Add dynamically multiple columns to data table

Question

Given a data table with 3 columns (the first two form the key), I'm trying to add a bunch of columns x_k with k running from 1 to N

df
>    a    b    c 
1:   1    1    1
2:   1    2    1
3:   1    3    2

For each new column x_k, its value is calculated by the equation cos(2 * pi * c / k) grouped by a and b.

My approach doesn't seem to work: generating a list of column names then parsing it

df[, eval(parse(text = paste0(paste0("x_", 1:N), " := cos(2 * pi * c / ", 1:N, ")"))), by = .(a,b)]

R said c not found. How should i correct my approach other than looping through list of column names ?

chinsoon12 · Accepted Answer · 2020-05-23 08:26:52Z

4

Maybe try something like:

df[, paste0("x_", 1L:N) := lapply(1L:N, function(k) cos(2 * pi * c / k)), .(a, b)]

answered May 23, 2020 at 8:26

chinsoon12

25.2k4 gold badges27 silver badges35 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

That's exactly what i came to after posting the question :D

Pls feel free to answer your own question or delete.

I think i'll leave it here for later reference.

I would use seq_len(N)