How do I map unique values from a multi-level array to a hash of value=>array?

Question

I have an array that looks something like this:

[[100, "one"],
 [101, "one"],
 [102, "one"],
 [103, "two"],
 [104, "three"],
 [105, "three"]]

What I would like to do is create an array of hashes that looks like this

[{"one" => [100,101,102]},
 {"two" => [103]},
 {"three" => [104,105]}]

The number portion will always be unique, the string portion will have duplicates. Every way I think about doing this I get some long function, I would like to know the "rails way" of going about this, I'm sure there's some obscure function I am missing.

Answer 1

There are no really function to do that. You need use inject and create yourself you hash.

Answer 2

Not a Rails helper, but a common Ruby idiom can get you there. A little

arr.inject({}) { |h,(v,k)| h[k] ||= []; h[k] << v; h }

will do the trick.

Answer 3

array = [[100, "one"], [101, "one"], [102, "one"], [103, "two"], [104, "three"], [105, "three"]]
h = Hash.new { |h,k| h[k] = []}
array.each { |a| h[a[1]] << a[0] }

Answer 4

As shingara points out, this is pretty specific to the format of the array(s). You can do what you need like so:

a = [...your data...]
r = a.inject( {} ) do |h, el|
  h[el.last] ||= []
  h[el.last] << el.first
  h
end

That gives a result like: {'one' => [101, 102], ... } which is better than your request for an array of one-key hashes, IMO.

Answer 5

Here's one implementation

your_array.inject(Hash.new{|h,k| h[k] = []}) do |result, (a, b)|
  result[b] << a
  result
end

Answer 6

If what you need is only group stuff then you can use the Rails group_by function :

[[100, "one"],
 [101, "one"],
 [102, "one"], 
 [103, "two"],
 [104, "three"],
 [105, "three"]].group_by { |a| a[1] }

 => #<OrderedHash {"three"=>[[104, "three"], [105, "three"]],
                   "two"=>[[103, "two"]], 
                   "one"=>[[100, "one"], [101, "one"], [102, "one"]]}

Not to far from what you need. So if you can use it as it stand, I guess that's fine, but if you need exaclty the format you said. I think it easier to do it yourself rather than usign this and converting.

Answer 7

Is there a specific reason why you need that specific format, i.e. an array of single-element hashes instead of just a bog-standard hash? Because in that case, it would literally be just

arr.group_by(&:last)

Answer 8

The shortest way to get exactly what you ask for that I can come up with is:

a = [[100, "one"],
     [101, "one"],
     [102, "one"], 
     [103, "two"],
     [104, "three"],
     [105, "three"]]

b = a.group_by(&:pop)
#=> {"three"=>[[104], [105]], "two"=>[[103]], "one"=>[[100], [101], [102]]}

which is probably what you want.

please note that a gets ruined by this

a
#=> [[100], [101], [102], [103], [104], [105]]

if that bothers you, you can write

b = a.map(&:dup).group_by &:pop

instead.

And if you really want that format you wrote then you can add another map:

b.map{|h,k| [h => k]}
#=> [{"one" => [100,101,102]}, {"two" => [103]}, {"three" => [104,105]}]

So to sum up:

[[100, "one"],
 [101, "one"],
 [102, "one"], 
 [103, "two"],
 [104, "three"],
 [105, "three"]].group_by(&:pop).map{ |h,k| [h => k] }