Question

I have a matrix from which I would like to select an observation together with its column name.

If I use subset() or matrix[]%in% then I get the whole row.

I would only like to get that single observation and its column name.
Is there a way to do this?

Here’s an example:

matrix:

        col1 col2 col3
row 1    10   20    30 
row 2    30   30    40

Now I would like to select the value and column name with the highest value:

max <- max(matrix)
subset(matrix, matrix==max)

This gives the output:

        col1 col2 col3
row 2    30   30    40

However what I would like to get is:

       col3
        40

Answer 1

As you already know the value, you just need to get the column names.

Calling your matrix m for the obvious reason:

m <- matrix(c(10,30,20,30,30,40), ncol=3)
colnames(m) <- c('col1','col2','col3')

To see which columns contain the max (or any particular value), and their names, apply can be used to return a logical vector with names:

apply(m==10, 2, any)
col1  col2  col3 
TRUE FALSE FALSE 

apply(m==30, 2, any)
col1 col2 col3 
TRUE TRUE TRUE 

which tells you which values are true:

which(apply(m==40, 2, any))
col3 
   3 

And just the names, if that is what you need:

names(which(apply(m==40, 2, any)))
[1] "col3"