interate over python pandas dataframe from last row to first

Question

I have a dataframe with statistical data that is cumulated with each new row. Every day a new row is added. Now I want to iterate over my column so that each row (starting from last) is substracted from the row above. The new value should be put into a new column. This is how my dataframe looks and the values in the column 'diff' are my desired outcome:

     time        In     diff
0  2017-06-26  7.086      
1  2017-06-27  8.086     1
2  2017-06-28  10.200    2.114

this is what I came up with:

for x in df['In']:
    df['diff'] = df.iloc[-1] - df.iloc[-2]

but thats not it. How do I start the loop from the last row and how do I make the iloc more dynamic? Can someone help? thank you!

Answer 1

You can use Series.diff :

df['diff'] = df['In'].diff()
print (df)
         time      In   diff
0  2017-06-26   7.086    NaN
1  2017-06-27   8.086  1.000
2  2017-06-28  10.200  2.114

Answer 2

使用pd.Series.diff

df.assign(Diff=df.In.diff())

Answer 3

This can be done using shift() :

df
       In        time
0   7.086  2017-06-26
1   8.086  2017-06-27
2  10.200  2017-06-28


df.sort_values('time', inplace=True)

df['diff'] = df['In'] - df['In'].shift(1)

df
       In       time   diff
0   7.086 2017-06-26    NaN
1   8.086 2017-06-27  1.000
2  10.200 2017-06-28  2.114

Answer 4

Here is all you need to do.

 df['diff'] = df.In - df.In.shift(1)

 # In [16]: df
 # Out[16]:
 #        time      In   diff
 # 0  2017-06-26   7.086    NaN
 # 1  2017-06-27   8.086  1.000
 # 2  2017-06-28  10.200  2.114