create a formula in a data.table environment in R

Question

I would like to run a regression within a data.table. The formula needs to be constructed dynamically. I have tried the following method:

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Answer 1

lm can accept a character string as the formula so combine that with .SD like this:

> x[, as.list(coef(lm("a ~ b", .SD))), by = id]
   id (Intercept)  b
1:  1          21 -1
2:  2          21 -1
3:  3          21 -1
4:  4          21 -1
5:  5          21 -1