Ruby array to hash

Question

I have the following array:

myarray = [
  ['usa','primary', 'john'],
  ['france','primary', 'lira'],
  ['usa', 'secondary', 'steve'],
  ['germany', 'primary', 'jeff'],
  ['france', 'secondary', 'ben']
]

I want to convert it to an array of hash like:

[
  {:country => 'usa', :primary => 'john', :secondary => 'steve'},
  {:country => 'france', :primary => 'lira', :secondary => 'ben'},
  {:country => 'germany', :primary => 'jeff', :secondary => ''}
]

I can do it by looping through the array and putting the values into a hash for "primary". How can I add "secondary" to the existing hash that may already be inserted into the array?

Jakub Hampl · Accepted Answer · 2012-10-18 23:49:57Z

11

With a bit of Higher order programming:

myarray.reduce({}) do |accu, (country, func, name)| 
  accu[country] ||= {}
  accu[country][func.to_sym] = name
  accu 
end.map{|k, h| h[:country] = k; h}

Explanation:

Reduce will take an accumulator, in this case we start with an empty hash and go through the array. We match the triples in the array to the variables country, func, name. Since we want to group things by country we create that as our first Hash key. It should contain a hash so we make sure that is corresponds to an array with accu[country] ||= {}.

Then we add our key value pair converting func to a symbol. Finally we return our modified accumulator that will be passed to the next iteration.

This will return a data structure like this:

{"usa"=>{:primary=>"john", :secondary=>"steve"}, 
 "france"=>{:primary=>"lira", :secondary=>"ben"}, 
 "germany"=>{:primary=>"jeff"}}

Now we need to transform it into a array of hashes rather than a big hash. We do this by calling map on it and in the process we add country as a key to the hash.

Now one thing that the algorithm above doesn't do is to check for missing values, resp. it doesn't guarantee that both :primary and :secondary are present. You can do that by modifying the map to this:

.map do |k, h|
  h[:country] = k
  h[:primary] ||= ""
  h[:secondary] ||= ""
  h
end

edited Oct 18, 2012 at 23:49

answered Oct 18, 2012 at 23:29

Jakub Hampl

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4 Comments

JVK Over a year ago

Thanks. But here you put country name outside e.g. "usa"=>{:primary=>"john", :secondary=>"steve"} while I wish to have {:country => 'usa', :primary => 'john', :secondary => 'steve'}

Jakub Hampl Over a year ago

Try running the code, it does exactly that. What I talk about in the answer is the intermediate step between reduce and map. Please read carefully.

JVK Over a year ago

you rock man :) but let me ask you, what if we have more than one identity to group by. For example, myarray = [ ['usa', 'california', 90072, 'primary', 'john'], ['usa', 'california', 90072, 'secondary', 'paul'], ['france', 'brittany', 73829, primary', 'lira'] that means here if I have grouping with more than one column, not only country. Group by country, state and zipcode :)

Jakub Hampl Over a year ago

You can use an array as a hash key, so instead of having simply accu[country] you could have a key made out of an array of all those values.

Pan Thomakos · Accepted Answer · 2012-10-19 00:22:24Z

5

Here's a fun, albeit confusing, way to do it.

format = Hash.new{ |h,k| h[k] = {:country => k, :primary => '', :secondary => ''} }

myarray.inject(format){ |result, (c,k,v)| result[c][k.to_sym] = v; result }.values

# => [
  {:country=>"usa", :primary=>"john", :secondary=>"steve"},
  {:country=>"france", :primary=>"lira", :secondary=>"ben"},
  {:country=>"germany", :primary=>"jeff", :secondary=>""} ]

Basically the format hash defines the way you want to generate your output. The inject call collects the results by country. The values call just grabs the actual results without the country hash keys.

answered Oct 19, 2012 at 0:22

Pan Thomakos

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1 Comment

Jakub Hampl Over a year ago

+1 I was wondering whether someone would use the Hash.new thing.

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