python - How to update the value of one record based on another record with specific criteria in the same dataframes -


in python, have dataframes table of gdp records this

quarter    vaule percentage 2017q1-q4  100   18% 2017q1-q3  60    20% 2017q1-q2  30    15% 2017q1-q1  10    10% 2016q1-q4  10    28% 2016q1-q3  6     50% 2016q1-q2  3     45% 2016q1-q1  1     20% 

i want output this:

quarter    vaule percentage 2017q4     40    18% 2017q3     30    20% 2017q2     20    15% 2017q1     10    10% 2016q4     4     28% 2016q3     3     50% 2016q2     2     45% 2016q1     1     20% 

that is, value updated based on computing of other records percentage keeps unchanged.

are there efficient way deal case. thanks!

df.iloc[:-1, 1] = df['vaule'].diff(-1)[:-1] >>> df      quarter  vaule percentage 0  2017q1-q4     40        18% 1  2017q1-q3     30        20% 2  2017q1-q2     20        15% 3  2017q1-q1      0        10% 4  2016q1-q4      4        28% 5  2016q1-q3      3        50% 6  2016q1-q2      2        45% 7  2016q1-q1      1        20% 

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