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@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.
