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

cut is possibly useful here.

out <- cut(elements, t(intervals[c("start","end")]))
levels(out)[c(FALSE,TRUE)]  <- NA
intervals$phase[out]
#[1] "a" "a" "a" NA  "b" "b" "c"