[Solved]Append different datasets

Hi everybody ,

Let's assume that I have two datasets , S1 (which the features are a_1,a_2,a_3) and S2 which the features are (a_1,a_2,a_3,a_4)

a_1 , a_2 , a_3 are of similar types and roles , but the only difference is that S2 has one more attribute a_4 ,

how can I append these two datasets into 1 dataset (with a_1,a_2,a_3,a_4) having missing values of a_4 attribute for instances of S1  ?

(is there a way to ADD an attribute to a dataset with missing values ?)

Thanks ,
Arian