Confused about using data.table package,

On Sun, Feb 19, 2017 at 3:01 PM, David Winsemius <dwinsemius at comcast.net>
wrote:

On Feb 19, 2017, at 11:37 AM, C W <tmrsg11 at gmail.com> wrote:

Hi R,

I am a little confused by the data.table package.

library(data.table)

df <- data.frame(w=rnorm(20, -10, 1), x= rnorm(20, 0, 1), y=rnorm(20,

10, 1),

z=rnorm(20, 20, 1))

df <- data.table(df)

  df <- setDT(df) is preferred.

Don't you mean just

setDT(df)

?

setDT() modifies by reference.

df_3 <- df[, a := x-y] # created new column a using x minus y, why are

we

using colon equals?

You need to do more study of the extensive documentation. The behavior

of the ":=" function is discussed in detail there.

You can get to that documentation with ?":="

Hadley

--
http://hadley.nz