doug - 1 year ago 70
R Question

# How to Correctly Use Lists in R?

Brief background: Many (most?) contemporary programming languages in widespread use have at least a handful of ADTs [abstract data types] in common, in particular,

• string (a sequence comprised of characters)

• list (an ordered collection of values), and

• map-based type (an unordered array that maps keys to values)

In the R programming language, the first two are implemented as
`character`
and
`vector`
, respectively.

When I began learning R, two things were obvious almost from the start:
`list`
is the most important data type in R (because it is the parent class for the R
`data.frame`
), and second, I just couldn't understand how they worked, at least not well enough to use them correctly in my code.

For one thing, it seemed to me that R's
`list`
data type was a straightforward implementation of the map ADT (
`dictionary`
in Python,
`NSMutableDictionary`
in Objective C,
`hash`
in Perl and Ruby,
`object literal`
in Javascript, and so forth).

For instance, you create them just like you would a Python dictionary, by passing key-value pairs to a constructor (which in Python is
`dict`
not
`list`
):

``````x = list("ev1"=10, "ev2"=15, "rv"="Group 1")
``````

And you access the items of an R List just like you would those of a Python dictionary, e.g., x['ev1']. Likewise, you can retrieve just the 'keys' or just the 'values' by:

``````names(x)    # fetch just the 'keys' of an R list
# [1] "ev1" "ev2" "rv"

unlist(x)   # fetch just the 'values' of an R list
#   ev1       ev2        rv
#  "10"      "15" "Group 1"

x = list("a"=6, "b"=9, "c"=3)

sum(unlist(x))
# [1] 18
``````

but R
`list`
s are also unlike other map-type ADTs (from among the languages I've learned anyway). My guess is that this is a consequence of the initial spec for S, i.e., an intention to design a data/statistics DSL [domain-specific language] from the ground-up.

three significant differences between R
`list`
s and mapping types in other languages in widespread use (e.g,. Python, Perl, JavaScript):

first,
`list`
s in R are an ordered collection, just like vectors, even though the values are keyed (ie, the keys can be any hashable value not just sequential integers). Nearly always, the mapping data type in other languages is unordered.

second,
`list`
s can be returned from functions even though you never passed in a
`list`
when you called the function, and even though the function that returned the
`list`
doesn't contain an (explicit)
`list`
constructor (Of course, you can deal with this in practice by wrapping the returned result in a call to
`unlist`
):

``````x = strsplit(LETTERS[1:10], "")     # passing in an object of type 'character'

class(x)                            # returns 'list', not a vector of length 2
# [1] list
``````

A third peculiar feature of R's
`list`
s: it doesn't seem that they can be members of another ADT, and if you try to do that then the primary container is coerced to a
`list`
. E.g.,

``````x = c(0.5, 0.8, 0.23, list(0.5, 0.2, 0.9), recursive=TRUE)

class(x)
# [1] list
``````

my intention here is not to criticize the language or how it is documented; likewise, I'm not suggesting there is anything wrong with the
`list`
data structure or how it behaves. All I'm after is to correct is my understanding of how they work so I can correctly use them in my code.

Here are the sorts of things I'd like to better understand:

• What are the rules which determine when a function call will return a
`list`
(e.g.,
`strsplit`
expression recited above)?

• If I don't explicitly assign names to a
`list`
(e.g.,
`list(10,20,30,40)`
) are the default names just sequential integers beginning with 1? (I assume, but I am far from certain that the answer is yes, otherwise we wouldn't be able to coerce this type of
`list`
to a vector w/ a call to
`unlist`
.

• why do these two different operators,
`[]`
, and
`[[]]`
, return the same result?

`x = list(1, 2, 3, 4)`

both expressions return "1":

`x[1]`

`x[[1]]`

• why do these two expressions not return the same result?

`x = list(1, 2, 3, 4)`

`x2 = list(1:4)`

Please don't point me to the R Documentation (
`?list`
,
`R-intro`
)--I have read it carefully and it does not help me answer the type of questions I recited just above.

(lastly, I recently learned of and began using an R Package (available on CRAN) called
`hash`
which implements conventional map-type behavior via an S4 class; I can certainly recommend this Package.)

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"
``````
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