I have a data.table with which I'd like to perform the same operation on certain columns. The names of these columns are given in a character vector. In this particular example, I'd like to multiply all of these columns by -1.
Some toy data and a vector specifying relevant columns:
library(data.table)
dt <- data.table(a = 1:3, b = 1:3, d = 1:3)
cols <- c("a", "b")
Right now I'm doing it this way, looping over the character vector:
for (col in 1:length(cols)) {
dt[ , eval(parse(text = paste0(cols[col], ":=-1*", cols[col])))]
}
Is there a way to do this directly without the for loop?
This seems to work:
dt[ , (cols) := lapply(.SD, "*", -1), .SDcols = cols]
The result is
a b d
1: -1 -1 1
2: -2 -2 2
3: -3 -3 3
There are a few tricks here:
(cols) :=, the result is assigned to the columns specified in cols, instead of to some new variable named "cols"..SDcols tells the call that we're only looking at those columns, and allows us to use .SD, the Subset of the Data associated with those columns.lapply(.SD, ...) operates on .SD, which is a list of columns (like all data.frames and data.tables). lapply returns a list, so in the end j looks like cols := list(...).EDIT: Here's another way that is probably faster, as @Arun mentioned:
for (j in cols) set(dt, j = j, value = -dt[[j]])