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Today's Topics:
1. Re: Sorting (Erlend Hamberg)
2. Re: Sorting (mike h)
----------------------------------------------------------------------
Message: 1
Date: Tue, 13 Dec 2016 16:01:41 +0000
From: Erlend Hamberg <erl...@hamberg.no>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Sorting
Message-ID:
<ca+g9oxn3n3+rpfjekhxfqsb_jxowk+5sooejcjndarxebnm...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
There is a really nice solution that takes advantage of Ordering's Monoid
instance (see https://wiki.haskell.org/Monoid).
The imports you need:
import Data.List (sortBy)
import Data.Ord (Down(..), comparing)
import Data.Monoid ((<>)) -- the “mappend” operator
You can then combine two calls to `comparing`
sortBy (comparing (Down . snd) <> comparing fst) xs
(`Down` is just a newtype that reverses the ordering, since you wanted the
first element in descending order and the second in ascending order.)
On Tue, 13 Dec 2016 at 16:30 Francesco Ariis <fa...@ariis.it> wrote:
> On Tue, Dec 13, 2016 at 02:36:39PM +0000, mike h wrote:
> > Hi,
> >
> > I’m trying to sort a list of tuples. A char and a count of that char
> (Char , Int)
> > e.g.
> >
> > [ ('r',2), ('c',2),('a', 2), ('b',3), ('f',2)]
> >
> > e.g. ‘r’ occurs twice etc.
> > The order should be based on the count first and then ties broken by the
> > natural ordering of char.
>
> You should provide sortBy with an appropriate compare function, e.g.
>
> comp (a,b) (c,d) | a > c = GT
> | -- etc etc.
>
> or go with the manky but working hack:
>
> λ> :m Data.List
> λ> sortOn (\(a, b) -> b*(-100) + fromEnum a) [('r',2), ('c',2),('a', 2),
> ('b',3), ('f',2)]
> [('b',3),('a',2),('c',2),('f',2),('r',2)]
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
--
Erlend Hamberg
erl...@hamberg.no
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Message: 2
Date: Tue, 13 Dec 2016 16:15:18 +0000
From: mike h <mike_k_hough...@yahoo.co.uk>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Sorting
Message-ID: <877cdeb9-e4a5-4daa-b43e-c44bb1e63...@yahoo.co.uk>
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Thanks folks.
Francesco
Cool - I just came to that conclusion too and did
tupleOrder :: (Char, Int) -> (Char, Int) -> Ordering
tupleOrder (c1, x1) (c2, x2)
-- char compared by ord and a is less than b!
| x1 == x2 && c1 <= c2 = GT
| x1 == x2 && c1 >= c2 = LT
| x1 < x2 = LT
| x1 > x2 = GT
and then did sortBy.
Erlend
I’ll try that - Monoids have such an understated elegance. :)
> On 13 Dec 2016, at 16:01, Erlend Hamberg <erl...@hamberg.no> wrote:
>
> There is a really nice solution that takes advantage of Ordering's Monoid
> instance (see https://wiki.haskell.org/Monoid
> <https://wiki.haskell.org/Monoid>).
>
> The imports you need:
>
> import Data.List (sortBy)
> import Data.Ord (Down(..), comparing)
> import Data.Monoid ((<>)) -- the “mappend” operator
>
> You can then combine two calls to `comparing`
>
> sortBy (comparing (Down . snd) <> comparing fst) xs
>
> (`Down` is just a newtype that reverses the ordering, since you wanted the
> first element in descending order and the second in ascending order.)
>
> On Tue, 13 Dec 2016 at 16:30 Francesco Ariis <fa...@ariis.it
> <mailto:fa...@ariis.it>> wrote:
> On Tue, Dec 13, 2016 at 02:36:39PM +0000, mike h wrote:
> > Hi,
> >
> > I’m trying to sort a list of tuples. A char and a count of that char (Char
> > , Int)
> > e.g.
> >
> > [ ('r',2), ('c',2),('a', 2), ('b',3), ('f',2)]
> >
> > e.g. ‘r’ occurs twice etc.
> > The order should be based on the count first and then ties broken by the
> > natural ordering of char.
>
> You should provide sortBy with an appropriate compare function, e.g.
>
> comp (a,b) (c,d) | a > c = GT
> | -- etc etc.
>
> or go with the manky but working hack:
>
> λ> :m Data.List
> λ> sortOn (\(a, b) -> b*(-100) + fromEnum a) [('r',2), ('c',2),('a', 2),
> ('b',3), ('f',2)]
> [('b',3),('a',2),('c',2),('f',2),('r',2)]
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org <mailto:Beginners@haskell.org>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> <http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners>
> --
> Erlend Hamberg
> erl...@hamberg.no
> <mailto:erl...@hamberg.no>_______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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