How to simplify hash intersection with array

Question

I have hash data (originally in json) and an array of selected hash keys:

jsondata ='{"1":
          {"name": "Tax Exempt"},
       "2":
          {"name": "Tax on Purchases"},
       "3":
          {"name": "Tax on Sales"},
       "4":
          {"name": "Service Tax"}              
        }'
parseddata = JSON.parse(jsondata);    
selectedtax = ["2","3"]

My code maps the keys and returns the value of the hash that exist in the array. Here is the code:

selectedtaxdetails = Array.new
parseddata.map do |key,value|  
  if selectedtax.include? key 
    selectedtaxdetails << value               
  end
end

Output of selectedtaxdetails is:

[{"name": "Tax on Purchases"},{"name": "Tax on Sales"}]

How can I improve my code?

you should use some underscores: selected_tax_details is easier to read — Stefan
– Stefan, Commented May 21, 2014 at 8:15
Is json parsing relevant at all to your question? If not, remove it. Give the hash in Ruby. Give only the relevant parts. — sawa
– sawa, Commented May 21, 2014 at 8:38
whats wrong with you sawa ? which part of the code is not working! i just ran this in ruby with of course require 'json' and it works. — Axil
– Axil, Commented May 21, 2014 at 8:42
@Axil - You missed an opening quote here: selectedtax = ["2",3"] — BroiSatse
– BroiSatse, Commented May 21, 2014 at 8:52

BroiSatse · Accepted Answer · 2014-05-21 09:34:25Z

4

Solution:

You can do (rails):

parseddata.slice(*selectedtax).values

or even simpler (pure ruby):

parseddata.values_at(*selectedtax)

Explanation:

Both slice and values_at methods expect a list of keys. If you just pass an array it will search for values where this array is a key, whcih obviously is not what you want. Instead you can use a splat operator (*). It will take each element of an array and will pass it into a method as a separate argument, which is exactely what we want here.

Update:

To achieve structure: [{"code":"2", "name": "Tax on Purchase"},{"code":"3", "name": "Tax on Sales"}] you can do (rails):

parseddata.slice(*selectedtax).map {|key, value| value.dup.tap {|h| h['code'] = key}}

or with pure ruby:

parseddata.select{|key,_| selectedtax.include? key}.map {|key, value| value.dup.tap {|h| h['code'] = key}}

edited May 21, 2014 at 9:34

answered May 21, 2014 at 8:08

BroiSatse

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7 Comments

Mike Campbell Over a year ago

This answer would be better if you explained the use of the splat operator.

Stefan Over a year ago

+1 for values_at

BroiSatse Over a year ago

@MikeCampbell - here it is. :)

Axil Over a year ago

thanks! works perfectly. got another question, if i wanted to return {"code":"2", "name": "Tax on Purchase"},{"code":"3", "name": "Tax on Sales"}. how do i construct a new structure for the returned array ?

BroiSatse Over a year ago

@Axil - This is rails (ActiveSupport core extension) method, I'll update with non-rails code in a second. :)

|

Gustavo Chaín · Accepted Answer · 2014-05-21 08:13:42Z

0

The following should do the same in one line

parseddaata.values_at(*selectedtax)

answered May 21, 2014 at 8:13

Gustavo Chaín

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