R data.table loop subset by factor and do lm()

Question

I am trying to create a function or even just work out how to run a loop using data.table syntax where I can subset the table by factor, in this case the id variable, then r

Answer 1

> df <- data.frame(id = letters[1:3], 
                   +                  cyl = sample(c("a","b","c"), 30, replace = TRUE),
                   +                  factor = sample(c(TRUE, FALSE), 30, replace = TRUE),   
                   +                  hp = sample(c(20:50), 30, replace = TRUE))
> by(df,df$id,function(x) lm(hp ~ cyl + factor, data = x))
df$id: a

Call:
  lm(formula = hp ~ cyl + factor, data = x)

Coefficients:
  (Intercept)         cylb         cylc   factorTRUE  
41.571        4.143       -3.429       -5.571  

------------------------------------------------------------------------------------------ 
  df$id: b

Call:
  lm(formula = hp ~ cyl + factor, data = x)

Coefficients:
  (Intercept)         cylb         cylc   factorTRUE  
46.040       -1.133      -19.467       -7.720  

------------------------------------------------------------------------------------------ 
  df$id: c

Call:
  lm(formula = hp ~ cyl + factor, data = x)

Coefficients:
  (Intercept)         cylc   factorTRUE  
31.850        3.917       -8.200  

    enter code here