[Haskell-beginners] computing percentile

[Nick Vanderweit]

Here is my implementation, which sorts the list of values. If there are N 
values, the first gets a percentile of 0/N, the second gets 1/N, and so on, so 
that the last value gets a percentile of (N-1)/N. It then generates the 
percentiles list from this mapping.

import Data.List (sort)
import Data.Maybe (fromJust)
toPercentile xs =
    let pairs = zip (sort xs) .
          map (/(fromIntegral $ length xs)) . map fromInteger $ [0..]
      in map (fromJust . (flip lookup) pairs) xs


Nick

On Monday, October 01, 2012 03:11:40 PM Dennis Raddle wrote:
> Just thought I would toss the list a problem I need to solve and see what I
> can learn from the solutions here.
> 
> My problem is that I need to take a list of Double and map them into
> "percentile" form (not sure if I have that terminology correct) - so that
> the lowest number is mapped to 0, the highest number is mapped to 1, the
> median is mapped to 0.5, and in general given a fixed delta D and an
> interval (x, x +D) in the percentile domain, the same number of elements
> get mapped into that interval for any x. Except there aren't an infinity of
> values and there may be ties, so it gets a little fuzzier how to define it.
> 
> so I need:
> 
> toPercentile :: [Double] -> [Double]
> 
> I shall now work on it and report back.
> 
> Dennis