apply over matrix by column - any way to get column name?

Question

I'm calling apply(data, 2, function(x) {...}) where data is a matrix. Is there any way for the function to know the name of the column whose values are present in x? Or perhaps there's another function to accomplish the same task?

Answer 1

One common way to deal with this is to loop over the column names themselves:

m <- matrix(runif(16),4,4)
colnames(m) <- letters[1:4]
sapply(colnames(m),function(x){mean(m[,x])})
        a         b         c         d 
0.4720319 0.4992337 0.5026318 0.3514267

Answer 2

You can use sapply on colnames(data):

set.seed(21)
Data <- matrix(rnorm(10),5,2)
colnames(Data) <- c("one","two")
sapply(colnames(Data), function(x) sum(Data[,x]))
#       one       two 
#  3.987540 -2.010875 
colSums(Data)
#       one       two 
#  3.987540 -2.010875