# [R] adding matrices with common column names

Murali.MENON at fortisinvestments.com Murali.MENON at fortisinvestments.com
Fri Mar 27 14:24:42 CET 2009

```folks,

if i have three matrices, a, b, cc with some colnames in common, and i
want to create a matrix which consists of the common columns added up,
and the other columns tacked on, what's a good way to do it? i've got
the following roundabout code for two matrices, but if the number of
matrices increases, then i'm a bit stymied.

> a <- matrix(1:20,ncol=4); colnames(a) <- c("a","b","c","d") b <-
> matrix(1:20,ncol=4); colnames(b) <- c("b","c","d", "e")
> cbind(a[,!(colnames(a) %in% colnames(b)), drop = FALSE],
a[,intersect(colnames(a),colnames(b))] +
b[,intersect(colnames(a),colnames(b)), drop = FALSE],
b[,!(colnames(b) %in% colnames(a)), drop = FALSE])

a  b  c  d  e
[1,] 1  7 17 27 16
[2,] 2  9 19 29 17
[3,] 3 11 21 31 18
[4,] 4 13 23 33 19
[5,] 5 15 25 35 20

now, what if i had a matrix cc? i want to perform the above operation on
all three matrices a, b, cc.

> cc <- matrix(1:10,ncol=2); colnames(cc) <- c("e","f")

i need to end up with:

a  b  c  d  e  f
[1,] 1  7 17 27 17  6
[2,] 2  9 19 29 19  7
[3,] 3 11 21 31 21  8
[4,] 4 13 23 33 23  9
[5,] 5 15 25 35 25 10

and, in general, with multiple matrices with intersecting colnames?

thanks,

murali

```