Odp: Matrix

A=matrix(10,ncol=2,nrow=2)
B <- 2*A
C <- rbind(A, B)

C[ as.vector(t(matrix(seq(1,nrow(C)),ncol=2))), ]  # trick to create  
the vector of permutations

     [,1] [,2]
[1,]   10   10
[2,]   20   20
[3,]   10   10
[4,]   20   20

On Mon, Feb 2, 2009 at 4:05 PM, Petr PIKAL <petr.pikal at precheza.cz>  
wrote:

Hi

r-help-bounces at r-project.org napsal dne 02.02.2009 16:52:06:

Hi,

I have a very basic question on merging two matrices by  
alternating the
rows. For illustration, assume two matrices -

A looks like:

10 10
10 10

B looks like:

20 20
20 20

How do I combine them such that I get alternating rows from A and  
B? My
final result should be C which looks like:

10 10
20 20
10 10
20 20

I would rbind both matrices and then indexed the resulting matrix.

x<-rep(10,4)
y<-rep(20,4)
dim(x)<-c(2,2)
dim(y)<-c(2,2)
z<-rbind(x,y)

z

   [,1] [,2]
[1,]   10   10
[2,]   10   10
[3,]   20   20
[4,]   20   20

ind<-c(seq(1,4,2), seq(2,4,2))
z[ind,]

z[ind,]

   [,1] [,2]
[1,]   10   10
[2,]   20   20
[3,]   10   10
[4,]   20   20

Another solution:

m1 <- matrix(10,4,2)
m2 <- matrix(20,2,2)
m1[seq(2,4,2),] <- m2

Paul