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] 1 6 8
> mean( mtrx[ sub("r|c", "", rep(rownames(mtrx),times=3) ) ==
sub("r|c", "", rep(colnames(mtrx), each=3))] )
[1] 5
Obviously you need to substitute NROW() and NCOL() vlues for the rep
arguments but that should be simple.
Thanks, Yoav
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