it is necessary to extract features before or after normalization? I'm doing a kaggle contest and I have discounted price and non-discounted price data in the columns. To get the discount value, I subtract the column with the price to from the column with the price at the discount and then submit it for normalization. Am I doing the right thing or should I first normalize the data and then calculate the discount?

Both are possible. But, if you do after normalization, the new feature loses its meaning. In your case, it has a particular meaning I suppose, i.e. the discount. You may as well want to produce additional features like percentage discount etc. with that. None of these interpretations are possible if you do the normalization first.
