python - Create new column in pandas dataframe by calculation from previous index columns -


hi new , learning pandas data analysis. have 2 columns data

a   b 1   2    2   3 3   4 4   5

i want create third column c result calculated column b , subtracting upper value current 1 , dividing current.

a   b   c 1   2    2   3   0.33 3   4   0.25 4   5   0.2

for example first row value c column empty because there no value above 2 . 

0.33 = > 3 - 2 / 3 ,  0.25 = > 4 - 3 / 4 , 0.2 = > 5 - 4 / 5 , on

i stuck while getting upper value of current column. need how achieve that.

use shift shift column , remaining operations regular ones (sub , div):

df['b'].sub(df['b'].shift()).div(df['b']) out:  0         nan 1    0.333333 2    0.250000 3    0.200000 name: b, dtype: float64

this can done without chaining methods, if prefer.

(df['b'] - df['b'].shift()) / df['b'] out[48]:  0         nan 1    0.333333 2    0.250000 3    0.200000 name: b, dtype: float64

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