From: Massimiliano Mirra <list@...>
Date: 2002-11-17T19:16:30+09:00
Subject: Re: ruby-dev summary 18613-18710

On Sun, Nov 17, 2002 at 05:18:40AM +0900, Florian Frank wrote:
> > Tried that with hashes already? :-)
> 
> I just did:
> 
> h = Hash[*(1..10)]
>     ==>{5=>6, 1=>2, 7=>8, 3=>4, 9=>10}
> h.zip
>     ==>[[5, 1, 7, 3, 9], [6, 2, 8, 4, 10]]
> h.zip == [h.keys, h.values]
>     ==>true

That's because you happen to have numeric keys.  Try with {:one =>
'two', :three => 'four'}.

> In Haskell its only possible to zip two or three "lists" with the
> standard library functions.

(Just curious: why?)

> There is a difference between the collection of "lists" and the
> "lists".

Sorry I don't understand what you mean. :-(

> > module Enumerable
> >   def zip(*enumerables)
> >     zipped = []
> >     # was like this:
> >     # all = [self, *enumerables] 
> >     # now is:
> >     all = [self.to_a, *enumerables.collect {|e| e.to_a}]
> >     max_length = all.collect {|a| a.length}.max
> > 
> >     0.upto(max_length-1) do |i|
> >       zipped << all.collect {|a| a[i]}
> >     end
> >     zipped
> >   end
> > end
> 
> That's another possibility. The construction of "all" wastes a lot of
> memory in some cases, 

True.  That's why I called it `a quick hack' in another thread.

> but it is perhaps more flexible because the
> arguments only have to be enumerables. I don't know what happens more
> often that an object can #[] or includes Enumerable and/or can #each.
> If it can't #[] you can always do #to_a to them before using zip.

You're assuming that #[](n) returns the element of a collection.
While this is often so, there's no assurance.  Strings and hashes are
the most notable cases.  I'd say, rely on enumerables instead.

> > You needn't obey String#each default blindly, either:
> > 
> > $/ = 'o'
> 
> You can't shock me -- I confess: I'm a perl programmer. *g*

LOL Well, confession heard, my son, now you can repent... (just
kidding)

> > p "one\ntwo\n".zip("three\nfour\n")
> > #=> [["o", "three\nfo"], ["ne\ntwo", "ur\n"], ["\n", nil]]
> 
> To pass another iterator has perhaps an advantage, this comes to my
> mind:
> 
> tree.zip(:preorder)
> tree.zip(:inorder)
> tree.zip(:postorder)

Cool.  Maybe, since each gets extra parameters already in String, one
could also have Tree#each(order), where order can be any of the above
three, and zip would just forward the parameter to #each.


Massimiliano