Question

I have a matrix A which is:

A <- matrix(c(1:15), byrow=T, nrow=5)
A
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
[3,]    7    8    9
[4,]   10   11   12
[5,]   13   14   15

Now I want to create a matrix B, which is 8×8 dimensions (or 10×10, or 15×15, etc), which would look like this:

     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,]    1    2    3    0    0    0    0    0
[2,]    4    5    6    0    0    0    0    0
[3,]    7    8    9    0    0    0    0    0
[4,]   10   11   12    0    0    0    0    0
[5,]   13   14   15    0    0    0    0    0
[6,]    0    0    0    0    0    0    0    0
[7,]    0    0    0    0    0    0    0    0
[8,]    0    0    0    0    0    0    0    0

So starting from A, I want to add columns and rows, to an 8×8 dimensions, all replaced with zero values…
Any idea?
Thank you in advance!!

Answer 1

Try this assuming A has at least one row and one column:

B <- matrix(0, 8, 8)
B[1:nrow(A), 1:ncol(A)] <- A

or as a single statement:

B <- "[<-"(matrix(0, 8, 8), 1:nrow(A), 1:ncol(A), value = A)

If A can have zero rows or zero columns then use seq_len(nrow(A)) and seq_len(ncol(A)) in place of 1:nrow(A) and 1:ncol(A).

Alternately, this works even in the case that A has zero rows or columns:

B <- matrix(0, 8, 8)
B[cbind(c(row(A)), c(col(A)))] <- A

or

B <- "[<-"(matrix(0, 8, 8), cbind(c(row(A)), c(col(A))), value = A)

or

B <- replace(matrix(0, 8, 8), cbind(c(row(A)), c(col(A))), A)