从 pandas 内部获取上一行值apply()函数 [英] Getting previous row values from within pandas apply() function
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问题描述
import pandas as pd
def greater_or_less(d):
if d['current'] > d['previous']:
d['result']="Greater"
elif d['current'] < d['previous']:
d['result']="Less"
elif d['current'] == d['previous']:
d['result']="Equal"
else:
pass
return d
df=pd.DataFrame({'current':[1,2,2,8,7]})
# Duplicate the column with shifted values
df['previous']=df['current'].shift(1)
df['result']=""
df=df.apply(greater_or_less,axis=1)
结果是:
current previous result
1 NaN
2 1 Greater
2 2 Equal
8 2 Greater
7 8 Less
然后我将删除previous
列,因为它不再需要了.结束于:
I'd then drop the previous
column as it's no longer needed. Ending up with:
current result
1
2 Greater
2 Equal
8 Greater
7 Less
如何在不添加额外列的情况下执行此操作?
How can I do this without adding the extra column?
我想做的是知道如何从greater_or_less
函数中引用上一行的值.
What i'd like to do, is know how to reference the previous row's value from within the greater_or_less
function.
推荐答案
使用diff()
方法:
import pandas as pd
import numpy as np
df=pd.DataFrame({'current':[1,2,2,8,7]})
np.sign(df.current.diff()).map({1:"Greater", 0:"Equal", -1:"Less"})
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