Calculate the difference between the largest and smallest column for each row

Question

The title is pretty straight forward - how can I calculate the difference between the largest and smallest value column-wise, for each row?

Let\'s assume this is my data

Answer 1

Here's an attempt using my old favourite max.col with a bit of matrix indexing:

rw <- seq_len(nrow(dat))
dat[cbind(rw, max.col(dat))] - dat[cbind(rw, max.col(-dat))]
#[1] 3 9 3 3

This should be much faster on large datasets, as per:

# 5 million big enough?
dat <- dat[sample(1:4,5e6,replace=TRUE),]

system.time({
  rw <- seq_len(nrow(dat))
  dat[cbind(rw, max.col(dat))] - dat[cbind(rw, max.col(-dat))]
})
#   user  system elapsed 
#   2.43    0.20    2.63 

system.time({
  apply(X = dat, MARGIN = 1, function(x) diff(range(x)))
})
#   user  system elapsed 
#  94.91    0.17   95.16