如何在r中的data.table中评估（或创建）动态列

I want to create a new data.table or maybe just add some columns to a data.table. It is easy to specify multiple new columns but what happens if I want a third column to calculate a value based on one of the columns I am creating. I think plyr package can do something such as that. Can we perform such iterative (sequential) column creation in data.table? I want to create a new data.table or maybe just