Update datatable-intro.Rmd

A powerful feature of data.table is the ability to store lists as elements (a facility not available in data.frame).  Here is a simple illustration of storing a fitted model as data.table element.

When statistical models are fit to different groups within a data.table, these models can be stored in a result data.table.  Now this resulting data.table can be used to access fitted models.

vignettes/datatable-intro.Rmd

@@ -639,6 +639,17 @@ DT[, print(list(c(a,b))), by = ID] # (2)
 
 In (1), for each group, a vector is returned, with length = 6,4,2 here. However, (2) returns a list of length 1 for each group, with its first element holding vectors of length 6,4,2. Therefore, (1) results in a length of ` 6+4+2 = `r 6+4+2``, whereas (2) returns `1+1+1=`r 1+1+1``.
 
+Flexibility of j allows us to store any list object as an element of data.table (a facility not available in data.frames).  For example, when statistical models are fit to groups, these models can be stored in a data.table.  
+
+```{r}
+## Do long distance flights cover up departure delay more than short distance flights?
+## Does cover up vary by month?
+flights[, `:=`(makeup = dep_delay - arr_delay)]
+
+makeup.models <- flights[, .(fit = list(lm(makeup ~ distance))), by = .(month)]
+makeup.models[, .(coefdist = coef(fit[[1]])[2], rsq = summary(fit[[1]])$r.squared), by = .(month)]
+```
+
 ## Summary
 
 The general form of `data.table` syntax is: