Sorting Without Duplicates, Revisited
February 10, 2017
In the comments to the previous exercise, reader Kevin gave a beautiful solution in Haskell; it sorts the input items, groups them into sub-lists of like items, transposes the rows and columns of the sub-lists, and concatenates the sub-lists back into a single list:
import Data.List sortwodup :: (Ord a) => [a] -> [a] sortwodup = concat . transpose . group . sort
Here’s the pipeline of functions applied to the sample input (2 9 1 5 1 4 9 7 2 1 4):
Sort converts the input list to (1 1 1 2 2 4 4 5 7 9 9).
Group converts the output of sort to a list of sub-lists: ((1 1 1) (2 2) (4 4) (5) (7) (9 9)).
Transpose swaps rows and columns, ignoring missing items, like this:
1 1 1 1 2 4 5 7 9
2 2 1 2 4 9
4 4 1
5
7
9 9
Thus, the output from transpose is ((1 2 4 5 7 9) (1 2 4 9) (1)). Finally, concatenate removes the sub-list structure, producing (1 2 4 5 7 9 1 2 4 9 1).
Your task is to write a Haskell-style program to solve the problem of sorting a list, moving duplicates to the end. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.
Programming Praxis 
Don’t you want
“`
(define (transpose l) (apply map list l))
“`
?
@Gambiteer: Your suggestion works if the list of lists is rectangular. For instance, (apply map list ‘((1 2 3) (4 5 6))) gives ((1 4) (2 5) (3 6)). But the result of the group-by is not necessarily rectangular. For instance, in the sample problem, the result of (group-by = (sort < '(2 9 1 5 1 4 9 7 2 1 4))) is ((1 1 1) (2 2) (4 4) (5) (7) (9 9)), which is not rectangular. The function you suggest is in the Standard Prelude with the name zip, where it would be called as (zip '(1 2 3) '(4 5 6)).
A Haskell version. The sorting and grouping are the same as in the previous problem. The foldr builds up two lists: the first consists of the head of each sublist produced by group; the second is the remaining elements (if any) of the sublists. Finally, we append the two lists from foldr.
With warnings enabled the compiler will complain that step doesn’t include a pattern match on [] (the empty list), but it can never be called that way since the sublists produced by group are always non-empty.
Equivalent Python version.
Uses the ’roundrobin’ recipe from the itertools documentation (https://docs.python.org/3.5/library/itertools.html#itertools-recipes)
from itertools import groupby from itertools_recipes import roundrobin def sortwithoutdups2(lst): return list(roundrobin(*(list(v) for _,v in groupby(sorted(d)))))Fixed a typo
from itertools import groupby from itertools_recipes import roundrobin def sortwithoutdups2(lst): return list(roundrobin(*(list(v) for _,v in groupby(sorted(lst)))))