From: Robert Klemme <shortcutter@...>
Date: 2010-09-15T21:16:01+09:00
Subject: Re: Iterate over a cartesian product.

2010/9/15 Jörg W Mittag <JoergWMittag+Ruby@googlemail.com>:
> Robert Klemme wrote:
>> 2010/9/14 Jörg W Mittag <JoergWMittag+Ruby@googlemail.com>:
>>> Handy Gandy wrote:
>>>> I have an array of N arrays A_i: [A_1,A_2,...,A_N].
>>>>
>>>> I want to iterate over the cartesian product A_1xA_2...xA_N.
>>>>
>>>> Is there a simple way of doing this without specifying N?
>>>    ary = [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
>>>    ary.first.product(*ary.drop(1))
>>>    # => [
>>>    # =>   [1, 3, 6], [1, 3, 7], [1, 3, 8], [1, 3, 9],
>>>    # =>   [1, 4, 6], [1, 4, 7], [1, 4, 8], [1, 4, 9],
>>>    # =>   [1, 5, 6], [1, 5, 7], [1, 5, 8], [1, 5, 9],
>>>    # =>   [2, 3, 6], [2, 3, 7], [2, 3, 8], [2, 3, 9],
>>>    # =>   [2, 4, 6], [2, 4, 7], [2, 4, 8], [2, 4, 9],
>>>    # =>   [2, 5, 6], [2, 5, 7], [2, 5, 8], [2, 5, 9]
>>>    # => ]
>> This looks nice.  How did you do the formatting?
>
> By hand :-)

14:09:29 ~$ gem19 install 'By hand'
ERROR:  could not find gem By hand locally or in a repository
14:09:58 ~$

Hm...

> Though I assume Hirb or awesome_print could probably do it
> automatically.
>
>> If destruction is allowed, we can do:
>>
>> irb(main):013:0> ary = [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
>> => [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
>> irb(main):014:0> pp ary.shift.product(*ary)
>> [[1, 3, 6],
>> ...
>> => nil
>
> That's nice. Having toyed with Scala and Haskell, mutation just feels
> *so* dirty to me. I guess I need some re-education to get back into
> the idiomatic Ruby mindset.

Unfortunately there is no #slice which has two return values.  If
there was we could do

a,b = arr.slice 0,1

But we can do this to achieve the same (i.e. non destructively partitioning):

irb(main):016:0> a=(1..5).to_a
=> [1, 2, 3, 4, 5]
irb(main):017:0> y=(x=a.dup).slice! 0,1
=> [1]
irb(main):018:0> x
=> [2, 3, 4, 5]
irb(main):019:0> a
=> [1, 2, 3, 4, 5]

Well, I guess the simplest approach is still

irb(main):001:0> a=(1..5).to_a
=> [1, 2, 3, 4, 5]
irb(main):002:0> x,*y = a
=> [1, 2, 3, 4, 5]
irb(main):003:0> x
=> 1
irb(main):004:0> y
=> [2, 3, 4, 5]

So we can do

irb(main):026:0> ary = [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
=> [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
irb(main):027:0> x, *y = ary
=> [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
irb(main):028:0> pp x.product(*y)
[[1, 3, 6],
 [1, 3, 7],
 [1, 3, 8],
 [1, 3, 9],
 [1, 4, 6],
 [1, 4, 7],
 [1, 4, 8],
 [1, 4, 9],
 [1, 5, 6],
 [1, 5, 7],
 [1, 5, 8],
 [1, 5, 9],
 [2, 3, 6],
 [2, 3, 7],
 [2, 3, 8],
 [2, 3, 9],
 [2, 4, 6],
 [2, 4, 7],
 [2, 4, 8],
 [2, 4, 9],
 [2, 5, 6],
 [2, 5, 7],
 [2, 5, 8],
 [2, 5, 9]]
=> nil

:-)

Kind regards

robert


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