converting mixed dataframe with list to pure dataframe - r

Question

what is the easiest way to extract information from a list embedded within a dataframe?
a<-data.frame(cyl=c(4,6,8),k=c("A","B","C"))
j<-by(data=mtcars,INDICES=mtcars$cyl,function(x) lm(mpg~disp,data=x))
a$l<-j
t(sapply(a$l,coef))->a$t
But this results in a matrix embedded within the dataframe and it needs some massaging in order to have it as two columns in a with their associated column names.
What I'd like is an easier method to extract this information and have it stored in dataframe a with the associated column names.
EDIT_ This is what I had in mind, but I just found the procedure somewhat cumbersome.
t(sapply(a$l,coef))->a$t
as.data.frame(a$t)->g
g$cyl<-as.numeric(rownames(g))
merge(x = a,y = g)->a2
a2[,-c(3,4)]->a3
Any simpler ways of doing this?
Now, to complicate matters- What If I´d like to get the residuals from a$l by cylinder.
sapply(a$l,function(x) x[['residuals']])->a$t
How can I generate a new dataframe in a long format with two columns: cyl and residual that later can be merged with the original dataframe a?

Answer 1

Well--see my previous edit for the first answer. This is for my second problem:
It does solve my problem, but I´m sure there must be a quicker and more intuitive way of solving this.
flat.list.df<-function(list,sublist){
nm<-names(list)
i<-do.call(rbind,lapply(nm,function(x){
u<-list[[x]][[sublist]]
g<-length(u)
j<-rep(x,g)
m<-data.frame(var=j,val=u)
m
})
)
return(i)
}
flat.list.df(a$l,"residuals")->w
w
merge(w,a,by.x="var",by.y="cyl")