How to do it…

In order to extract any element of a matrix, we can specify the position of that element in R using square brackets. For example, mat[2,3] will extract the element under the second row and the third column. The first numeric value corresponds to the row and the second numeric value corresponds to a column [row, column].

Similarly, to replace an element, we can type the following lines in R:

mat[2,3] = 16

To select all the elements of the second row, we can use mat[2, ]. If we do not specify any numeric value for a column, R will automatically assume all columns.