Ruby- creating an hash from an array of hashes

Question

I have an array of hashes:

@operating_systems = [
  {"title"=>"iPhone", "value_percent"=>"42.6"},
  {"title"=>"Windows 7", "value_percent"=>"21.3"},
  {"title"=>"Android", "value_percent"=>"12.8"},
  {"title"=>"Mac OS X", "value_percent"=>"8.5"},
  {"title"=>"Windows 8.1", "value_percent"=>"6.4"},
  {"title"=>"Windows XP", "value_percent"=>"4.3"},
  {"title"=>"Linux", "value_percent"=>"2.1"},
  {"title"=>"Windows Vista", "value_percent"=>"2.1"}
]

and want to create the following hash:

 {"iphone" => "42.6", "windows 7" => "21.3", ... "windows vista" => "2.1"}

What is the best way to accomplish this?

Write the logic behind the conversion. Don't let the reader guess. As of now, none of the given answers got your intention correctly, and that is because you have not made it clear. — sawa
– sawa, Commented May 16, 2014 at 23:35
I'm a bit confused by your comment. All three answers correctly solve my question, so it would appear that they indeed understand the intention, no? — dmt2989
– dmt2989, Commented May 16, 2014 at 23:39
@dmt2989 I don't understand your mind. All the three answers given at the time of my comment above give different result from what you wanted. — sawa
– sawa, Commented May 16, 2014 at 23:45

sawa · Accepted Answer · 2014-05-16 23:42:25Z

6

[
  {"title"=>"iPhone", "value_percent"=>"42.6"},
  {"title"=>"Windows 7", "value_percent"=>"21.3"},
  {"title"=>"Android", "value_percent"=>"12.8"},
  {"title"=>"Mac OS X", "value_percent"=>"8.5"},
  {"title"=>"Windows 8.1", "value_percent"=>"6.4"},
  {"title"=>"Windows XP", "value_percent"=>"4.3"},
  {"title"=>"Linux", "value_percent"=>"2.1"},
  {"title"=>"Windows Vista", "value_percent"=>"2.1"}
]
.map{|h| h.values.map(&:downcase)}.to_h
# =>
# {
#   "iphone"=>"42.6",
#   "windows 7"=>"21.3",
#   "android"=>"12.8",
#   "mac os x"=>"8.5",
#   "windows 8.1"=>"6.4",
#   "windows xp"=>"4.3",
#   "linux"=>"2.1",
#   "windows vista"=>"2.1"
# }

answered May 16, 2014 at 23:42

sawa

169k51 gold badges287 silver badges398 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Cary Swoveland Over a year ago

I don't see how to improve on this. +1.

SteveTurczyn · Accepted Answer · 2014-05-16 23:51:39Z

2

Here's one way

 op_sys =  [{"title"=>"iPhone", "value_percent"=>"42.6"}, {"title"=>"Windows 7", "value_percent"=>"21.3"}, {"title"=>"Android", "value_percent"=>"12.8"}, {"title"=>"Mac OS X", "value_percent"=>"8.5"}, {"title"=>"Windows 8.1", "value_percent"=>"6.4"}, {"title"=>"Windows XP", "value_percent"=>"4.3"}, {"title"=>"Linux", "value_percent"=>"2.1"}, {"title"=>"Windows Vista", "value_percent"=>"2.1"}] 

    new_hash = op_sys.inject({}) {|r,e| r[e['title']] = e['value_percent']; r}

    p new_hash

EDIT

You may have wanted the new hash keys to be in downcase... so an alternative:

new_hash = op_sys.inject({}) {|r,e| r[e['title'].downcase] = e['value_percent']; r}

edited May 16, 2014 at 23:51

answered May 16, 2014 at 23:15

SteveTurczyn

37k6 gold badges46 silver badges56 bronze badges

11 Comments

SteveTurczyn Over a year ago

(shrug) the Ruby style guide disagrees... "Rely on the fact that as of Ruby 1.9 hashes are ordered." However I take your point that explicitly examining for "title" and "value_percent" keys are more robust. I'll rework the answer.

user2864740 Over a year ago

I've been bit by relying on the insertion order before (when dealing with outside data like JSON), it hurt :|

sawa Over a year ago

@user2864740 Are you sure this is correct? It doesn't look so to me.

user2864740 Over a year ago

@sawa It is semantically correct (ideone.com/8xukR8), although the variable names are a bit confusing as r is the accumulator hash and e is each item (hash) in the original array ..

mu is too short Over a year ago

each_with_object might be a little nicer than inject in this case, that would let you get rid of the ;r at the end of your block.

|

the Tin Man · Accepted Answer · 2014-05-17 00:11:30Z

@Sawa came the closest to what I'd do:

operating_systems = [
  {"title"=>"iPhone", "value_percent"=>"42.6"},
  {"title"=>"Windows 7", "value_percent"=>"21.3"},
  {"title"=>"Android", "value_percent"=>"12.8"},
  {"title"=>"Mac OS X", "value_percent"=>"8.5"},
  {"title"=>"Windows 8.1", "value_percent"=>"6.4"},
  {"title"=>"Windows XP", "value_percent"=>"4.3"},
  {"title"=>"Linux", "value_percent"=>"2.1"},
  {"title"=>"Windows Vista", "value_percent"=>"2.1"}
] 
operating_systems.map(&:values).to_h 
# => {"iPhone"=>"42.6",
#     "Windows 7"=>"21.3",
#     "Android"=>"12.8",
#     "Mac OS X"=>"8.5",
#     "Windows 8.1"=>"6.4",
#     "Windows XP"=>"4.3",
#     "Linux"=>"2.1",
#     "Windows Vista"=>"2.1"}

Which works on Ruby 2.1+.

Or, on older versions of Ruby:

Hash[operating_systems.map(&:values)] 
# => {"iPhone"=>"42.6",
#     "Windows 7"=>"21.3",
#     "Android"=>"12.8",
#     "Mac OS X"=>"8.5",
#     "Windows 8.1"=>"6.4",
#     "Windows XP"=>"4.3",
#     "Linux"=>"2.1",
#     "Windows Vista"=>"2.1"}

If folding the keys to lowercase is needed use these to replace the above commands:

operating_systems.map{ |h| k, v = h.values; [k.downcase, v] }.to_h
# => {"iphone"=>"42.6",
#     "windows 7"=>"21.3",
#     "android"=>"12.8",
#     "mac os x"=>"8.5",
#     "windows 8.1"=>"6.4",
#     "windows xp"=>"4.3",
#     "linux"=>"2.1",
#     "windows vista"=>"2.1"}

Hash[operating_systems.map{ |h| k, v = h.values; [k.downcase, v] }]
# => {"iphone"=>"42.6",
#     "windows 7"=>"21.3",
#     "android"=>"12.8",
#     "mac os x"=>"8.5",
#     "windows 8.1"=>"6.4",
#     "windows xp"=>"4.3",
#     "linux"=>"2.1",
#     "windows vista"=>"2.1"}

bjhaid · Accepted Answer · 2014-05-16 23:22:22Z

0

Combine Enumerable#reduce with Hash#merge

op_sys.reduce({}) { |hsh, itx| hsh.merge({ itx["title"] => itx["value_percent"]}) }
 => {"iPhone"=>"42.6", "Windows 7"=>"21.3", "Android"=>"12.8", "Mac OS X"=>"8.5", "Windows 8.1"=>"6.4", "Windows XP"=>"4.3", "Linux"=>"2.1", "Windows Vista"=>"2.1"}

answered May 16, 2014 at 23:22

bjhaid

9,8502 gold badges41 silver badges47 bronze badges

Comments

user2864740 · Accepted Answer · 2014-05-17 00:15:00Z

0

While there are a number of more clever approaches, a simple loop over the source array with modification of the output hash will suffice; consider

arr = [{"title"=>"iPhone", "value_percent"=>"42.6"}, ..]

hash = {}
arr.each do |item|
   key = item["title"]
   value = item["value_percent"]
   hash[key] = value
end

and see the ideone demo. This is a very easy task, and I hope that how it was "solved" is made readily apparent above - there is no "best" until there is "working".

Once the basic approach is understood, the same process that was done (a transformation and then building a map from pairs) can be done as so

hash = Hash[arr.map do |v| [v["title"], v["value_percent"]] end]

Is this "better"? shrug, I guess it's "shorter" and still clear.. (and any additional transformation can be trivially added)

edited May 17, 2014 at 0:15

answered May 16, 2014 at 23:05

user2864740

62.5k15 gold badges159 silver badges234 bronze badges

7 Comments

user2864740 Over a year ago

@dmt2989 Then the arr variable (or whatever is substituting it in your code) evaluates to nil and it ends up as nil.each do .. which results in the descriptive exception.

dmt2989 Over a year ago

yea, that is odd, because I can display @arr in my view, and it's [{"title"=>"iPhone", "value_percent"=>"42.6"}, ..]

user2864740 Over a year ago

@dmt2989 The error message is telling the truth. Either @arr hasn't been set to a non-nil value yet (or has been reset?) or that code is running in a different scope.

dmt2989 Over a year ago

My mistake- this also worked, but a different error was confounding the problem. Appreciate the help.

user2864740 Over a year ago

@dmt2989 Glad the problem is solved! Make sure to write/use code that you (and others) can understand later.

|

Collectives™ on Stack Overflow

Ruby- creating an hash from an array of hashes

5 Answers 5

1 Comment

11 Comments

Comments

Comments

7 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

1 Comment

11 Comments

Comments

Comments

7 Comments

Your Answer

Sign up or log in

Post as a guest

Related