R Optimizing double for loop, matrix manipulation

Question

I am trying to manipulate column data in a two column matrix and output it as a data.frame.

The matrix that I have is in this format where both the values in the sta

Answer 1

Here is a simple base R version

with(as.data.frame(dat), {
  data.frame(
    Start=tapply(Start, cut(Start, c(0, End)), c),
    End=na.omit(End)
  )
})
#        Start End
# 1    1, 2, 3   6
# 2       7, 8   9
# 3 11, 12, 14  15

Another

with(as.data.frame(dat), {
  group <- as.integer(cut(Start, c(0, End)))                  # assign Start values to End groups
  data.frame(
    Start=unclass(by(dat, group, function(g) g[["Start"]])),  # combine Start groups
    End=unique(na.omit(End))                                  # Remove duplicate/NA End values
  )
})