R help - Feb 2007 - Efficient way to repeat rows (or columns) of a matrix?

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!

--  TMK  --
212-460-5430	home
917-656-5351	cell

Hi Talbot,

ma <- matrix(1:6, nrow  = 2)
rma <- ma[rep(1:2, c(2,3)),]

Best,

Jim



Talbot Katz wrote:
> 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!
> 
> --  TMK  --
> 212-460-5430	home
> 917-656-5351	cell
> 
> ______________________________________________
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-- 
James W. MacDonald
University of Michigan
Affymetrix and cDNA Microarray Core
1500 E Medical Center Drive
Ann Arbor MI 48109
734-647-5623



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