Finding all unique pairings in a list

Comparing all pairs of items is a common idiom in data analysis. This recipe will cover how to create a list of element pairs out of a list of elements. For example, if there is a list [1, 2, 3], we will create a list of every possible pair-ups [(1, 2), (1, 3), (2, 3)].

Notice that the order of pairing does not matter. We will create a list of unique tuple pairs so that we can compare each item to every other item in the list.

How it works…

Create a new file, which we call Main.hs, and insert the code explained in the following steps:

1. Import the following packages:

   import Data.List (tails, nub, sort)

2. Construct all unique pairs from a list of items as follows:

   pairs xs = [(x, y) | (x:ys) <- tails (nub xs), y <- ys]

3. Print out all unique pairings of the following list:

   main = print $ pairs [1,2,3,3,4]

4. The output will be as follows:

   [(1,2),(1,3),(1,4),(2,3),(2,4),(3,4)]
   

See also

We can apply the pairs algorithm to the Using the Pearson correlation coefficient...