The implementation of findInterval() in R from
Advanced R by Hadley Wickham. Given a vector of non-decreasing
breakpoints in v, find the interval containing each element of x; i.e.,
if i <- findInterval3(x,v), for each index j in x,
v[i[j]] <= x[j] < v[i[j] + 1], where v[0] := -Inf,
v[N+1] := +Inf, and N = length(v).
Usage
findInterval3(x, v)
Value
A vector of length(x) with values in 0:N where
N = length(v).