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

An ugly dplyr solution:

library(dplyr)
df <- as.data.frame(df)

df %>% mutate(End = V2[findInterval(V1, na.omit(V2)) + 1]) %>%
       group_by(End) %>%
       summarise(Start = paste(V1, collapse=", "))

Edit - using findInterval thanks to @bgoldst