在应用函数中使用 shift() 函数来比较 Pandas Dataframe 中的行
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Using a shift() function within an apply function to compare rows in a Pandas Dataframe
提问by user2242044
I would like to use shift()to pull in data from the previous index, provided values in one of the columns, Letter, is the same.
我想使用shift()从前一个索引中提取数据，前提是其中一列中的值Letter相同。
import pandas as pd
df = pd.DataFrame(data=[['A', 'one'],
['A', 'two'],
['B', 'three'],
['B', 'four'],
['C', 'five']],
columns=['Letter', 'value'])
df['Previous Value'] = df.apply(lambda x : x['value'] if x['Letter'].shift(1) == x['Letter'] else "", axis=1)
print df
I am getting the error:
我收到错误：
AttributeError: ("'str' object has no attribute 'shift'", u'occurred at index 0')
Desired Output:
期望输出：
Letter value Previous Value
0 A one
1 A two one
2 B three
3 B four three
4 C five
回答by EdChum
Use whereon your condition where the current row matches previous row using shift:
使用where您的病情在当前行使用前一行匹配shift：
In [11]:
df = pd.DataFrame(data=[['A', 'one'],
['A', 'two'],
['B', 'three'],
['B', 'four'],
['C', 'five']],
columns=['Letter', 'value'])
?
df['Previous Value'] = df['value'].shift().where(df['Letter'].shift() == df['Letter'], '')
df
?
Out[11]:
Letter value Previous Value
0 A one
1 A two one
2 B three
3 B four three
4 C five
回答by Dmitry Andreev
You are trying to apply .shift() to a value of a given column of a given row instead of a Series. I would do this, using groupby:
您正在尝试将 .shift() 应用于给定行的给定列的值，而不是系列。我会这样做，使用 groupby：
In [6]: df['Previous letter'] = df.groupby('Letter').value.shift()
In [7]: df
Out[7]:
Letter value Previous letter
0 A one NaN
1 A two one
2 B three NaN
3 B four three
4 C five NaN
