What does "with=" mean in data.table?

with()in base R and data.table

Suppose we have a data.frame: DF, and we want to subset all rows where x <1. In base R, we can do the following:

DF <- data.frame(x = c(1,1,1,2,2,3,3,3), y = 1:8)
## (1) normal way
DF[DF$x > 1, ] # data.frame needs that ',' as well
#   x y
# 4 2 4
# 5 2 5
# 6 3 6
# 7 3 7
# 8 3 8

## (2) using with()
DF[with(DF, x > 1), ]
#   x y
# 4 2 4
# 5 2 5
# 6 3 6
# 7 3 7
# 8 3 8

In the example(2) above, the x is regarded as a variable or column name when the codes/elements are inside the function with(). In data.table, the argument with= works in a similar way. It is TRUE in default, in which the codes/elements are regarded as variables/column names, hence the argument with=FALSE disables the ability to refer to columns, thereby restoring the “data.frame mode”.