2021/8/14   11 
2021/8/16   7   TRUE    2021/8/16   7 
2021/8/17   19  FALSE   2021/8/16   7 
2021/8/18   9   TRUE    2021/8/18   9 
2021/8/19   8   TRUE    2021/8/19   8 
2021/8/20   12  TRUE    2021/8/20   12 

Answer 1

df['expected'] = df['A'].map(df['price'])