Re: Perl6 Contest #2: P6 That Doesn't Look Like P5

What you're searching for is the outer product. Following code should do it, and works in current Pugs :)

  use v6;

  sub outer(*@vals) {
    my &helper = -> @prev, @rest {
      if @rest {
        # We've still got a rest to interate over.
        # We add all items of @rest[0] to the new @prev
        # in our sub-helper, and use @rest[1...] as new
        # rest, i.e. all elements of @rest except the
        # first one.
        helper [ *@prev, $_ ], @rest[1...] for @rest[0];
      } else {
        # We don't have to recurse further, so we
        # simply "return" @prev.
        take @prev;
      }
    };

    # @prev: Empty array
    # @rest: @vals
    gather { helper [], @vals };
  }

  my @a = outer [1,2,3],[4,5,6],[7,8,9];
  say join "\n", @a;
  # 1 4 7
  # 1 4 8
  # [...]
  # 3 6 8
  # 3 6 9
[download]

--Ingo

Comment on Re: Perl6 Contest #2: P6 That Doesn't Look Like P5 Download Code

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Re^2: Perl6 Contest #2: P6 That Doesn't Look Like P5 by dpuu (Chaplain) on Jun 03, 2005 at 21:45 UTC
`my &helper = -> @prev, @rest { ... }` [download] Too much syntax! You can use lexically scoped subs in P6 -- you don't have to use pointy-blocks for everything: `my sub helper (@prev, @rest) { ... }` [download] Now I'm wondering if its possible to get rid of the "if" statment using multi-subs; And after that if its possible to get rid of the indexing into @rest. Something like `sub outer(@vals) { my multi sub helper (@prev) { take @prev } my multi sub helper (@prev, $current, @rest) { $current.map: { helper [@prev, $_], @rest } } gather { helper [], @vals } }` [download] This is totally untested -- I haven't been able to install pugs yet. --Dave.	[reply] [d/l] [select]
Re^2: Perl6 Contest #2: P6 That Doesn't Look Like P5 by Limbic~Region (Chancellor) on Jun 03, 2005 at 19:50 UTC
iblech, Call it what you want, what I am trying to replicate is NestedLoops from Algorithm::Loops. That is, the ability to loop over an arbitrary number of lists iteratively with a relatively small memory footprint. Your solution certainly produces the same output but not 1 at a time and has everything in memory at once. That aside, it certainly helps me learn more Perl6 and exposes others as well. Cheers - L~R	[reply]
Re^3: Perl6 Contest #2: P6 That Doesn't Look Like P5 by iblech (Friar) on Jun 03, 2005 at 20:24 UTC
That is, the ability to loop over an arbitrary number of lists iteratively with a relatively small memory footprint. Sure, my `outer` does that, unless I've misunderstood something. It does not return a Code, though, but a lazy list, and it hasn't the same API as `NestedLoops`, as I was concentrating on the algorithm. Your solution certainly produces the same output but not 1 at a time and appears to hold everything in memory at once. Yep, this is as to get even only one item, the `helper` sub has to be executed recursively several times. I.e., in general: `{ ...; recurse ...; take ...; ...; } # not so good { ...; take ...; recurse ...; ...; } # better (items are yielded ASAP)` [download] Therefore my solution should probably not be included in a library, as the other solutions presented scale much better. I do think that my solution is rather elegant though (no `for`, few lines of code). --Ingo	[reply] [d/l] [select]