Implementing a frequency table using Data.List

A frequency map of values is often useful to detect outliers. We can use it to identify frequencies that seem out of the ordinary. In this recipe, we will be counting the number of different colors in a list.

How to do it...

Create a new file, which we will call Main.hs, and perform the following steps:

1. We will use the group and sort functions from Data.List:

   import Data.List (group, sort)

2. Define a simple data type for colors:

   data Color = Red | Green | Blue deriving (Show, Ord, Eq)

3. Create a list of these colors:

   main :: IO ()
   main = do
     let items = [Red, Green, Green, Blue, Red, Green, Green]

4. Implement the frequency map and print it out:

     let freq = 
        map (\x -> (head x, length x)) . group . sort $ items
     print freq

How it works...

Grouping identical items after sorting the list is the central idea.

See the following step-by-step evaluation in ghci:

Prelude> sort items

[Red,Red,Green,Green,Green,Green,Blue]
Prelude> group it

[[Red,Red],[Green,Green,Green,Green],[Blue]]

Prelude> map (\x -> (head x, length x)) it

[(Red,2),(Green,4),(Blue,1)]

Tip

As we may expect, sorting the list is the most expensive step.

See also

A cleaner version of the code is possible by using Data.MultiSet described in the next recipe, Implementing a frequency table using Data.MultiSet.