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

Question

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 

Solution 1

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)

Solution 2

Subsetting the matrix first results in a one-element vector that has no names, as you show in your question. Remember that subsetting creates a completely new object via copying. There's no way to reference the original mdat after subsetting.

This is more clear if you assign the result of subsetting to another object.

> m <- mdat[4]
> m
[1] 12
> names(m)  # no names were printed above... so
NULL

You really want to access the column/row names first and subset them.

> colnames(mdat)[3]
[1] "C.3"
> rownames(mdat)[2]
[1] "row2"

You can re-assign column/row names similarly.

> colnames(mdat)[3] <- "C3"
> rownames(mdat)[2] <- "row.2"
> mdat
      C.1 C.2 C3
row1    1   2  3
row.2  11  12 13

Comments

[Aaron left Stack Overflow]

In reading this answer again, I wonder if the OP only had the overall index because they ran which without arr.ind=TRUE. Future readers: if you're getting that index from which, use arr.ind=TRUE to get the row and column indices, instead of using arrayInd.