# [Rd] findInterval Documentation Suggestion

Martin Maechler m@ech|er @end|ng |rom @t@t@m@th@ethz@ch
Fri Mar 6 14:56:51 CET 2020

```>>>>> brodie gaslam via R-devel
>>>>>     on Thu, 5 Mar 2020 22:18:33 +0000 (UTC) writes:

> I've found over time that R documentation that comes off as terse at
> first blush is usually revealed to be precise, concise, and complete
> on close reading.  I'm sure this is also true of `?findInterval`, but
> for whatever reason my brain simply refuses to extract meaning from it.

> Part of the problem may be that we interact with the function via a
> compressed form of the bounds of the intervals (only specify left bounds
> for 2nd interval onwards), but the semantics are described mostly in
> terms of the intervals themselves.  This requires indirections to map
> the parameters to the concepts.

> An alternative is to first describe what the function does directly in
> terms of its inputs, and subsequent relate that to the intervals.  If I
> understand correctly (in default mode) the function can be described as:

>      Given a vector of non-decreasing values 'vec', for each value in
>      'x' return the highest position in 'vec' that corresponds to a
>      value less than or equal to that 'x' value, or zero if none are.
>      Equivalently, if the values in 'vec' are taken to be the closed
>      left-bounds of contiguous half-open intervals, return which of
>      those intervals each value of 'x' lies in.

> Compared to the original:

>      Given a vector of non-decreasing breakpoints in ‘vec’, find the
>      interval containing each element of ‘x’; i.e., if ‘i <-
>      findInterval(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)’.  At the two boundaries, the returned index may differ
>      by 1, depending on the optional arguments ‘rightmost.closed’ and
>      ‘all.inside’.

Note that the  * -> LaTex -> PDF rendered version looks a bit
nicer.

See lower part of page 206 of (the 33nn pages of)
https://cran.r-project.org/doc/manuals/r-release/fullrefman.pdf

I wrote the function and that help page originally.  Of
course, I'm interested to hear how to improve the documentation.
However, the help pages make up the "Reference Manual", and so
-- as you mention initially -- should be precise and (mostly)
comprehensive.

For that reason, replacing the well defined precise
inequality-based definition by *much* less precise English prosa
is out of the question.

Extending that very long first sentence
"Given .... .... .... length(v)'.
by adding some helper words or other means may be fine and
indeed an improvement, .. so I'm happy for another try.

Martin

> Obviously you would be right to question whether someone who claims not
> to understand the documentation should venture to re-write it.
> Nonetheless I attach a proposed alternate version in the hopes that
> someone who clearly understand the original might use or adapt parts of it to
> make `?findInterval` more accessible to those comprehension-challenged
> like me.

> Best,
> Brodie
> ______________________________________________
> R-devel using r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

```