pandas "diff()"带线 [英] Pandas "diff()" with string

问题描述

每当列更改其字符串值时，如何标记数据框中的行?

How can I flag a row in a dataframe every time a column change its string value?

例如:

输入

ColumnA   ColumnB
1            Blue
2            Blue
3            Red
4            Red
5            Yellow


#  diff won't work here with strings....  only works in numerical values
dataframe['changed'] = dataframe['ColumnB'].diff()        


ColumnA   ColumnB      changed
1            Blue         0
2            Blue         0
3            Red          1
4            Red          0
5            Yellow       1

推荐答案

使用ne可以获得更好的性能，而不是使用实际的!=比较:

I get better performance with ne instead of using the actual != comparison:

df['changed'] = df['ColumnB'].ne(df['ColumnB'].shift().bfill()).astype(int)

时间

使用以下设置来产生更大的数据帧:

Using the following setup to produce a larger dataframe:

df = pd.concat([df]*10**5, ignore_index=True) 

我得到以下计时:

%timeit df['ColumnB'].ne(df['ColumnB'].shift().bfill()).astype(int)
10 loops, best of 3: 38.1 ms per loop

%timeit (df.ColumnB != df.ColumnB.shift()).astype(int)
10 loops, best of 3: 77.7 ms per loop

%timeit df['ColumnB'] == df['ColumnB'].shift(1).fillna(df['ColumnB'])
10 loops, best of 3: 99.6 ms per loop

%timeit (df.ColumnB.ne(df.ColumnB.shift())).astype(int)
10 loops, best of 3: 19.3 ms per loop

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