# How to delete a row by reference in R data.table?

My question is related to assignment by reference versus copying in data.table. I want to know if one can delete rows by reference, similar to

DT[,someCol:=NULL]

I want to know about

DT[someRow:=NULL, ]

I guess there's a good reason for why this function doesn't exist, so maybe you could just point out a good alternative to the usual copying approach, as below. In particular, going with my favourite from example(data.table),

DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9) x y v [1,] a 1 1 [2,] a 3 2 [3,] a 6 3 [4,] b 1 4 [5,] b 3 5 [6,] b 6 6 [7,] c 1 7 [8,] c 3 8 [9,] c 6 9

say I want to delete the first row from this data.table. I know I can do this

DT = DT[-1, ]

but often we may want to avoid that, because we are copying the object (and that requires about 3*N memory, if N object.size(DT), as pointed out here.

Now I found set(DT,i,j,value). I know how to set specific values (like here: set all values in rows 1 and 2 and columns 2 and 3 to zero)

set(DT,1:2,2:3,0) DT x y v [1,] a 0 0 [2,] a 0 0 [3,] a 6 3 [4,] b 1 4 [5,] b 3 5 [6,] b 6 6 [7,] c 1 7 [8,] c 3 8 [9,] c 6 9

but how can I erase the first two rows, say? Doing

set(DT,1:2,1:3,NULL)

sets the entire DT to NULL.

My SQL knowledge is very limited, so you guys tell me: given data.table uses SQL technology, is there an equivalent to the SQL command

DELETE FROM table_name WHERE some_column=some_value

in data.table?