ruby set key of hash for array of hashes

Question

Given the following array of hashes, how can I make a new hash with friend_id as the key and dist the value?

results = [
 {"user_id"=>"18", "friend_id"=>"17", "dist"=>"1"},
 {"user_id"=>"18", "friend_id"=>"42", "dist"=>"1"},
 {"user_id"=>"18", "friend_id"=>"43", "dist"=>"1"},
 {"user_id"=>"18", "friend_id"=>"46", "dist"=>"2"}
]

desired_hash = {"17" => "1", "42" => "1", "43" => "1", "46" => "2"}

I've tried map but the values are then in an array. I also tried to flatten that result but it flattened the key instead of the value

Cary Swoveland · Accepted Answer · 2015-05-03 03:20:15Z

2

results.each_with_object({}) { |g,h| h[g["friend_id"]] = g["dist"] }

or

results.each_with_object({}) { |g,h| h.update(g["friend_id"]=> g["dist"]) }

answered May 3, 2015 at 3:20

Cary Swoveland

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Comments

Sid · Accepted Answer · 2015-05-03 02:58:21Z

1

For this use case, I think it would be simpler and more readable to just use #each:

desired_hash = Hash.new
results.each {|h| desired_hash[h["friend_id"]] = h["dist"]}

Then, desired_hash is:

#=> {"17"=>"1", "42"=>"1", "43"=>"1", "46"=>"2"}

answered May 3, 2015 at 2:58

Sid

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Comments

Yu Hao · Accepted Answer · 2015-05-03 03:12:14Z

1

You can use Enumerable#inject method.

results.inject({}) {|sum, e| sum.merge({e["friend_id"] => e["dist"]})}
# => {"17"=>"1", "42"=>"1", "43"=>"1", "46"=>"2"}

edited May 3, 2015 at 3:12

Yu Hao

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answered May 3, 2015 at 2:49

pangpang

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3 Comments

Aleksei Matiushkin Over a year ago

merge on each iteration is quite inefficient.

pangpang Over a year ago

Why ? Could you please tell me the reason?

Aleksei Matiushkin Over a year ago

h1 = { :a => 1 } ; h2 = { :b => 2 } ; n = 5000000 ; Benchmark.bm do |x| x.report { for i in 1..n; h1.merge h2; end } ; x.report { n.times do   ; h1[:b] = h2[:b]; end } ; end

⇒ 5.870000 vs. 0.660025. The reason: hash is updated on every merge.

Myst · Accepted Answer · 2015-05-03 04:25:29Z

1

results.map {|h| [h['friend_id'], h['dist']]} .to_h

Although I probably like @CarySwoveland 's answer better, on the lines of:

results.each_with_object({}) {|h, n| n[h['friend_id']] = h['dist']}

answered May 3, 2015 at 4:25

Myst

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Comments

G. Allen Morris III · Accepted Answer · 2015-05-03 14:02:02Z

1

Simply:

desired_hash = Hash[results.map{ |h| [ h['friend_id'], h['dist']] }]

or as Victor suggests

desired_hash = Hash[results.map{ |x| x.values_at('friend_id', 'dist') }]

edited May 3, 2015 at 14:02

answered May 3, 2015 at 3:10

G. Allen Morris III

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2 Comments

Sid Over a year ago

I like this approach but you would have to set a new variable to results and use that in the map method instead. As it is, this mutates results array by adding the desired_hash to it:

[{"user_id"=>"18", "friend_id"=>"17", "dist"=>"1"}, {"user_id"=>"18", "friend_id"=>"42", "dist"=>"1"}, {"user_id"=>"18", "friend_id"=>"43", "dist"=>"1"}, {"user_id"=>"18", "friend_id"=>"46", "dist"=>"2"}]

Victor Moroz Over a year ago

h.values_at('friend_id', 'dist') is probably easier

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