The centroid of a list of points

Question

New to Haskell

The problem is

-- The centroid of a list of points is a point whose x (and y) coordinates are
-- the means of the x (and y) coordinates of the points in the list.
--
-- You may assume the list contains at least one point.
--
-- > centroid [Pt 1 1, Pt 2 2]
-- Pt 1.5 1.5
-- > centroid [Pt (-1.5) 0, Pt 3 2, Pt 0 1]
-- Pt 0.5 1.0

Try to code like this

data Point = Pt Double Double deriving (Show, Eq)

centroid :: [Point] -> Point

pointX :: Point -> Double
pointX (Pt x y) = x
pointY :: Point -> Double
pointY (Pt x y) = y

pointsX :: [Point] -> [Double]
pointsX xs = map pointX xs
pointsY :: [Point] -> [Double]
pointsY xs = map pointY xs

average :: [Double] -> Double
average xs = (sum xs) `div` (genericLength xs)

centroid cenpoint = (Pt average(pointsX cenpoint) average(pointsY cenpoint))

And I got

Project1.hs:35:22: error:
    • Couldn't match expected type ‘([Double] -> Double)
                                    -> [Double] -> Point’
                  with actual type ‘Point’
    • The function ‘Pt’ is applied to four arguments,
      but its type ‘Double -> Double -> Point’ has only two
      In the expression:
        (Pt average (pointsX cenpoint) average (pointsY cenpoint))
      In an equation for ‘centroid’:
          centroid cenpoint
            = (Pt average (pointsX cenpoint) average (pointsY cenpoint))
   |
35 | centroid cenpoint = (Pt average(pointsX cenpoint) average(pointsY cenpoint))
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Project1.hs:35:25: error:
    • Couldn't match expected type ‘Double’
                  with actual type ‘[Double] -> Double’
    • Probable cause: ‘average’ is applied to too few arguments
      In the first argument of ‘Pt’, namely ‘average’
      In the expression:
        (Pt average (pointsX cenpoint) average (pointsY cenpoint))
      In an equation for ‘centroid’:
          centroid cenpoint
            = (Pt average (pointsX cenpoint) average (pointsY cenpoint))
   |
35 | centroid cenpoint = (Pt average(pointsX cenpoint) average(pointsY cenpoint))
   |                         ^^^^^^^

Project1.hs:35:33: error:
    • Couldn't match expected type ‘Double’ with actual type ‘[Double]’
    • In the second argument of ‘Pt’, namely ‘(pointsX cenpoint)’
      In the expression:
        (Pt average (pointsX cenpoint) average (pointsY cenpoint))
      In an equation for ‘centroid’:
          centroid cenpoint
            = (Pt average (pointsX cenpoint) average (pointsY cenpoint))
   |
35 | centroid cenpoint = (Pt average(pointsX cenpoint) average(pointsY cenpoint))
   |

user_67579 · Answer

The main problem here is that you call the functions, like you would do in a language like Java, C++, or Python.

You need to wrap average into parenthesis, like:

centroid cenpoint = Pt (average (pointsX cenpoint)) (average (pointsY cenpoint))

As @dfeuer says, the outer parenthesis are not necessary here.

Furthermore you probably want to use (/) here over div, since you are working with Doubles, whereas div works on Integral types:

average :: [Double] -> Double
average xs = sum xs / genericLength xs

but like @leftroundabout says, we can better use length, and then use fromIntegral:

average :: [Double] -> Double
average xs = sum xs / fromIntegral (length xs)

user_8475054 · Answer

You were doing pretty well - note how the compilation errors all point to your final line (the definition of centroid). And they're all due to incorrect bracketing. This is what it should be instead:

centroid cenpoint = Pt (average (pointsX cenpoint)) (average (pointsY cenpoint))

That is, the x co-ordinate of the resulting Pt value is the average of the x co-ordinates of the corresponding points (pointsX cenpoint is the list of x-coordinates, so average (pointsX cenpoint) is their average), and similarly for the y co-ordinate.

Comparing to your incorrect version (which I have tidied up a little without changing how it is interpreted by the compiler):

Pt average (pointsX cenpoint) average (pointsY cenpoint)

This means you apply the Pt function to 4 arguments: average, pointsX cenpoint, average again, and pointsY cenpoint. This simply doesn't work because the Pt function only takes 2 arguments.

user_745903 · Answer

Not really an answer to the question, but – this is the preferred way of doing it:

{-# LANGUAGE DeriveGeneric, DeriveAnyClass #-}

import Data.VectorSpace
import GHC.Generics

data Point = Pt Double Double
   deriving (Eq, Show, Generic, AdditiveGroup, VectorSpace)

average :: (VectorSpace v, Fractional (Scalar v)) => [v] -> v
average ps = sumV ps ^/ fromIntegral (length xs)

Then you can directly do

> average [Pt (-1.5) 0, Pt 3 2, Pt 0 1]
Pt 0.5 1.0

I.e. centroid ≡ average.

The centroid of a list of points

3 Answers