Advice for R newcomers like me : beware, the following is a list of a single object :
> mylist <- list (1:10)
> length (mylist)
[1] 1
In such a case you are not looking for the length of the list, but of its first element :
> length (mylist[[1]])
[1] 10
This is a "true" list :
> mylist <- list(1:10, rnorm(25), letters[1:3])
> length (mylist)
[1] 3
Also, it seems that R considers a data.frame as a list :
> df <- data.frame (matrix(0, ncol = 30, nrow = 2))
> typeof (df)
[1] "list"
In such a case you may be interested in ncol() and nrow() rather than length() :
> ncol (df)
[1] 30
> nrow (df)
[1] 2
Though length() will also work (but it's a trick when your data.frame has only one column) :
> length (df)
[1] 30
> length (df[[1]])
[1] 2