How to change array of arrays into hashmap?

Question

I stuck in a situation, I have an array of array structure how can I make it hash like below:-

Example:

[[378, 1], [144, 1], [144, 1], [144, 3], [144, 7], [144, 6], [144, 8], [144, 8], [809, 1], [809, 1], [809, 8]]

Convert to:

{378=>[1], 144=>[1,3,7,6,8], 809=>[1,8]}

means take the first element as key and make the second element in another array as value matched to key.

I tried below method but failed in one use case:

raw_hash = arr.group_by { |sub_arr| sub_arr[0] }
modified_hash = {}
raw_hash.each do |k, arr|
   modified_hash[k] = [ arr.flatten.uniq - [ k ] ].flatten
end

failed use-case is if first element and second element both will same then it will make unique and subtract second value

Ilya · Accepted Answer · 2019-09-27 09:04:56Z

4

Just use each_with_object, it is faster and consumes less memory than @mrzasa's approach:

array.each_with_object(Hash.new { |h, k| h[k] = [] }) do |(first, last), m|  
  m[first] |= [last]
end
#=> {378=>[1], 144=>[1, 3, 7, 6, 8], 809=>[1, 8]}

But for the uniqueness the simplest probably would be using Set:

array.each_with_object(Hash.new { |h, k| h[k] = Set.new }) do |(first, last), m|  
  m[first] << last 
end
#=> {378=>#<Set: {1}>, 144=>#<Set: {1, 3, 7, 6, 8}>, 809=>#<Set: {1, 8}>}

edited Sep 27, 2019 at 9:04

answered Sep 27, 2019 at 8:49

Ilya

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1 Comment

Viktor Over a year ago

|(first, last), m| nice

mrzasa · Accepted Answer · 2019-09-27 08:53:23Z

3

Group by the first element of the array and then chose the last second elements of the grouped values.

a = [[378, 1], [144, 1], [144, 1], [144, 3], [144, 7], [144, 6], [144, 8], [144, 8], [809, 1], [809, 1], [809, 8]] 
a.group_by(&:first).map{|k, v| [k, v.map(&:last)]}.to_h
# => {378=>[1], 144=>[1, 1, 3, 7, 6, 8, 8], 809=>[1, 1, 8]}

EDIT:

If you want to have a unique values, add uniq to the map that collects them:

a.group_by(&:first).map{|k, v| [k, v.map(&:last).uniq]}.to_h
#=> {378=>[1], 144=>[1, 3, 7, 6, 8], 809=>[1, 8]}

edited Sep 27, 2019 at 8:53

answered Sep 27, 2019 at 8:44

mrzasa

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7 Comments

mrzasa Over a year ago

Updated, see now.

Ilya Over a year ago

This solution is ok for the instance from the question, but extremely slow and inefficient if array is quite big.

mrzasa Over a year ago

How big is quite big to make the difference significant?

Ilya Over a year ago

@mrzasa you can perform benchmarking, it doesn't matter actually :) If we can do something obviously faster and consume less memory why just don't do it? :)

mrzasa Over a year ago

chaining maps & group_by in opposition to each and mutating arrays (I know that too many maps causes GC overhead).

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