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How to aggregate matrix columns by row names in R?
To aggregate matrix columns by row names, we can use colSums with sapply and transpose the output. For example, if we have a matrix called M then the aggregate matrix columns by row names can be done using t(sapply(by(M,rownames(M),colSums),identity)).
Example1
Consider the below matrix −
> M1<-matrix(rpois(40,2),ncol=2) > rownames(M1)<-sample(c("A","B","C","D"),20,replace=TRUE) > M1
Output
[,1] [,2] B 4 6 D 2 1 B 1 5 C 0 0 A 2 3 B 1 0 B 5 3 D 1 3 C 0 1 C 0 2 D 1 1 B 7 2 C 2 3 C 2 3 C 2 2 C 1 2 B 1 1 C 5 3 D 2 2 D 5 2
Finding the aggregate of M1 by row names −
> t(sapply(by(M1,rownames(M1),colSums),identity))
Output
V1 V2 A 2 3 B 19 17 C 12 16 D 11 9
Example2
> M2<-matrix(rpois(40,10),ncol=2) > rownames(M2)<-sample(c("S1","S2","S3"),20,replace=TRUE) > M2
Output
[,1] [,2] S1 6 10 S3 7 11 S3 13 6 S1 9 9 S2 11 9 S3 6 8 S1 4 5 S3 8 12 S2 14 8 S3 11 7 S1 15 12 S3 8 13 S3 9 10 S1 10 8 S2 14 18 S1 13 10 S1 10 4 S3 12 7 S2 5 9 S3 13 8
Finding the aggregate of M2 by row names −
> t(sapply(by(M2,rownames(M2),colSums),identity))
Output
V1 V2 S1 67 58 S2 44 44 S3 87 82
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