[R] Product of certain rows in a matrix

[arun]

Hi Bert,
Thanks.  It is a better solution.

If nrow() is not even.

Anew<- rbind(A,c(1,3,5))
j<-seq_len(nrow(Anew)/2)###
 Anew[j,]*Anew[j-1,]
#Error in Anew[j, ] * Anew[j - 1, ] : non-conformable arrays

t(sapply(split(as.data.frame(Anew),as.numeric(gl(nrow(Anew),2,7))),colProds))
  [,1] [,2] [,3]
1    4   10   18
2   63   64   63
3   18   10    4
4    1    3    5

A.K.






________________________________
From: Bert Gunter <gunter.berton at gene.com>
To: arun <smartpink111 at yahoo.com> 
Cc: R help <r-help at r-project.org> 
Sent: Monday, September 2, 2013 10:55 AM
Subject: Re: [R] Product of certain rows in a matrix



These elaborate manipulations are unnecessary and inefficient. Use indexing instead:

j <- 2*seq_len(nrow(A)/2)
b <- A[j,]*A[j-1,]
b
[,1] [,2] [,3]
[1,]    4   10   18
[2,]   63   64   63
[3,]   18   10    4

[,1] [,2] [,3]
[1,]    4   10   18
[2,]   63   64   63
[3,]   18   10    4
[,1] [,2] [,3]
[1,]    4   10   18
[2,]   63   64   63
[3,]   18   10    4[,1] [,2] [,3]
[1,]    4   10   18
[2,]   63   64   63
[3,]   18   10    4
[,1] [,2] [,3]
[1,]    4   10   18
[2,]   63   64   63
[3,]   18   10    4





On Mon, Sep 2, 2013 at 7:25 AM, arun <smartpink111 at yahoo.com> wrote:

Hi,
>You could try:
>
>A<- matrix(unlist(read.table(text="
>1 2 3
>4 5 6
>7 8 9
>9 8 7
>6 5 4
>3 2 1
>",sep="",header=FALSE)),ncol=3,byrow=FALSE,dimnames=NULL)
>
>library(matrixStats)
> res1<-t(sapply(split(as.data.frame(A),as.numeric(gl(nrow(A),2,6))),colProds))
> res1
>#  [,1] [,2] [,3]
>#1    4   10   18
>#2   63   64   63
>#3   18   10    4
>
>
> res2<-t(sapply(split(as.data.frame(A),((seq_len(nrow(A))-1)%/%2)+1),colProds))
> identical(res1,res2)
>#[1] TRUE
>
>#or
> t(sapply(split(as.data.frame(A),as.numeric(gl(nrow(A),2,6))),function(x) apply(x,2,prod)))
>
>#or
>library(plyr)
> as.matrix(ddply(as.data.frame(A),.(as.numeric(gl(nrow(A),2,6))),colProds)[,-1])
>#     V1 V2 V3
>#[1,]  4 10 18
>#[2,] 63 64 63
>#[3,] 18 10  4
>
>#or
>do.call(rbind,tapply(seq_len(nrow(A)),list(as.numeric(gl(nrow(A),2,6))),FUN=function(x) colProds(A[x,])))
>#or
>A1<- data.frame(A,ID=as.numeric(gl(nrow(At),2,6)))
> aggregate(A1[,-4],list(A1[,4]),colProds)[,-1]
>#  X1 X2 X3
>#1  4 10 18
>#2 63 64 63
>#3 18 10  4
>
>#or
>library(data.table)
>At<- data.table(A1,key='ID')
>subset(At[,lapply(.SD,colProds),by=ID],select=-1)
>#   X1 X2 X3
>#1:  4 10 18
>#2: 63 64 63
>#3: 18 10  4
>
>A.K.
>
>
>
>
>Hello,
>
>I have this matrix :
>A =
>1 2 3
>4 5 6
>7 8 9
>9 8 7
>6 5 4
>3 2 1
>
>I would like to have this matrix (product of rows 2 by 2) :
>A =
>4 10 18
>63 64 63
>18 10 4
>
>Is it possible to do that without a loop ?
>
>Thank you in advance !
>
>______________________________________________
>R-help at r-project.org mailing list
>https://stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.
>


-- 


Bert Gunter
Genentech Nonclinical Biostatistics

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