Idiom for dropping a single column in a data.table

If you are wanting to remove the column permanently use := NULL

dat[, z := NULL]

If you have your columns to drop as a character string use () to force evaluation as a character string, not as the character name.

toDrop <- c('z')

dat[, (toDrop) := NULL]

If you want to limit the availability of the columns in .SD, you can pass the .SDcols argument

dat[,lapply(.SD, somefunction) , .SDcols = setdiff(names(dat),'z')]

However, data.table inspects the j arguments and only gets the columns you use any way. See FAQ 1.12

When you write X[Y,sum(foo*bar)], data.table automatically inspects the j expression to see which columns it uses.

and doesn't try and load all the data for .SD (unless you have .SD within your call to j)

subset.data.table is processing the call and eventually evaluating dat[, c('x','y'), with=FALSE]

using := NULL should be basically instantaneous, howveer t does permanently delete the column.

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