So I have created this variable that is called mc_split_device inside the datatable called mc_with_devices. However, If I type mc_with_devices$mc_split I get the values of the column mc_split_device while I never created any variable with the name mc_split.
See Hadley Wickham's Advanced R:
$ is a shorthand operator, where x$y is equivalent to x[["y", exact = FALSE]]. It’s often used to access variables in a data frame, as in mtcars$cyl or diamonds$carat.
So the exact=FALSE is the reason why $mc_split works despite there not being a column with that exact name.
As an aside, I don't believe mc_with_devices[,.(mc_split)] will work without doublequotes. The following will work:
mc_with_devices[,"mc_split_resp"]
It matches the name of the column partially. From ?Extract
names : For extraction, this is normally (see under ‘Environments’) partially matched to the names of the object.
Character indices can in some circumstances be partially matched (see pmatch) to the names or dimnames of the object being subsetted
Thus the default behaviour is to use partial matching only when extracting from recursive objects (except environments) by $.
Hence, when you do
mtcars$m
You get
#[1] 21.0 21.0 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 17.8 16.4 17.3 15.2 10.4 10.4
#[17] 14.7 32.4 30.4 33.9 21.5 15.5 15.2 13.3 19.2 27.3 26.0 30.4 15.8 19.7 15.0 21.4
which is same as mtcars$mpg
This can be sometimes confusing and if you want to make sure to be notified when such partial matching is done. You can turn on the warning by
options(warnPartialMatchDollar = TRUE)
mtcars$m
# [1] 21.0 21.0 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 17.8 16.4 17.3 15.2 10.4 10.4
#[17] 14.7 32.4 30.4 33.9 21.5 15.5 15.2 13.3 19.2 27.3 26.0 30.4 15.8 19.7 15.0 21.4
Warning message: In
$.data.frame(mtcars, m) : Partial match of 'm' to 'mpg' in data frame
According to ?Extract
name - A literal character string or a name (possibly backtick quoted). For extraction, this is normally (see under ‘Environments’) partially matched to the names of the object.
and exact
exact - Controls possible partial matching of [[ when extracting by a character vector (for most objects, but see under ‘Environments’). The default is no partial matching. Value NA allows partial matching but issues a warning when it occurs. Value FALSE allows partial matching without any warning.
So, when we do
mtcars$m
#[1] 21.0 21.0 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 17.8 16.4 17.3 15.2 10.4 10.4 14.7 32.4 30.4 33.9 21.5 15.5 15.2 13.3 19.2 27.3
#[27] 26.0 30.4 15.8 19.7 15.0 21.4
mtcars$d
#NULL
Because there are multiple names that starts with 'd'
names(mtcars)
#[1] "mpg" "cyl" "disp" "hp" "drat" "wt" "qsec" "vs" "am" "gear" "carb"
If we are specific, it does the partial match for the 'disp' column
mtcars$di
#[1] 160.0 160.0 108.0 258.0 360.0 225.0 360.0 146.7 140.8 167.6 167.6 275.8 275.8 275.8 472.0 460.0 440.0 78.7 75.7 71.1 120.1
#[22] 318.0 304.0 350.0 400.0 79.0 120.3 95.1 351.0 145.0 301.0 121.0