Question

So if I have a data frame that looks like:

             A     B      C  
  rowname1   4.5   4      3.2
  rowname2   3     23     9

How do I get R to give me the name(s) of the row/columns that contain a particular number?

i.e. if I give the value 3, it gives back

 rowname2,A

Answer 1

Assuming no duplicates, you can use which combined with the arr.ind argument:

df <- data.frame(matrix(sample(1:100,12), ncol=3))
#    X1 X2 X3
# 1  84 58 36
# 2   9 40 92
# 3 100 28 78
# 4  15 98 29

index <- which(df==36, arr.ind=TRUE)
#      row col
# [1,]   1   3

If you must have the actual row and column names of the location, then just index into them appropriately:

paste(rownames(df)[index[1]], colnames(df)[index[2]], sep=", ")
# [1] "1, X3"