[Rd] findInterval

[Gabor Grothendieck]

Suppose we have `dat` shown below and we want to find the the `y` value
corresponding to the last value in `x` equal to the corresponding component
of `seek` and we wish to return an output the same length as `seek` using
`findInterval` to perform  the search.  This returns the correct result:

  dat <- data.frame(x = c(2, 2, 3, 4, 4, 4),
    y = c(37, 12, 19, 30, 6, 15),
    seek = 1:6)

  zero2na <- function(x) replace(x, x == 0, NA)
  dat |>
    transform(dat, result = y[ zero2na(findInterval(seek, x)) ] ) |>
    _$result
   ## [1] NA 12 19 15 15 15

Since `findInterval` returns an index it is natural that the next step be
to use the index and it is also common that we want a result that is the
same length as the input.

The extra step here is to convert the 0 which `findInterval`
hard codes as missing to NA.

If, like `match`, the `findInterval` function had a `nomatch=` argument we
could have written this as follows which is shorter, more understandable
and avoids the need for zero2na:

  # if nomatch= were implemented
  seek |>
    transform(result = y[ findInterval(x, nomatch = NA) ] ) |>
    _$result

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