Assign a vector to a specific existing row of data table in R

Question

I've been looking through tutorials and documentation, but have not figured out how to assign a vector of values for all columns to one existing row in a data.table.

I start with an empty data.table that has already the correct number of columns and rows:

dt <- data.table(matrix(nrow=10, ncol=5))

Now I calculate some values for one row outside of the data.table and place them in a vector vec, e. g.:

vec <- rnorm(5)

How could I assign the values of vec to e. g. the first row of the data.table while achieving a good performance (since I also want to fill the other rows step by step)?

Answer 1

First you need to get the correct column types, as the NA matrix you've created is logical. The column types won't be magically changed by assigning numerics to them.

dt[, names(dt) := lapply(.SD, as.numeric)] 

Then you can change the first row's values with

dt[1, names(dt) := as.list(vec)]

That said, if you begin with a numeric matrix you wouldn't have to change the column types.

dt <- data.table(matrix(numeric(), 10, 5))
dt[1, names(dt) := as.list(vec)]