Matrix in R Programming Language: Definition and Examples

A matrix is a collection of data elements arranged in a two-dimensional rectangular layout. The following is an example of a matrix with 2 rows and 3 columns.

 

We reproduce a memory representation of the matrix in R with the matrix function. The data elements must be of the same basic type.

> A = matrix( +   c(2, 4, 3, 1, 5, 7), # the data elements +   nrow=2,              # number of rows +   ncol=3,              # number of columns +   byrow = TRUE)        # fill matrix by rows  > A                      # print the matrix      [,1] [,2] [,3] [1,]    2    4    3 [2,]    1    5    7

An element at the m^th row, n^th column of A can be accessed by the expression A[m, n].

> A[2, 3]      # element at 2nd row, 3rd column [1] 7

The entire m^th row A can be extracted as A[m, ].

> A[2, ]       # the 2nd row [1] 1 5 7

Similarly, the entire n^th column A can be extracted as A[ ,n].

> A[ ,3]       # the 3rd column [1] 3 7

We can also extract more than one rows or columns at a time.

> A[ ,c(1,3)]  # the 1st and 3rd columns      [,1] [,2] [1,]    2    3 [2,]    1    7

If we assign names to the rows and columns of the matrix, than we can access the elements by names.

> dimnames(A) = list( +   c("row1", "row2"),         # row names +   c("col1", "col2", "col3")) # column names  > A                 # print A      col1 col2 col3 row1    2    4    3 row2    1    5    7  > A["row2", "col3"] # element at 2nd row, 3rd column [1] 7