How to remove hashes from array based on if all keys in hash match another hash

Question

I have an array of hashes where each hash is a list of URL parameters from URI::decode_www_form. I want to remove duplicates in this array so that all hashes inside the array have unique parameter keys.

For example if I have

arr = [{"update" => "1", "reload" => "true"},
       {"update" => "5", "reload" => "false"},
       {"update" => "9", "reload" => "false"},
       {"update" => "7", "reload" => "true", "newvalue" => "11111"},
       {"page" => "1"}]

I would expect to have an array containing only:

arr = [{"update" => "1", "reload" => "true"},
       {"update" => "7", "reload" => "true", "newvalue" => "11111"},
       {"page" => "1"}]

Where the first three entries are duplicates of each other so only keep one of them, the fourth being unique since it has an extra unique key the first three did not have, and the fifth being unique since it is not the same as any of them.

How would I attempt to solve this problem?

How did you attempt to solve this problem? And why do you think that the first three are duplicates? They are all there different. — vgoff
– vgoff, Commented Jul 9, 2016 at 20:00
@vgoff Their keys are the same, which is what I am trying to remove duplicates of. — randy newfield
– randy newfield, Commented Jul 9, 2016 at 20:05
You can edit the question to clarify that, rather than entries, to unique keys.. I did not rely on the title giving that hint as strongly as I should have. :( — vgoff
– vgoff, Commented Jul 9, 2016 at 22:19

Albin · Accepted Answer · 2016-07-09 20:05:13Z

1

You could solve it like this:

tmp = {}
b = arr.select do |h|
  if tmp[h.keys]
    false
  else
    tmp[h.keys] = true
    true
  end
end

answered Jul 9, 2016 at 20:05

Albin

3,0321 gold badge24 silver badges33 bronze badges

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

2 Comments

randy newfield Over a year ago

Could you add a bit more detail to your answer as to why it works for other readers?

Jake Shorty Over a year ago

The tmp hash is used to track whether we've seen a specific set of keys before. We iterate over each hash in the array and use its keys to return a value from tmp. If we get nil we haven't seen it before, so we set it to tmp[h.keys] = true and return true. Otherwise we'll just return false. Array#select will return only the values returning true in the supplied block— the first hash with any unique set of keys.

Cary Swoveland · Accepted Answer · 2016-07-10 05:53:03Z

1

arr = [{"update" => "1", "reload" => "true"},
       {"update" => "5", "reload" => "false"},
       {"update" => "9", "reload" => "false"},
       {"update" => "7", "reload" => "true", "newvalue" => "11111"},
       {"page" => "1"}]

arr.uniq(&:keys)
  #=> [{"update"=>"1", "reload"=>"true"},
  #    {"update"=>"7", "reload"=>"true", "newvalue"=>"11111"},
  #    {"page"=>"1"}]

See the doc for Array#uniq for the case where uniq takes a block. In effect, Ruby is doing the following to determine which elements of arr to select:

a = arr.map(&:keys) 
  #=> [["update", "reload"],
  #    ["update", "reload"],
  #    ["update", "reload"],
  #    ["update", "reload", "newvalue"],
  #    ["page"]] 

a.uniq
  #=> [["update", "reload"], ["update", "reload", "newvalue"], ["page"]]

arr.uniq(&:keys) has the same effect as:

arr.uniq { |h| h.keys }
  #=> [{"update"=>"1", "reload"=>"true"},
  #    {"update"=>"7", "reload"=>"true", "newvalue"=>"11111"},
  #    {"page"=>"1"}]

Many view arr.uniq(&:keys) as just a short-hand way of writing the above expression with the block. That's OK, but actually arr.uniq(&:keys) converts the method (represented by the symbol) :keys to a proc and then calls the proc.

edited Jul 10, 2016 at 5:53

answered Jul 10, 2016 at 4:10

Cary Swoveland

111k6 gold badges69 silver badges105 bronze badges

1 Comment

randy newfield Over a year ago

Thanks for the answer, can you explain what &: does in this instance?

Collectives™ on Stack Overflow

How to remove hashes from array based on if all keys in hash match another hash

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related