# [R] get cells by the combination of their column and row names

David Winsemius dwinsemius at comcast.net
Wed Apr 20 22:40:10 CEST 2011

```On Apr 20, 2011, at 3:52 PM, yoav baranan wrote:

>
> Hi,
> I have a (correlation) matrix and I want to select a subset of its
> cells depending on the combination of their column and row names.
> This illustrates my problem: mtrx <- matrix(c(1,2,3,4,5,6,7,8,9),
> nrow=3, ncol=3, dimnames = list(c('c132','c432', 'c233'),
> c('r132','r233', 'r432')))> mtrx     r132 r233 r432c132    1    4
> 7c432    2    5    8c233    3    6    9
> At this point I want to compute the mean of the cells in which the
> column and the row names share the same suffix (i.e., the cells
> mtrx[1,1], mtrx[2,3], mtrx[3,2]). By suffix I mean the last three
> digits of the row/col name.
> I need a generic code (and not vec<-c(mtrx[1,1], mtrx[2,3],
> mtrx[3,2])) because I don't know in advance how many rows and
> columns I will have (currently 118).

> mtrx[ sub("r|c", "", rep(rownames(mtrx),times=3) ) ==
sub("r|c", "", rep(colnames(mtrx), each=3))]
 1 6 8

> mean( mtrx[ sub("r|c", "", rep(rownames(mtrx),times=3) ) ==
sub("r|c", "", rep(colnames(mtrx), each=3))] )
 5

Obviously you need to substitute NROW() and NCOL() vlues for the rep
arguments but that should be simple.

> Thanks, Yoav
> 	[[alternative HTML version deleted]]
>
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