CSC121 - R Console
Friday March 2nd
---------------------------
> # Combining Matrices
> X <- matrix(1:6, nrow=2, ncol=3)
> X
     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6
> Y <- matrix(3, nrow=2, ncol=4)
> Y
     [,1] [,2] [,3] [,4]
[1,]    3    3    3    3
[2,]    3    3    3    3
> # To combine two matrices with the same numbers of rows, we can use the
> # function cbind()
> cbind(X, Y)
     [,1] [,2] [,3] [,4] [,5] [,6] [,7]
[1,]    1    3    5    3    3    3    3
[2,]    2    4    6    3    3    3    3
> # This 'binds' the columns together - cbind
> # If same number of columns, use rbind
> Z <- matrix(7, nrow=4, ncol=3)
> rbind(X, Z)
     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6
[3,]    7    7    7
[4,]    7    7    7
[5,]    7    7    7
[6,]    7    7    7
> rbind(X, c(5,6,7))
     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6
[3,]    5    6    7
> # rbind can easily add a row to a matrix
> # Vectors can be treated as single columns or rows
> 
> # You can change matrix elements in place:
> A <- matrix(0, nrow=3, ncol=3)
> A
     [,1] [,2] [,3]
[1,]    0    0    0
[2,]    0    0    0
[3,]    0    0    0
> A[2, 1] <- 5
> A[1, 3] <- 7
> A[3, 3] <- 9
> A
     [,1] [,2] [,3]
[1,]    0    0    7
[2,]    5    0    0
[3,]    0    0    9
> # You can change individual rows or columns
> A[,1] <- c(1, 2, 3)
> A
     [,1] [,2] [,3]
[1,]    1    0    7
[2,]    2    0    0
[3,]    3    0    9
> A[2,] <- c(1, 2, 3)
> A
     [,1] [,2] [,3]
[1,]    1    0    7
[2,]    1    2    3
[3,]    3    0    9
> # When we index a single row or column, we get a vector
> A[2,]
[1] 1 2 3
> # Can repackage as a matrix, but it will give you back a column
> matrix(A[2,])
     [,1]
[1,]    1
[2,]    2
[3,]    3
> # Instead, can set drop=FALSE to maintain row shape
> A[2, , drop=FALSE]
     [,1] [,2] [,3]
[1,]    1    2    3
> B <- matrix(1:9, nrow=3)
> B
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9
> A[1:2, 2:3]
     [,1] [,2]
[1,]    0    7
[2,]    2    3
> B[1:2, 2:3]
     [,1] [,2]
[1,]    4    7
[2,]    5    8
> # To get any sub-matrix, you need to specify the rows and the columns
> # that you want to extract using either a sequence
> B[c(1,3), c(1,3)]
     [,1] [,2]
[1,]    1    7
[2,]    3    9
> # ...or any numeric vector
> B
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9
> source('~/Desktop/matrix_functions.R')
> PrintMatrix(B)
1 4 7 
2 5 8 
3 6 9 
> B
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9
> B[nrow(B),]
[1] 3 6 9
> source('~/Desktop/matrix_functions.R')
> ChangeEdgesTo(B)
Error in ChangeEdgesTo(B) : argument "n" is missing, with no default
> ChangeEdgesTo(B, 1)
     [,1] [,2] [,3]
[1,]    1    1    1
[2,]    1    5    1
[3,]    1    1    1
> source('~/Desktop/matrix_functions.R')
> LowerOnes(4)
     [,1] [,2] [,3] [,4]
[1,]    1    0    0    0
[2,]    1    1    0    0
[3,]    1    1    1    0
[4,]    1    1    1    1
> LowerOnes(6)
     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    1    0    0    0    0    0
[2,]    1    1    0    0    0    0
[3,]    1    1    1    0    0    0
[4,]    1    1    1    1    0    0
[5,]    1    1    1    1    1    0
[6,]    1    1    1    1    1    1
> source('~/Desktop/matrix_functions.R')
> XMatrix(3)
     [,1] [,2] [,3]
[1,]    1    0    1
[2,]    0    1    0
[3,]    1    0    1
> XMatrix(7)
     [,1] [,2] [,3] [,4] [,5] [,6] [,7]
[1,]    1    0    0    0    0    0    1
[2,]    0    1    0    0    0    1    0
[3,]    0    0    1    0    1    0    0
[4,]    0    0    0    1    0    0    0
[5,]    0    0    1    0    1    0    0
[6,]    0    1    0    0    0    1    0
[7,]    1    0    0    0    0    0    1