[R] Efficient way to repeat rows (or columns) of a matrix?

[Talbot Katz]

Hi.

If I have a vector, v_1, and another vector of positive integers, i_1, the 
same length as v_1, then rep(v_1,i_1) will repeat v_i[j] exactly i_1[j] 
times, like so:

>rep(c(1,2,3),c(3,2,1))
[1] 1 1 1 2 2 3
>

I'd like to do the same sort of thing where I replace v_1 with a matrix, and 
the jth row of the matrix is repeated i_1 times.

Obviously, I could do this with for loops, like the following:

>(ma1=matrix(1:6,nrow=2))
     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6
>vr1=c(2,3)
>rma1=NULL
>for(i in 1:length(vr1)){for(j in 1:vr1[i]){rma1=rbind(rma1,ma1[i,])}}
>rma1
     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    1    3    5
[3,]    2    4    6
[4,]    2    4    6
[5,]    2    4    6
>

I just thought some of you clever programmers could show me a more efficient 
way.  (I apologize if this question has come up before and you're tired of 
answering it ;-)

Thanks!

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