apply a function over groups of columns

Question

How can I use apply or a related function to create a new data frame that contains the results of the row averages of each pair of columns in a very large data

Answer 1

mean for rows from vectors a,b,c

 rowMeans(dat[1:3])

means for rows from vectors d,e,f

 rowMeans(dat[4:6])

all in one call you get

results<-cbind(rowMeans(dat[1:3]),rowMeans(dat[4:6]))

if you only know the names of the columns and not the order then you can use:

rowMeans(cbind(dat["a"],dat["b"],dat["c"]))
rowMeans(cbind(dat["d"],dat["e"],dat["f"]))

#I dont know how much damage this does to speed but should still be quick