How do I retrieve a matrix column and row name by a matrix index value?

r matrix indexing rows
user1301593 picture user1301593 · Mar 29, 2012 · Viewed 49.1k times · Source

So let's say I have a matrix, mdat and I only know the index number. How do I retrieve the column and row names? For example:

> mdat <- matrix(c(1,2,3, 11,12,13), nrow = 2, ncol=3, byrow=TRUE, 
    dimnames = list(c("row1", "row2"), c("C.1", "C.2", "C.3"))) 
> mdat[4] 
[1] 12 
> names(mdat[4]) 
NULL 
> colnames(mdat[4]) 
NULL 
> rownames(mdat[4])
NULL 
> dimnames(mdat[4]) 
NULL

Answer

Aaron left Stack Overflow picture Aaron left Stack Overflow · Mar 29, 2012

First you need to get the row and column of that index using arrayInd.

k <- arrayInd(4, dim(mdat))

You can then get the right name by getting that element of the row and column names

rownames(mdat)[k[,1]]
colnames(mdat)[k[,2]]

Or both at once using mapply:

mapply(`[[`, dimnames(mdat), k)

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