Array to Hash with conditional logic in Ruby?

Question

ruby 2.1.1

Is there a way to do the logic in this piece of code in one line or a more concise manner?

user = User.new
h = Hash.new
attrs = [:name, :foo, :bar]
attrs.each do |a|
    h[a] = user[a] if user.has_attribute? a
end
return h

From a matter of style, `Hash.new` with no options is almost always written as simply `{ }`. It's also unnecessary to use `return r` when `r` at the end of the method has exactly the same effect. The only place `return` is normally used is to break out of a method early. — tadman, May 31 '14 at 22:51
@ArupRakshit k, tagged, at first I thought ruby would have a complete solution to this issue. Didn't think rails would have such a better alternative. Now I know, thanks! — Derek, May 31 '14 at 23:14

score 3 · Accepted Answer · answered May 31 '14 at 23:14

3

If you're using Rails and User is an ActiveRecord model (which it looks like given your use of has_attribute?) then this will do the same thing:

user = User.new
...
return user.attributes.slice("name", "foo", "bar")

Or, if you really want symbols:

return user.attributes.with_indifferent_access.slice(:name, :foo, :bar)

answered May 31 '14 at 23:14

gwcoffey

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Your answer is compact one liner.. But just remove *return*, It is not need. – Arup Rakshit May 31 '14 at 23:22
1

I left the `return` there to make it clear to the asker that this is the final result, in place of what he was returning before. Nothing wrong with a `return` if it adds clarity. :) – gwcoffey May 31 '14 at 23:35
I concede… I just like my returns :) – gwcoffey Jun 02 '14 at 20:58

Arup Rakshit · Answer 2 · 2014-05-31T23:26:29.727

It seems you are on Rails. If so,then -

attrs = [:name, :foo, :bar]
# the result hash will be returned, if last line of the method.
user.attributes.extract!(*attrs)

Look these methods extract! and attributes.

Example :

arup@linux-wzza:~/Rails/app> rails c
Loading development environment (Rails 4.1.1)
2.0.0-p451 :001 > h = { a: 1, b: 2, c: 3, d: 4 }
 => {:a=>1, :b=>2, :c=>3, :d=>4}
2.0.0-p451 :002 > h.extract!(:a ,:b ,:x)
 => {:a=>1, :b=>2}
2.0.0-p451 :003 >

score 0 · Answer 3 · answered Jun 02 '14 at 12:44

Answers above are correct in Rails scope, I'l just add generic solution:

# assuming user[a] returns nil, if user have no a attribute
[:name, :foo, :bar].
  map{|a| [attr, user[a]]}.
  reject{|k, v| v.nil?}.
  to_h

# assuming user[a] can raise if not user.has_attribute?(a)
[:name, :foo, :bar].
   map{|a| [attr, user.has_attribute?(a) && user[a]]}.
   reject{|k, v| !v}.
   to_h

I've formatted them as NOT one-liners, but they are still one-statements :)

Basically, the trick is "invent the right method chain to convert one sequence to other", and requires to know all Enumerable sequence-transforming methods (map/select/reduce/reject/...), as well as a method to transform array of key-value pairs into hash (#to_h is standard in Ruby 2.1.1)

Array to Hash with conditional logic in Ruby?

3 Answers3