# [erlang-questions 83] Re: A sudoku solver in Erlang compared to Python

Andreas Pauley <>
Mon Mar 28 15:12:02 CEST 2011

```Cool, good catch!
https://github.com/apauley/sudoku-in-erlang/commit/9125147621faeb11cd46cea5100b2e1e88ff9732

This reduces the elimination count significantly :-)

Solved 50 of 50 puzzles from easy50.txt in 1.286811 secs (38.86 Hz)
(32833 total eliminations, avg 656.66, median 648, max 808, min 648).
Solved 95 of 95 puzzles from top95.txt in 8.572484 secs (11.08 Hz)
(210794 total eliminations, avg 2218.88, median 1536, max 10664, min 648).
Solved 11 of 11 puzzles from hardest.txt in 0.391032 secs (28.13 Hz)
(9516 total eliminations, avg 865.09, median 813, max 1227, min 648).

On Sun, Mar 27, 2011 at 11:23 PM, Ahmed Omar <> wrote:
> running test after fixing the counter gives the following
> Solved 95 of 95 puzzles from top95.txt in 2.235227 secs (42.50 Hz)
> (210794 total eliminations, avg 2218.88, median 1536, max 10664, min 648).
>
> On Sun, Mar 27, 2011 at 11:21 PM, Ahmed Omar <> wrote:
>>
>> No problem, as said before u can also replace these lines :
>> PeerSet = gb_sets:from_list(NonUniquePeers),
>> PeersWithSelf = gb_sets:to_list(PeerSet),
>> lists:delete(Square, PeersWithSelf).
>> with just
>> lists:delete(Square, lists:usort(NonUniquePeers)).
>> (don't forget to check the rest of the thread too ;))
>> On Sun, Mar 27, 2011 at 11:02 PM, Andreas Pauley <>
>> wrote:
>>>
>>> Thanks!
>>> I didn't think about the sets at all :-)
>>>
>>>
>>> https://github.com/apauley/sudoku-in-erlang/commit/a02fbcd001a9fbd876725008d58c52fcff9872d9
>>>
>>> On Sat, Mar 26, 2011 at 9:08 PM, Ahmed Omar <>
>>> wrote:
>>> > using gb_sets instead of sets could decrease eliminations a bit and
>>> > give u
>>> > some boost. However, i didn't dive deeper into the code or the
>>> > algorithm.
>>> >
>>> > On Sat, Mar 26, 2011 at 9:41 AM, Andreas Pauley <>
>>> > wrote:
>>> >>
>>> >> Thanks, I've changed the compile options and this definitely makes it
>>> >> somewhat faster:
>>> >>
>>> >>
>>> >> https://github.com/apauley/sudoku-in-erlang/commit/b7ce2abb6ca013850ed8f3e8fd7f5f6be7004cbb
>>> >>
>>> >> Strangely the native flag is not documented here:
>>> >> http://www.erlang.org/doc/man/compile.html#file-2
>>> >>
>>> >> The more interesting improvement would be to decrease the number of
>>> >> eliminations performed, but for that I'll have to go deep into the
>>> >> code :-)
>>> >>
>>> >>
>>> >> On Sat, Mar 26, 2011 at 12:16 AM, Evans, Matthew <>
>>> >> wrote:
>>> >> > Without going deep into the code, one thing to try is compile with
>>> >> > the
>>> >> > native flag (running the tests from the VM shell here):
>>> >> >
>>> >> > Without native set:
>>> >> > 18> c(sudoku).
>>> >> > {ok,sudoku}
>>> >> > 19>  sudoku:print_results("top95.txt", "\n").
>>> >> > Solved 95 of 95 puzzles from top95.txt in 2.038825 secs (46.60 Hz)
>>> >> > (922678 total eliminations, avg 9712.40, median 6596, max 55370, min
>>> >> > 1797).
>>> >> > ok
>>> >> >
>>> >> >
>>> >> > With native set:
>>> >> > 20> c(sudoku,[native]).
>>> >> > {ok,sudoku}
>>> >> > 21>  sudoku:print_results("top95.txt", "\n").
>>> >> > Solved 95 of 95 puzzles from top95.txt in 1.613416 secs (58.88 Hz)
>>> >> > (922678 total eliminations, avg 9712.40, median 6596, max 55370, min
>>> >> > 1797).
>>> >> >
>>> >> >
>>> >> >
>>> >> > -----Original Message-----
>>> >> > From:
>>> >> > [mailto:] On Behalf Of Andreas
>>> >> > Pauley
>>> >> > Sent: Friday, March 25, 2011 8:15 AM
>>> >> > To:
>>> >> > Subject: [erlang-questions 4] A sudoku solver in Erlang compared to
>>> >> > Python
>>> >> >
>>> >> > Hi all,
>>> >> >
>>> >> > In my quest to learn Erlang I've translated Peter Norvig's sudoku
>>> >> > solver into Erlang:
>>> >> > https://github.com/apauley/sudoku-in-erlang
>>> >> >
>>> >> > For comparison, my slightly modified version of Norvig's Python code
>>> >> > can be found here:
>>> >> > https://github.com/apauley/sudoku-by-norvig
>>> >> >
>>> >> > I like the pattern matching, it was fun to do the entire solution
>>> >> > without a single if or case statement :-)
>>> >> > Something that bothers me though is that the Python solution seems
>>> >> > to
>>> >> > be faster, even after I've made the Erlang solution use multiple
>>> >> > processors.
>>> >> >
>>> >> > In order to compare the two solutions I counted the number of
>>> >> > eliminations each performed.
>>> >> > The eliminate function is the core of the solution, for example
>>> >> > assigning a single value to a square is implemented as the
>>> >> > elimination
>>> >> > of all other values.
>>> >> >
>>> >> > With the Erlang implementation I get:
>>> >> > sudoku-in-erlang\$ ./sudoku
>>> >> > All tests passed :-)
>>> >> > Solved 50 of 50 puzzles from easy50.txt in 2.890978 secs (17.30 Hz)
>>> >> >  (93403 total eliminations, avg 1868.06, median 1810, max 2517, min
>>> >> > 1770).
>>> >> > Solved 95 of 95 puzzles from top95.txt in 22.004369 secs (4.32 Hz)
>>> >> >  (922678 total eliminations, avg 9712.40, median 6596, max 55370,
>>> >> > min
>>> >> > 1797).
>>> >> > Solved 11 of 11 puzzles from hardest.txt in 0.851678 secs (12.92 Hz)
>>> >> >  (32339 total eliminations, avg 2939.91, median 2894, max 4779, min
>>> >> > 1781).
>>> >> >
>>> >> > And with the Python implementation:
>>> >> > sudoku-by-norvig\$ ./sudoku.py
>>> >> > All tests pass.
>>> >> > Solved 50 of 50 puzzles from easy50.txt in 0.792008 secs (63.13 Hz)
>>> >> >  (33059 total eliminations, avg 661.00, median 648, max 830, min
>>> >> > 648).
>>> >> > Solved 95 of 95 puzzles from top95.txt in 5.903875 secs (16.09 Hz)
>>> >> >  (221997 total eliminations, avg 2336.00, median 1492, max 11512,
>>> >> > min
>>> >> > 648).
>>> >> > Solved 11 of 11 puzzles from hardest.txt in 0.237532 secs (46.31 Hz)
>>> >> >  (9436 total eliminations, avg 857.00, median 817, max 1198, min
>>> >> > 648).
>>> >> >
>>> >> > So according to the stats above, the Python solution performs less
>>> >> > computations when given exactly the same input.
>>> >> > The Erlang code is as close to the Python as I could make it, I've
>>> >> > done more or less a direct translation of the algorithms used.
>>> >> >
>>> >> > I suspect that there are some lazy evaluation happening in the
>>> >> > Python
>>> >> > version, possibly generators, although I haven't pinpointed it yet.
>>> >> >
>>> >> > How can I improve my Erlang code in this solution?
>>> >> >
>>> >> > Kind regards,
>>> >> > Andreas
>>> >> > _______________________________________________
>>> >> > erlang-questions mailing list
>>> >> >
>>> >> > http://erlang.org/mailman/listinfo/erlang-questions
>>> >> >
>>> >> _______________________________________________
>>> >> erlang-questions mailing list
>>> >>
>>> >> http://erlang.org/mailman/listinfo/erlang-questions
>>> >
>>> >
>>> >
>>> > --
>>> > Best Regards,
>>> > - Ahmed Omar
>>> > @spawn_think
>>> >
>>
>>
>>
>> --
>> Best Regards,
>> - Ahmed Omar
>> @spawn_think
>
>
>
> --
> Best Regards,
> - Ahmed Omar