[R] Building a Matrix

[Gabor Grothendieck]

Doran, Harold <HDoran <at> air.org> writes:

: 
: Dear List:
: 
: I am having some difficulty constructing a matrix that must take a
: specific form. The matrix must be have a lower block of non-zero values
: and the rest must all be zero. For example, if I am building an n X n
: matrix, then the first n/2 rows need to be zero and the first n/2
: columns must remain as zero with all other elements having a non-zero
: value that I specify.
: 
: For example, assume I start with the following 4 x 4 matrix:
: 
: vl.mat <- matrix(0,4,4)
: 
:      [,1] [,2] [,3] [,4]
: [1,]    0    0    0    0
: [2,]    0    0    0    0
: [3,]    0    0    0    0
: [4,]    0    0    0    0
: 
: I need for the for the first two columns (4/2) to remain as zero and the
: first two rows (4/2) to remain as zeros. But I need for the bottom block
: to include some values that I specify.
: 
:      [,1] [,2] [,3] [,4]
: [1,]    0    0    0    0
: [2,]    0    0    0    0
: [3,]    0    0  100  100
: [4,]    0    0  100  100
: 
: I know that if I use the following I can build a matrix with values
: along the diagonals where I need them, but I need for the lower block of
: off-diagonals to also be the same value.
: 
: vl.mat <- matrix(0,4,4)
: vl.mat[(col(vl.mat)%%4 == row(vl.mat)%%4)&col(vl.mat)%%4 !=1
: &col(vl.mat)%%4 !=2  ] <- 100
: 

If A is the 2x2 submatrix that is to go in the lower right hand
corner then:

kronecker(diag(0:1), A)