How to replace first n1 and last n2 rows with NA in specific columns

Question

Given the following example:

library(data.table)
mat <- data.table(x = c(1:10), y = c(11:20), z = c(21:30))

cut.head <- c(0, 2, 1) 
cut.tail <- c(3, 1,         


        

Answer 1

I'd just use a for loop with := (or set()) so it's fast and (fairly) easy to read.

> for (i in 1:3) mat[seq_len(cut.head[i]), (i):=NA]
> mat
     x  y  z
 1:  1 NA NA
 2:  2 NA 22
 3:  3 13 23
 4:  4 14 24
 5:  5 15 25
 6:  6 16 26
 7:  7 17 27
 8:  8 18 28
 9:  9 19 29
10: 10 20 30

Notice that the LHS of := accepts column numbers as well as names. As an aside, this is valid :

DT[, 2:=2]   # assign 2 to column 2

Wrapping the LHS of := with parenthesis, (i):=NA, tells it to use the variable's value rather than its name.

For the tail I first tried the following but .N isn't available in i. I've added that as a feature request, FR#724.
UPDATE: Now added to v1.9.3 on 11 Jul 2014

for (i in 1:3) mat[.N+1-seq_len(cut.tail[i]), (i):=NA]
# .N now works in i
> mat
     x  y  z
 1:  1 NA NA
 2:  2 NA 22
 3:  3 13 23
 4:  4 14 24
 5:  5 15 25
 6:  6 16 26
 7:  7 17 27
 8: NA 18 28
 9: NA 19 NA
10: NA NA NA
>

We no longer have to live with a repetition of the symbol mat :

> for (i in 1:3) mat[nrow(mat)+1-seq_len(cut.tail[i]), (i):=NA]