How to Correctly Use Lists in R?

Just to address the last part of your question, since that really points out the difference between a list and vector in R:

Why do these two expressions not return the same result?

x = list(1, 2, 3, 4); x2 = list(1:4)

A list can contain any other class as each element. So you can have a list where the first element is a character vector, the second is a data frame, etc. In this case, you have created two different lists. x has four vectors, each of length 1. x2 has 1 vector of length 4:

> length(x[[1]])[1] 1> length(x2[[1]])[1] 4

So these are completely different lists.

R lists are very much like a hash map data structure in that each index value can be associated with any object. Here's a simple example of a list that contains 3 different classes (including a function):

> complicated.list <- list("a"=1:4, "b"=1:3, "c"=matrix(1:4, nrow=2), "d"=search)> lapply(complicated.list, class)$a[1] "integer"$b[1] "integer"$c[1] "matrix"$d[1] "function"

Given that the last element is the search function, I can call it like so:

> complicated.list[["d"]]()[1] ".GlobalEnv" ...

As a final comment on this: it should be noted that a data.frame is really a list (from the data.frame documentation):

A data frame is a list of variables of the same number of rows with unique row names, given class ‘"data.frame"’

That's why columns in a data.frame can have different data types, while columns in a matrix cannot. As an example, here I try to create a matrix with numbers and characters:

> a <- 1:4> class(a)[1] "integer"> b <- c("a","b","c","d")> d <- cbind(a, b)> d a   b  [1,] "1" "a"[2,] "2" "b"[3,] "3" "c"[4,] "4" "d"> class(d[,1])[1] "character"

Note how I cannot change the data type in the first column to numeric because the second column has characters:

> d[,1] <- as.numeric(d[,1])> class(d[,1])[1] "character"

Regarding your questions, let me address them in order and give some examples:

1) A list is returned if and when the return statement adds one. Consider

 R> retList <- function() return(list(1,2,3,4)); class(retList()) [1] "list" R> notList <- function() return(c(1,2,3,4)); class(notList()) [1] "numeric" R> 

2) Names are simply not set:

R> retList <- function() return(list(1,2,3,4)); names(retList())NULLR> 

3) They do not return the same thing. Your example gives

R> x <- list(1,2,3,4)R> x[1][[1]][1] 1R> x[[1]][1] 1

where x[1] returns the first element of x -- which is the same as x. Every scalar is a vector of length one. On the other hand x[[1]] returns the first element of the list.

4) Lastly, the two are different between they create, respectively, a list containing four scalars and a list with a single element (that happens to be a vector of four elements).

Just to take a subset of your questions:

This article on indexing addresses the question of the difference between [] and [[]].

In short [[]] selects a single item from a list and [] returns a list of the selected items. In your example, x = list(1, 2, 3, 4)' item 1 is a single integer but x[[1]] returns a single 1 and x[1] returns a list with only one value.

> x = list(1, 2, 3, 4)> x[1][[1]][1] 1> x[[1]][1] 1