10 ms·

N-queen puzzle in 4 lines of Scala

• michaelaiello 10y ago
  This problem is also fun with perl regexes http://www.perlmonks.org/?node_id=297616 http://www.perlmonks.org/?node_id=297616
• dkopi 10y ago
  While definitely great proof of: 1. The author knowing a lot of the language's functional features (permutations, mapping, zipping, filters) 2. How powerful functional programming is when solving problems I find this type of code an anti pattern of how good code should be. This solution has a high "cognitive load" imho: http://chrismm.com/blog/how-to-reduce-the-cognitive-load-of-your-code/ http://chrismm.com/blog/how-to-reduce-the-cognitive-load-of-... I'd much rather see a 15-20 line solution that's clear and readable, than a 4 line solution that I have to reverse engineer.
  • tyingq 10y ago
    Here's < 15 lines, clear, but not functional programming :) #!/usr/local/bin/perl #borrowed from Nick Holloway <alfie@dcs.warwick.ac.uk>, 27th May 1993. my $size=8;my @cols=(0..$size-1); sub nextq { (print( "@_\n" ),return) if @_ == $size; foreach my $col(@cols) { next if grep($_ == $col, @_); my $row=@_;next if grep ($_ == $col-$row--, @_); my $row=@_;next if grep ($_ == $col+$row--, @_); nextq(@_,$col); } } nextq(); Not sure if it's slower or more resource intensive than the Scala solution. I'm sure it could be golfed down to be smaller than the Scala counterpart. It has fewer characters already, even with the comments intact.
    • username223 10y ago
      Indeed, it has been golfed: http://www.perlmonks.org/?node_id=196582 http://www.perlmonks.org/?node_id=196582 -l /.{@ARGV}/?print:map/((.).*)(.)(??{$2!=$3&&length$1!=abs$2-$3&&0})/||do$0,$_.1.."$_@ARGV"
      • _asummers 10y ago
        That's frightening. I hate to ask, but is that representative of Perl code?
        • dalke 10y ago
          No. Golf is a game where the winner has the lowest score. It's reused as the term for a programming challenge to implement an algorithm with the fewest number of characters. See https://en.wikipedia.org/wiki/Perl#Perl_pastimes https://en.wikipedia.org/wiki/Perl#Perl_pastimes . This is no more indicative of Perl than the Obfuscated C contest is indicative of C.
        • GregBuchholz 10y ago
          Javascript anyone? !function r(y,a,z,b,x){for(x=y--?-1:print(a);++x<n;z&c|b&d||r(y,a+1?a+' '+x:x,z|c,b|d))c=(1|1<<y+9)<<x,d=1<<9+x-y}(n=readline()) http://golf.shinh.org/reveal.rb?N+Queens/murky-satyr_1217229999&js http://golf.shinh.org/reveal.rb?N+Queens/murky-satyr_1217229...
        • username223 10y ago
          Not at all, but it is representative of the best of Perl culture: smart people who enjoy playing with unexpected corners of a complex system, while remembering the difference between "work" and "play." See Ton Hospel's insane Roman numeral calculator: http://perlmonks.org/?node_id=594299 http://perlmonks.org/?node_id=594299
    • laumars 10y ago
      It's actually more lines than it appears since the author has concatenated lines with a semi-colon: my $size=8;my @cols=(0..$size-1); my $row=@_;next if grep ($_ == $col-$row--, @_); my $row=@_;next if grep ($_ == $col+$row--, @_); Strictly speaking that's 6 lines of code concatenated into 3. But as you said, it would be relatively easy to golf that down and still keep the program logic.
      • tyingq 10y ago
        Perl being big on "more than one way to do things...", semicolons are easy to discard: sub nextq { my $sz=8; (print("@_\n"),return) if @_ == $sz; foreach my $col(0..$sz-1) { my $row; if (grep($_==$col,@_)) {next} else {$row=@_} if (grep($_==$col-$row--,@_)) {next} else {$row=@_} if (grep($_==$col+$row--,@_)) {next} else {$row=@_} nextq(@_,$col); } } nextq();
        • laumars 10y ago
          I still feel that's cheating as you're just replacing the semi-colon with braced blocks; effectively it's still being parsed as multiple lines. I was thinking more along the lines of using the one line conditional: condition ? first_expression : second_expression; eg sub nextq { (print( "@_\n" ),return) if @_ == (my $size = 8); foreach my $col(0..$size-1) { my ($row, $next); grep ($_ == $col, @_) ? $next=1 : $row=@_; grep ($_ == $col-$row--, @_) ? $next=1 : $row=@_; grep ($_ == $col+$row--, @_) ? $next=1 : $row=@_; nextq(@_,$col) unless $next; } } nextq(); The `$next` ugliness is due to not being allowed to use the `next` inside a one line conditional. There's probably cleaner ways of doing the above though. But that was only a quick hack.
          • shasta 10y ago
            It's almost as if "number of lines" isn't particularly meaningful.
            • 10y ago
    • ionforce 10y ago
      The lack of whitespace is maddening. It contributes to the reputation that Perl is write once read never. That "print and return with postfix" is the gold star. /s
      • tyingq 10y ago
        The context being a deliberate effort to make it compact. You can abuse features in most languages and make it hard to read.
        • ionforce 10y ago
          Who cares about "compactness"? You weren't golfing. It just sticks out as bad code.
          • tyingq 10y ago
            Not my code, it was copied (note the attribution) from a specific context where somebody cared about compactness. Edit: Perl's also not the only language that supports the comma operator, predicate if, ternary, and other things that seem to bother you.
  • CJefferson 10y ago
    Also, this is (I think, I'm not 100% up on Scala), insanely inefficient. Usually when people discuss the n-queens problem, they do a backtrack search, stopping as soon as some queen conflicts with an earlier queen. You can of course then go on and get increasingly clever, but I think you should at least aim for backtracking.
  • atemerev 10y ago
    Those are bread and butter of functional programming, like for loop, break and continue in imperative programming. There are really no advanced features here; not even a single monad! For someone completely unfamiliar with FP, the cognitive load might be high. However, even basic familiarity with FP constructs is enough to understand and appreciate the code.
    • hk__2 10y ago
      The fact that it needs two comments to clarify what it’s doing is a code smell in itself.
      • atemerev 10y ago
        So, comments are code smell?
        • deleted 10y ago
          [deleted]
        • hk__2 10y ago
          That’s not what I wrote.
      • Retra 10y ago
        It doesn't need those comments; you do.
  • dustingetz 10y ago
    I think a more reasonable comparison is 4 lines of code you have to think hard to decipher but is correct, vs 40 lines of code that has a bunch of bugs and edge cases and is just as jibberish but 10x more code to decipher. Shorter solutions often encode the authors insight from thinking about the problem, and longer solutions encode are more of a history of the path an author took while just hammering on it without any particular insight until it worked good enough.
  • mathetic 10y ago
    Arggh, every single time someone posts an incredible cool small piece of code that solves a difficult problem, comments like this appear. This whole HN attitude of pristine code is killing me. This is a piece of code in a GIST and solves the problem in three lines, it is clearly not intended for production use in a large code base. Please stop searching for something wrong and enjoy what is presented in all of its glory.
    • zyngaro 10y ago
      Maybe it's the case here but in the scala community in general I noted a tendency to emphasize cleverness over clarity and simplicity.
      • ADRIANFR 10y ago
        Relevant Stack Overflow question: http://stackoverflow.com/questions/7084212/is-scala-idiomatic-coding-style-just-a-cool-trap-for-writing-inefficient-code http://stackoverflow.com/questions/7084212/is-scala-idiomati...
    • deleted 10y ago
      [deleted]
    • haddr 10y ago
      The difference here is that when you read the title you think: "wow, scala features allows solving interesting problems in few steps, let's see". And the you realize that it's a cheat. This code is literaly squeezed into those 4 lines, full of less common idioms, and quite obscure. It should be called "0,5kb scala contest" than "solving x with scala", to avoid misunderstanding.
    • pyrale 10y ago
      This piece of code doesn't "solve" the problem, it's a crude bruteforce implementation. The problem is hard for a reason. Another way this code could be useful would be to showcase good readability practices, but it doesn't.
  • deleted 10y ago
    [deleted]
  • gawi 10y ago
    Better yet: 4 lines of code + a few lines of comments.
  • deleted 10y ago
    [deleted]
  • Veedrac 10y ago
    This has nothing to do with functional programming. Here's an iterative Python solution that's almost a direct translation but uses explicit iteration and avoids `filter`/`map`/etc. from itertools import permutations def n_queens(n): for p in permutations(range(n)): if len({c + i for i, c in enumerate(p)}) == len({c - i for i, c in enumerate(p)}) == n: yield p
    • atemerev 10y ago
      Looks quite functional to me. Except that it uses "for comprehension" instead of map/filter (a matter of preference).
      • Veedrac 10y ago
        Well then functional doesn't mean very much :P. Python's iterators are mutation-based, so not really functional. `yield` is based around pause-resume of function execution, so not really functional. `permutations` is just a function that wraps a list (or iterator coercible to one (which mutates the iterator)) and returns a (mutable) iterator. A `for` comprehension desugars to a `for` loop with `yield` if it's a lazy comprehension, `append` if it's a list comprehension or `add` if it's a set comprehension, which of course involves mutation. Basically nothing here is mutation free. That, at least IMO, means it's not functional.
        • willtim 10y ago
          You seem to be confusing interface and implementation. Lazy lists and for comprehensions appeared in functional programming languages first; and that Python code looks to have borrowed them.
          • Veedrac 10y ago
            Addition appeared in imperative languages first. Does that mean addition is imperative? It's great that functional languages were first to catch onto these ideas, but that doesn't mean that anything else that uses them becomes functional. The interface isn't even functional, since in code like foo = iter((1, 2, 3)) {x for x in foo} `foo` is mutated. No "true" functional language would accept that ;).
  • edejong 10y ago
    Such tight structures might be difficult to parse when the patterns used are still relatively unknown. However, once applied more widely, read more often and more familiar, you might start to think the longer version (which undoubtedly relies on ambiguous naming) becomes unreadable. Said in other words: 15 line solutions cannot break into the higher-order pattern-recognition space, since these solutions rely on context specific naming. Naming is one of the most difficult parts of software engineering, so we should try to rely on contextual patterns in order to minimize naming altogether.
    • richard_todd 10y ago
      Indeed, one thing I initially liked about Prolog was that you can't deeply nest expressions due to the way evaluation works. Simple examples looked nice and explicit. But, you quickly run into the problem of having to name 15 intermediate stages of a harder computation, and suddenly all your names are L3, L4, L5, ... which makes the code hard to understand again. So, complexity is complexity in the end, and you can pick your poison. The same issue arises when factoring Forth code into tiny "words" (one of the advantages of a concatenative language): you have to name sequences of operations that don't have an obvious problem-space concept behind them.
  • digsmahler 10y ago
    Ironically, the article linked, "Writing good code: how to reduce the cognitive load of your code", itself has a high cognitive load. It's grammatically irksome. The opening sentence has no verb, "Low bug count, good performance, easy modification." Further it uses poor non-examples of the points the author is trying to make. For example, Yoda conditionals are quirky, but do not increase cognitive load. They were also called out as a good habit in Code Complete, the very book the author sites as the long form of what he's writing about. OTOH, the gist has a low cognitive load provided one understands funtional notation. I can look at his four lines of code and see that this is a correct solution, because each atom of the function can be reasoned about separately and then composed into the whole. The same is much more difficult with the looping solutions that have been presented as peers to this comment. These solutions have a higher cognitive load because proving correctness requires a full trace of the algorithm, while keeping all of the variables in working memory as you go. tl;dr; The OP presented a clear and clever solution. I'll allow it. ;)
  • qznc 10y ago
    Maybe terse Scala solutions compare to longer C solution, like terse math equations compare to longer prose text? People who are not used to greek symbols, read equations in the same speed and mindset like english text and get confused quickly. Reading requires a different mode. I would always prefer a map/fold to a for-loop. Removing the "i" and its book keeping reduces cognitive load. However, a Python list comprehension often becomes too big an expression if you use too much filtering. Extracting the filter into its own function an giving it a good name compartmentalizes the complexity, though. All in all, it is quite subjective. If you are used to premutations, mapping, zipping, filters the code becomes quite readable. I'm pretty sure there is C code which confuses a Haskell programmer but would be considered "clear and readable" by the average C programmer.
  • tunesmith 10y ago
    This is honestly pretty clear to understand - there's hardly anything there other than those general functional features you listed. I think there's a difference between the cognitive load of the solution and the cognitive load of the implementation. The cognitive load of the general solution for queens is somewhat high, but that's why it's a well-known puzzle. But this implementation is practically a description of the problem itself. It's literally selecting (filter) from all queen arrangements (permutations) where the queens aren't conflicting (described via map and zipWithIndex).
  • AzzieElbab 10y ago
    it is collections api which has always been very well documented, as opposed to some scary "functional features". Nothing too advanced and absolutely no hacks
  • pfn 10y ago
    permutation, mapping, zipping and filtering are all completely idiomatic scala. There is next to no cognitive load in this 4-line solution. Brute-force attack on every permutation. I would say that anyone find there to be an excessive cognitive load would be someone that is not familiar with scala nor functional programming.
• pkolaczk 10y ago
  Honestly, I can see more than 4 lines of code there.
  • agumonkey 10y ago
    nqueens is ~4, the rest is output
    • eggy 10y ago
      One line of J code taken from the jsoftware site [1]: queenst =: (, #: I.@,@(</)&i.)~ (] #"1~ [ */@:~:&(|@-/) {)&.|: ! A.&i. ] This is the calculation, and you can output all 92 solutions for the 8 queens problems like this: queenst 8 0 4 7 5 2 6 1 3 0 5 7 2 6 3 1 4 0 6 3 5 7 1 4 2 .... each column is a row number, and the integer is which column the queen is in, or vice versa on rows and columns. ,or print how many solutions $queens 92 8 $ gives the 'shape' of the array, that is 92 rows (solutions) x 8 columns (8-queens problem). I always return to J, because I enjoy mathematics and functional programming. Like math, you work with symbols you learn to think as you code in an interpreted window, somewhat like when working in the Lisp REPL. Ken Iverson created the APL language in 1960, and won an ACM Turing Award and gave a lecture 'Notation as a Tool of Thought', which sums it up better than I could ever hope to express it [3]. J was a collaboration between him, Arthur Whitney (of finance's k language, and kdb fame), and Roger Hui, in an attempt to improve on APL, and to do away with needing a special character keyboard. It is under the GPL3 license today. J uses the ASCII character set, and is very functional and composable. [1] http://code.jsoftware.com/wiki/Essays/N_Queens_Problem [2] http://code.jsoftware.com/wiki/Essays/Queens_and_Knights [3] http://www.jsoftware.com/papers/tot.htm
      • agumonkey 10y ago
        Always good to see some APL/J
      • jxy 10y ago
        > queenst =: (, #: I.@,@(</)&i.)~ (] #"1~ [ */@:~:&(|@-/) {)&.|: ! A.&i. ] So this must be the origin of the one liner from [0] queens←{⍵{((+/b)⌿⍵),⍺|(,b)/⍳×/⍴b←~↑(⊂⍳⍺)∊¨(↓⍵)+[0]¨⊂(c-⍳c←1↓⍴⍵)∘.ׯ1 0 1}⍣⍵⍳1 0} Somehow J is shorter? I need someone who speaks both J and APL to tell me if they are the same. Never learned J, but I always return to APL. I guess J to APL is like latin alphabet to chinese characters. You don't need a special keyboard for APL, just like you don't need a special keyboard for Chinese. Sometimes I really don't understand why they started J. Native English speaker's mentality? We never needed special keyboard for diacritics, let alone east asian languages. With an input method for APL characters, J does not even save the number of keystrokes. [0] https://dfns.dyalog.com/n_queens.htm https://dfns.dyalog.com/n_queens.htm
• wslh 10y ago
  Not the shortest, but the most clear solving in Z3 because instead of an algorithm you define the solution: https://github.com/0vercl0k/z3-playground/blob/master/nqueens_z3.py https://github.com/0vercl0k/z3-playground/blob/master/nqueen...
  • FreeFull 10y ago
    And here is the solution using Picat, using the included sat library: http://picat-lang.org/exs/bqueens.pi http://picat-lang.org/exs/bqueens.pi
• est 10y ago
  8-queen puzzle in 1 line of Python with pretty print _=[__import__('sys').stdout.write("\n".join('.' * i + 'Q' + '.' * (8-i-1) for i in vec) + "\n===\n") for vec in __import__('itertools').permutations(xrange(8)) if 8 == len(set(vec[i]+i for i in xrange(8))) == len(set(vec[i]-i for i in xrange(8)))]
  • JoeAltmaier 10y ago
    Ah! That's similar to the solution I used in 1980 to solve this problem for a Byte Magazine contest. Except I trimmed the search tree by aborting the permutation recursion each time the condition failed in a previous step. It took 21 minutes to find and print all the solutions, if I remember right. But that was in Basic on a machine that ran at 10MHz.
  • e12e 10y ago
    I tried to make that a little more readable, and converted it to py3 -- This one produces the same output with python3, as your one-liner with python2: #!/usr/bin/env python3 from itertools import permutations as p for vec in p(range(8)): if 8 == len(set(vec[i]+i for i in range(8))) \ == len(set(vec[i]-i for i in range(8))): print("\n".join( '.' * i + 'Q' + '.' * (8-i-1) for i in vec), end="\n===\n") The string construction could/should probably also be refactored a bit, but at least this version I have a hope of following the logic... :-)
• brownbat 10y ago
  A fast algorithm that really isn't very complex: http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=4DC9292839FE7B1AFABA1EDB8183242C?doi=10.1.1.57.4685&rep=rep1&type=pdf http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=4DC...
• pmorici 10y ago
  Isn't the point of an n-queen solution to be fast? What does it matter how many new lines you have in the solution if it isn't fast?
• SagelyGuru 10y ago
  There is no search solution to N-queens, so any code doing search is poor and wasteful. I am not going to spoil the fun by giving you the details here, just the hint: start on the right square, depending on whether N is even or odd. Then place successive queens in a knight jump pattern. It is fascinating to me that whoever invented chess a long time ago may have been aware of these subtle complementary relationships between the different types of moves?
  • JoeAltmaier 10y ago
    That may find some solutions, but does it find them all?
    • tromp 10y ago
      obviously not
    • SagelyGuru 10y ago
      You can probably find them all by considering all the valid starting (boundary) positions, mostly obtainable by symmetry.
  • chongli 10y ago
    Yes, the knight is to the queen as the bishop is to the rook.
• Bladtman 10y ago
  By someone who clearly cannot count to 4?
  • pathikrit 10y ago
    OP here. Sorry for the code-golfy post on HN. But, some clarifications: 1. Yes, it is O(n!) - it says so in the description but not on the title of the post on HN. 2. It is 4 lines of code for `nQueen` function. I did not include the printing code in the line count. 3. And no, I did not cheat by using semicolons in same lines - these are 4 legit lines IMO. You can put the `&` clause in 1 line to reduce it further.
• master_yoda_1 10y ago
  Here is my 2 line solution to n-queen. #include <nqueensolver.h> int main(){int n=5;solve(n);} Please include the binaries for nqueen solver when compiling and running. The point I am trying to make is that, in such a high label language like scala, this kind of claim sounds foolish.
  • aconz2 10y ago
    I think many may take your response as a bit off-the-cuff, but I share this sentiment. You could even go so far as to write the longest C program you could dream of to solve it, then simply feed it into tr -d '\n' and call it one line! Again, many will find this pedantic and unsatisfactory but programmers should really be more precise in what they mean by lines of code. What I think most people are really referring to is information content. To achieve the same program in shorter length, the rest of the information has to lie somewhere else (in the scala implementation or nqeensolver.h). We can increase the density of the information needed to specify a program, but not decrease it. I actually just wrote something about this concept yesterday: https://aconz2.github.io/programming/2016/04/09/power_of_pl.html https://aconz2.github.io/programming/2016/04/09/power_of_pl....
• jxy 10y ago
  Another HN post becomes rosetta? Besides the poor search, it does not even try to remove duplications because of symmetry. Rosetta posts should always include a one liner from APL, so here is a link: https://dfns.dyalog.com/n_queens.htm https://dfns.dyalog.com/n_queens.htm
• junke 10y ago
  If you want a fine challenge, try "Queens and Knights" (http://archive.vector.org.uk/art10003900 http://archive.vector.org.uk/art10003900). It has some interesting corner cases to consider.
• digsmahler 10y ago
  Love it! That's a really clever use of the Set collection type to pair down non-solutions.
  • pklausler 10y ago
    pare
    • digsmahler 10y ago
      lol. ty!
• dblock 10y ago
  If you want to get serious about solving queens fast here's an x86 assembly implementation from 1995 - https://github.com/dblock/reines https://github.com/dblock/reines :)
  • userbinator 10y ago
    An even faster x86-64 Asm one that does it with no memory accesses - everything is in registers: https://github.com/davidad/8queens/blob/1989666c45baa639f152dfc89c70635f7007d20b/8queens.asm https://github.com/davidad/8queens/blob/1989666c45baa639f152...
• edejong 10y ago
  Personally, I love this simple, readable, short implementation in the EcliPse solver: http://www.hakank.org/minizinc/queens_viz.mzn http://www.hakank.org/minizinc/queens_viz.mzn
• user2994cb 10y ago
  Well, here's a fairly compact Haskell function that should be reasonably efficient (no fancy stuff mind): queens n = q n n [[]] where q 0 m ss = ss q n m ss = q (n-1) m (concatMap (\s->(map (:s)(filter (f 1 s) [0..m-1]))) ss) f _ [] a = True f n (b:s) a = a /= b && a /= b+n && a /= b-n && f (n+1) s a main = print $ queens 8
  • user2994cb 10y ago
    Actually, this is better (and no less clear): import Data.List queens n = q [] [0..n-1] where q s [] = [[]] q s as = concatMap (\a -> q (a:s) (delete a as)) (filter (f 1 s) as) f _ [] a = True f n (b:s) a = a /= b+n && a /= b-n && f (n+1) s a main = print $ length (queens 8) Undoubtedly functional, the question is whether it is improved eg. by using folds instead of explicit recursion, or replacing concatMap with monadic join.
    • user2994cb 10y ago
      That should be "q s [] = [s]", otherwise you get 92 copies of the empty list.
• todd8 10y ago
  Here's my Python3 version (4 line nQueens function): from itertools import permutations def nQueens(n): return (p for p in permutations(range(n)) if (len(set(c+d for c,d in enumerate(p))) == n and len(set(c-d for c,d in enumerate(p))) == n)) for num,solution in enumerate(nQueens(8)): print("Solution #", num+1) print("\n".join((" ".join("Q" if i == col else "-" for i in range(8))) for col in solution))