[R] Matrix manipulation

[Giovanni Petris]

> Hi, let's say I have this
> 
> A = matrix(c(1, 2, 4), nrow=1)
> colnames(A)=c("YOO1", "YOO2", "YOO3")

Why do you need A to be a matrix and not simply a vector?

> 
> # ie
> #      YOO1 YOO2 YOO3
> #[1,]    1    2    4
> 
> HELLO <- NULL
> HELLO$YOO1="BOO"
> HELLO$YOO2="BOO"
> HELLO$YOO3="HOO"
> 

Why do you need HELLO to be a list and not simply a character vector?

> and I want a matrix that will sum my categorization.. how can I do it
> efficiently without any loop?
> 
> #ie   BOO   HOO
> #[1,]  3         4
> 

Anyway, here is a solution:

x <- tapply(A, match(unlist(HELLO), unique(unlist(HELLO))), sum)
names(x) <- unique(unlist(HELLO))
x

HTH,
Giovanni

-- 

Giovanni Petris  <GPetris at uark.edu>
Associate Professor
Department of Mathematical Sciences
University of Arkansas - Fayetteville, AR 72701
Ph: (479) 575-6324, 575-8630 (fax)
http://definetti.uark.edu/~gpetris/