Question

I have problem with creating list of models.
Suppose I’ve created model:

> rp <- rpart(V41 ~ ., data=learnData, method="class")

If I’m creating list straight, thats OK:

> ll <- list(rp, rp, rp)
> class(ll[[1]])
[1] "rpart"
> class(ll[[2]])
[1] "rpart"
> class(ll[[3]])
[1] "rpart"

But if I’m trying to append model to already created list,
models changing their class to data.frame:

> ll <- list(rp)
> ll <- append(ll, rp)
> class(ll[[1]])
[1] "rpart"
> class(ll[[2]])
[1] "data.frame"

What’s a reason of this behavior and how can I append model to list?

Answer 1

The function append is used to add elements to a vector.

To add elements to a list, use list. Try:

fit1 <- lm(Sepal.Length ~ Sepal.Width, data=iris)
fit2 <- lm(Sepal.Length ~ Petal.Width, data=iris)

x <- list(fit1, fit2)
str(x, max.level=1)

List of 2
 $ :List of 12
  ..- attr(*, "class")= chr "lm"
 $ :List of 12
  ..- attr(*, "class")= chr "lm"

You should now have a list of lm objects:

> class(x[[1]])
[1] "lm"

To append to an existing list, use list as follows:

x <- list(fit1)
x <- list(x, fit2)