combining array of hashes based on specific key

Question

I need a group an array of hashes based on a particular key of each hash. For example, take this:

[
    [0] {
        :status => "pending",
             :x => 1,
             :y => 2
    },
    [1] {
        :status => "pending",
             :x => 33,
             :y => 74
    },
    [2] {
        :status => "done",
             :x => 33,
             :y => 74
    }
]

I need to convert it to this:

{
    "pending" => [
        [0] {
            :status => "pending",
                 :x => 1,
                 :y => 2
        },
        [1] {
            :status => "pending",
                 :x => 33,
                 :y => 74
        }
    ],
       "done" => [
        [0] {
            :status => "done",
                 :x => 33,
                 :y => 74
        }
    ]
}

I am grouping the array by :status key. I have done this (it works):

a.inject({}) {|a, b| (a[b[:status]] ||= []) << b; a }

But, is there a simpler, less cryptic one-liner that can do the same thing?

Doesn't your way work?

Sergio Tulentsev
– Sergio Tulentsev

2013-04-10 08:00:51 +00:00
Commented Apr 10, 2013 at 8:00 — Sergio Tulentsev
– Sergio Tulentsev, Commented Apr 10, 2013 at 8:00

Beat Richartz · Accepted Answer · 2013-04-10 08:29:46Z

2

Why not use group_by? It does exactly what you need.

a.group_by {|b| b[:status] }

answered Apr 10, 2013 at 8:29

Beat Richartz

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1 Comment

Beat Richartz Over a year ago

always think of group_by before you inject each_with_object, i'd say :)

Jon Cairns · Accepted Answer · 2013-04-10 08:17:24Z

It's not a common enough operation to warrant a built-in descriptive method, but I would tweak your line slightly.

Instead of using #inject(), how about using #each_with_object()? It's more appropriate for passing the same object over an iteration, as that's exactly what it does - it's also more descriptive than "inject", IMO.

That has the added benefit of removing the ; a from the end of the block: this is the problem with using inject to pass the same object between each iteration. Therefore, the final line becomes (with some variable name tweaking):

ary.each_with_object({}) {|e, obj| (obj[e[:status]] ||= []) << e }

The return value of each_with_object is the hash that's being built up, so you can assign the above to a variable, or return it from your method.

In the end, if you want it to be more descriptive in your application, wrap that line in a method that is descriptive:

def index_with_status(ary)
    ary.each_with_object({}) {|e, obj| (obj[e[:status]] ||= []) << e }
end

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