[R] Building a Matrix

[Patrick Burns]

Unless I'm missing something, all you need to do is create the
large matrix and then replace the submatrix via subscripting
the rows and columns:

ans <- matrix(0, n, n)
sub.seq <- floor(n/2 + 1):n
ans[sub.seq, sub.seq] <- submatrix


Patrick Burns

Burns Statistics
patrick at burns-stat.com
+44 (0)20 8525 0696
http://www.burns-stat.com
(home of S Poetry and "A Guide for the Unwilling S User")

Doran, Harold wrote:

>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
>
>Can anyone offer a suggestion? Thank you.
>
>-Harold
>
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