Create new array of arrays from key

Question

I have an array:

[{
    "name": "Category 1",
    "entries": [{
        "question": "Question 1",
        "answer": "Answer 1"
    }, {
        "question": "Question 2",
        "answer": "Answer 2"
    }, {
        "question": "Question 3",
        "answer": "Answer 3"
    }]
}, {
    "name": "Category 2",
    "entries": [{
        "question": "Question 1",
        "answer": "Answer 1"
    }, {
        "question": "Question 2",
        "answer": "Answer 2"
    }, {
        "question": "Question 3",
        "answer": "Answer 3"
    }]
}]

What I want to do is create a new array of just the entries.

So I end up with this:

[{
    "question": "Question 1",
    "answer": "Answer 1"
}, {
    "question": "Question 2",
    "answer": "Answer 2"
}, {
    "question": "Question 3",
    "answer": "Answer 3"
}, {
    "question": "Question 1",
    "answer": "Answer 1"
}, {
    "question": "Question 2",
    "answer": "Answer 2"
}, {
    "question": "Question 3",
    "answer": "Answer 3"
}]

In PHP I would of just do a push into a new array... but in Ruby is it possible to use group_by or collect to achieve this?

When you give an example, it's helpful to assign a variable to each input object (e.g., arr = [{ "name": "Category 1",...]). That way, readers can refer to the variable(s) (arr) in answers and comments without having to define it (them), and all readers will use the same variable(s). I suggest you edit your question to do that. Generally there's no need to do that for desired or expected output objects. — Cary Swoveland
– Cary Swoveland, Commented Sep 15, 2016 at 18:06

Andrey Deineko · Accepted Answer · 2016-09-15 11:15:17Z

7

Use Enumerable#flat_map:

array = [{
    "name": "Category 1",
    "entries": [{
        "question": "Question 1",
        "answer": "Answer 1"
    }, {
        "question": "Question 2",
        "answer": "Answer 2"
    }, {
        "question": "Question 3",
        "answer": "Answer 3"
    }]
}, {
    "name": "Category 2",
    "entries": [{
        "question": "Question 1",
        "answer": "Answer 1"
    }, {
        "question": "Question 2",
        "answer": "Answer 2"
    }, {
        "question": "Question 3",
        "answer": "Answer 3"
    }]
}]
array.flat_map { |hash| hash[:entries] }
   #=> [{:question=>"Question 1", :answer=>"Answer 1"},
   #  {:question=>"Question 2", :answer=>"Answer 2"},
   #  {:question=>"Question 3", :answer=>"Answer 3"},
   #  {:question=>"Question 1", :answer=>"Answer 1"},
   #  {:question=>"Question 2", :answer=>"Answer 2"},
   #  {:question=>"Question 3", :answer=>"Answer 3"}]

edited Sep 15, 2016 at 11:15

answered Sep 15, 2016 at 11:07

Andrey Deineko

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5 Comments

dp7 Over a year ago

Instead of flat_map, map and flatten can be used here to get exactly an array of hashes like array.map {|hash| hash[:entries]}.flatten

Andrey Deineko Over a year ago

@dkp flat_map is about 2 times faster, that map + flatten :)

dp7 Over a year ago

[[{:question=>"Question 1", :answer=>"Answer 1"}, {:question=>"Question 2", :answer=>"Answer 2"}, {:question=>"Question 3", :answer=>"Answer 3"}], [{:question=>"Question 1", :answer=>"Answer 1"}, {:question=>"Question 2", :answer=>"Answer 2"}, {:question=>"Question 3", :answer=>"Answer 3"}]]

- I thought flat_map would result array inside array here

Cary Swoveland Over a year ago

@dkp, re your first comment, that's obvious, considering that flat_map's raison d'être is to avoid the need for map and flatten.

Cary Swoveland Over a year ago

Maybe just say "If array is your array," and delete array, which is a bit distracting.

dp7 · Accepted Answer · 2016-09-15 11:55:41Z

1

map and flatten would give you desired output:

your_array.map {|hash| hash[:entries]}.flatten

#=> [{:question=>"Question 1", :answer=>"Answer 1"},
# {:question=>"Question 2", :answer=>"Answer 2"},
# {:question=>"Question 3", :answer=>"Answer 3"},
# {:question=>"Question 1", :answer=>"Answer 1"},
# {:question=>"Question 2", :answer=>"Answer 2"},
# {:question=>"Question 3", :answer=>"Answer 3"}]

answered Sep 15, 2016 at 11:55

dp7

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