[R] Building a Matrix

[Spencer Graves]

      Does the following do what you want: 

 > vl.mat <- matrix(0,4,4)
 > i34 <- 3:4
 > vl.mat[i34,i34] <- 100
 > vl.mat
     [,1] [,2] [,3] [,4]
[1,]    0    0    0    0
[2,]    0    0    0    0
[3,]    0    0  100  100
[4,]    0    0  100  100
 >
      spencer graves

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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>
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