Skip to main content

Indices and the Apply Commands in R

Giving Names to Elements​

We can name elements of vectors and data frames in R using the names command.

Examples​

Example
> X <- c(41, 3, 73)
> names(X) <- c("One", "Two", "Three")

View the results by simply typing X and the output of X is given as follows:

> X
  One   Two Three
   41     3    73

With this we can refer to the elements by name as well as locations using:

> X[1]
One
 41

> X["Three"]
Three
   73

Regular Matrix Indices and Naming​

A matrix is a table of numbers. Typical matrix indexing: mat[i,j], mat[1:2,] etc. A matrix can have row and column names Indexing with row and column names: mat["a","B"].

Details​

Definition

A matrix is a (two-dimensional) table of numbers, indexed by row and column numbers.

Note

Note that a matrix can also have row and column names, so that the matrix can be indexed by its names rather than numbers.

Examples​

Example

Consider a matrix with 2 rows and 3 columns. Consider extracting first element (1,2)(1,2), then all of line 2 and then columns 2-3 in an R session:

> mat <- matrix(1:6,ncol=3)

> mat
     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6

> mat[1,2]
[1] 3

> mat[2,]
[1] 2 4 6

> mat[,2:3]
     [,1] [,2]
[1,]    3    5
[2,]    4    6

Next, consider the same matrix, but give names to the rows and columns. The rows will get the names aa and bb and the columns will be named AA, BB and CC.

The entire R session could look like this:

> mat <- matrix(1:6,ncol=3)

> dimnames(mat) <- list(c("a","b"),c("A","B","C"))

> mat
  A B C
a 1 3 5
b 2 4 6

> mat["b",c("B","C")]
B C
4 6

The apply Command​

apply(mat,2,sum)\verb|apply(mat,2,sum)| -- applies the sum function within each column

apply(mat,1,mean)\verb|apply(mat,1,mean)| -- computes the mean within each row

The tapply Command​

Commonly one has a data vector and another vector of the same length giving categories for the measurements. In this case one often wants to compute the mean or variance (or median etc) within each category. To do this we use the tapply command in R.

Examples​

Example
> z <- c(5,7,2,9,3,4,8)
> i <- c("m","f","m","m","f","m","f")

A. Find the sum within each group

> tapply(z,i,sum)
 f  m
18 20

B. Find the sample sizes

> tapply(z,i,length)
f m
3 4

C. Store outputs and use names

> n <- tapply(z,i,length)

> n
f m
3 4

> n["m"]
m
4

Logical Indexing​

A logical vector consists of TRUE\verb|TRUE| (1) or FALSE\verb|FALSE| (0) values. These can be used to index vectors or matrices.

Examples​

Example
> i <- c("m","f","m","m","f","m","f")

> z <- c(5,7,2,9,3,4,8)

> i=="m"
[1]  TRUE FALSE  TRUE  TRUE FALSE  TRUE FALSE

> z[i=="m"]
[1] 5 2 9 4

> z[c(T,F,T,T,F,T,F)]
[1] 5 2 9 4

Lists, Indexing Lists​

A list is a collection of objects. Thus, data frames are lists.

Examples​

Example
> x <- list(y=2,z=c(2,3),w=c("a","b","c"))

> x[["z"]]
[1] 2 3

> names(x)
[1] "y" "z" "w"

> x["w"]
$w
[1] "a" "b" "c"

> x$w
[1] "a" "b" "c"