Find which interval row in a data frame that each element of a vector belongs in

Question

I have a vector of numeric elements, and a dataframe with two columns that define the start and end points of intervals. Each row in the dataframe is one interval. I want to

Answer 1

Just lapply works:

l <- lapply(elements, function(x){
    intervals$phase[x >= intervals$start & x <= intervals$end]
})

str(l)
## List of 7
##  $ : chr "a"
##  $ : chr "a"
##  $ : chr "a"
##  $ : chr(0) 
##  $ : chr "b"
##  $ : chr "b"
##  $ : chr "c"

or in purrr, if you purrrfurrr,

elements %>% 
    map(~intervals$phase[.x >= intervals$start & .x <= intervals$end]) %>% 
    # Clean up a bit. Shorter, but less readable: map_chr(~.x[1] %||% NA)
    map_chr(~ifelse(length(.x) == 0, NA, .x))
## [1] "a" "a" "a" NA  "b" "b" "c"