python - Pandas 从查找 DataFrame 列中减去 DataFrame 列

Question

给定一个数据框 test和另一个数据框 lookup

test = pd.DataFrame( [['a',1],
                      ['a',2], 
                      ['b',9]], columns = ['id', 'n'])

lookup_mins = pd.DataFrame( [['a',1], 
                             ['b',9], 
                             ['c',7]] , columns = ['id', 'n'])

尝试减去每个值 n在test按值 n与匹配id在lookup_mins使用

s = lookup_mins.groupby(['id'])['n'].transform('min')
test['n2'] = test['n'] - s

预期结果，

id n n2
a  1 0
a  2 1
b  9 0

但是却得到了

id n n2
a  1 0
a  2 -7
b  9  9

如何减去test和lookup_mins得到如上所述的预期结果？

Answer 1

使用Series.map聚合 min:

s = lookup_mins.groupby(['id'])['n'].min()
test['n2'] = test['n'] - test['id'].map(s)
print (test)
  id  n  n2
0  a  1   0
1  a  2   1
2  b  9   0