1 Extracting Data from a Matrix

In workshop week 6 you were introduced to subsetting a vector using the square brackets [ ]. We also use this technique to extract information from a matrix, but we need to provide the row number and the column number of the matrix element that we are interested in. In general, to extract the value contained in the $m^{\text{th}}$ row and $n^{\text{th}}$ column cell of matrix $F$, we would execute the command:

F[ m, n ]

Using the matrices that you have created in this workshop, execute the following extraction commands. Remember that the row number is first and the column number is second.

A[ 3, 3 ]

B[ 2, 1 ]

D[ 4, 2 ]

E[ 1, 3 ]

Note that the number you put into the square brackets cannot be greater than the dimension of the matrix. For example, the command C[ 2, 3 ] produces the message Error: subscript out of bounds. Note that we can obtain the dimensions of a matrix using the function dim, e.g. dim(A) returns c(3,2).

Now that we know how to extract data from a matrix, we can edit its content by using subsetting to identify a particular element and assigning a new value to that position. For example:

C

C[ 2, 2 ] <- 4

C

Note that the number in the second row and second column had changed from 2 to 4.