Remove constant columns with or without NAs

Question

I am trying to get many lm models work in a function and I need to automatically drop constant columns from my data.table. Thus, I want to keep only columns wit

Answer 1

Here is an option:

df[,which(df[,
       unlist(
        sapply(.SD,function(x) length(unique(x[!is.na(x)])) >1))]),
   with=FALSE]

    x
1:  1
2:  2
3:  3
4: NA
5:  5

For each column of the data.table we count the number of unique values different of NA. We keep only column that have more than one value.