pandas - read and write array at the same time python -

i want recalculate column "a" of given dataframe = df. way of doing not fill in new, calculated values on old ones.

 import pandas pd  import numpy np  numpy.random import randn   df = pd.dataframe(randn(100))  df["a"] = np.nan  df["b"] = randn()  df.a[0] = 0.5   df.a= df.a.shift(1) * df.b 

do have ideas how can fill solve that?

i want calculate "a" depending on previous value "b":

            b            0.5      2            #set starting value df.a[0] = 0.5  since there no value prior that, there's no calculation performed.      1.5      3            # = previous value of *b (0.5*3) =1.5      15       10           # = previous value of *b (1.5*10) =15      45       3            # = previous value of *b (15*3) =45 

the problem calcation not perform / results of calcutation not overwrite set values.

how this?

df = pd.dataframe({'a': [none] * 4, 'b': [2, 3, 10, 3]}) df.a.iloc[0] = 0.5 df.a.iloc[1:] = (df.b.shift(-1).cumprod() * df.a.iat[0])[:-1].values  >>> df        b 0  0.5   2 1  1.5   3 2   15  10 3   45   3