6 Lists and data frames

6.1 Lists

An R list is an object consisting of an ordered collection of objects known as its components.

There is no particular need for the components to be of the same mode or type, and, for example, a list could consist of a numeric vector, a logical value, a matrix, a complex vector, a character array, a function, and so on. Here is a simple example of how to make a list:

> Lst <- list(name="Fred", wife="Mary", no.children=3,
              child.ages=c(4,7,9))

Components are always numbered and may always be referred to as such. Thus if Lst is the name of a list with four components, these may be individually referred to as Lst[[1]], Lst[[2]], Lst[[3]] and Lst[[4]]. If, further, Lst[[4]] is a vector subscripted array then Lst[[4]][1] is its first entry.

If Lst is a list, then the function length(Lst) gives the number of (top level) components it has.

Components of lists may also be named, and in this case the component may be referred to either by giving the component name as a character string in place of the number in double square brackets, or, more conveniently, by giving an expression of the form

> name$component_name

for the same thing.

This is a very useful convention as it makes it easier to get the right component if you forget the number.

So in the simple example given above:

Lst$name is the same as Lst[[1]] and is the string "Fred",

Lst$wife is the same as Lst[[2]] and is the string "Mary",

Lst$child.ages[1] is the same as Lst[[4]][1] and is the number 4.

Additionally, one can also use the names of the list components in double square brackets, i.e., Lst[["name"]] is the same as Lst$name. This is especially useful, when the name of the component to be extracted is stored in another variable as in

> x <- "name"; Lst[[x]]

It is very important to distinguish Lst[[1]] from Lst[1]. [[...]] is the operator used to select a single element, whereas [...] is a general subscripting operator. Thus the former is the first object in the list Lst, and if it is a named list the name is not included. The latter is a sublist of the list Lst consisting of the first entry only. If it is a named list, the names are transferred to the sublist.

The names of components may be abbreviated down to the minimum number of letters needed to identify them uniquely. Thus Lst$coefficients may be minimally specified as Lst$coe and Lst$covariance as Lst$cov.

The vector of names is in fact simply an attribute of the list like any other and may be handled as such. Other structures besides lists may, of course, similarly be given a names attribute also.

6.2 Constructing and modifying lists

New lists may be formed from existing objects by the function list(). An assignment of the form

> Lst <- list(name_1=object_1, ..., name_m=object_m)

sets up a list Lst of m components using object_1, …, object_m for the components and giving them names as specified by the argument names, (which can be freely chosen). If these names are omitted, the components are numbered only. The components used to form the list are copied when forming the new list and the originals are not affected.

Lists, like any subscripted object, can be extended by specifying additional components. For example

> Lst[5] <- list(matrix=Mat)

6.2.1 Concatenating lists

When the concatenation function c() is given list arguments, the result is an object of mode list also, whose components are those of the argument lists joined together in sequence.

> list.ABC <- c(list.A, list.B, list.C)

Recall that with vector objects as arguments the concatenation function similarly joined together all arguments into a single vector structure. In this case all other attributes, such as dim attributes, are discarded.

6.3 Data frames

A data frame is a list with class "data.frame". There are restrictions on lists that may be made into data frames, namely

The components must be vectors (numeric, character, or logical), factors, numeric matrices, lists, or other data frames.
Matrices, lists, and data frames provide as many variables to the new data frame as they have columns, elements, or variables, respectively.
Vector structures appearing as variables of the data frame must all have the same length, and matrix structures must all have the same number of rows.

A data frame may for many purposes be regarded as a matrix with columns possibly of differing modes and attributes. It may be displayed in matrix form, and its rows and columns extracted using matrix indexing conventions.

6.3.1 Making data frames

Objects satisfying the restrictions placed on the columns (components) of a data frame may be used to form one using the function data.frame:

> accountants <- data.frame(home=statef, loot=incomes, shot=incomef)

A list whose components conform to the restrictions of a data frame may be coerced into a data frame using the function as.data.frame()

The simplest way to construct a data frame from scratch is to use the read.table() function to read an entire data frame from an external file. This is discussed further in Reading data from files.

6.3.2 `attach()` and `detach()`

The $ notation, such as accountants$home, for list components is not always very convenient. A useful facility would be somehow to make the components of a list or data frame temporarily visible as variables under their component name, without the need to quote the list name explicitly each time.

The attach() function takes a ‘database’ such as a list or data frame as its argument. Thus suppose lentils is a data frame with three variables lentils$u, lentils$v, lentils$w. The attach

> attach(lentils)

places the data frame in the search path at position 2, and provided there are no variables u, v or w in position 1, u, v and w are available as variables from the data frame in their own right. At this point an assignment such as

> u <- v+w

does not replace the component u of the data frame, but rather masks it with another variable u in the workspace at position 1 on the search path. To make a permanent change to the data frame itself, the simplest way is to resort once again to the $ notation:

> lentils$u <- v+w

However the new value of component u is not visible until the data frame is detached and attached again.

To detach a data frame, use the function

> detach()

More precisely, this statement detaches from the search path the entity currently at position 2. Thus in the present context the variables u, v and w would be no longer visible, except under the list notation as lentils$u and so on. Entities at positions greater than 2 on the search path can be detached by giving their number to detach, but it is much safer to always use a name, for example by detach(lentils) or detach("lentils")

Note: In R lists and data frames can only be attached at position 2 or above, and what is attached is a copy of the original object. You can alter the attached values via assign, but the original list or data frame is unchanged.

6.3.3 Working with data frames

A useful convention that allows you to work with many different problems comfortably together in the same workspace is

gather together all variables for any well defined and separate problem in a data frame under a suitably informative name;
when working with a problem attach the appropriate data frame at position 2, and use the workspace at level 1 for operational quantities and temporary variables;
before leaving a problem, add any variables you wish to keep for future reference to the data frame using the $ form of assignment, and then detach();
finally remove all unwanted variables from the workspace and keep it as clean of left-over temporary variables as possible.

In this way it is quite simple to work with many problems in the same directory, all of which have variables named x, y and z, for example.

6.3.4 Attaching arbitrary lists

attach() is a generic function that allows not only directories and data frames to be attached to the search path, but other classes of object as well. In particular any object of mode "list" may be attached in the same way:

> attach(any.old.list)

Anything that has been attached can be detached by detach, by position number or, preferably, by name.

6.3.5 Managing the search path

The function search shows the current search path and so is a very useful way to keep track of which data frames and lists (and packages) have been attached and detached. Initially it gives

> search()
[1] ".GlobalEnv"   "Autoloads"    "package:base"

where .GlobalEnv is the workspace.¹

¹ See the on-line help for autoload for the meaning of the second term.

After lentils is attached we have

> search()
[1] ".GlobalEnv"   "lentils"      "Autoloads"    "package:base"
> ls(2)
[1] "u" "v" "w"

and as we see ls (or objects) can be used to examine the contents of any position on the search path.

Finally, we detach the data frame and confirm it has been removed from the search path.

> detach("lentils")
> search()
[1] ".GlobalEnv"   "Autoloads"    "package:base"

Footnotes