CODEWITHSUNDEEP
rubyhash
Len
Len

Reputation: 2145

Ruby Hashes: getting a set of keys from sorted values

Hi all I've made myself quite a complicated (at least, for me) construction. I have a Hash, looking like (example):

states_with_depths = {
 #State => depth
 "S0" => 0,
 "S1" => 1,
 "S2" => 2,
 "S3" => 2,
 "S4" => 2,
 "S5" => 3
}

(note: its a Hash, so its not sorted yet)

Now what I would like: Having a loop starting at depth = 0 with a set of all states having depth 0 or less, next iteration of the loop, having a set of all states with depth 1 or less, etc.

What would be a neat way to get such a construction?

Please let me know if my question is unclear.

Upvotes: 1

Views: 353

Answers (3)

Phrogz
Phrogz

Reputation: 303224

Here's a variation that works for Ruby before 1.8.7 (when Enumerable#group_by was added):

states_by_depth = Hash.new{|h,depth| h[depth]=[] }
states_with_depths.each{ |state,depth| states_by_depth[depth] << state }
#=> {0=>["S0"], 1=>["S1"], 2=>["S3", "S4", "S2"], 3=>["S5"]}
min_depth = states_by_depth.keys.min
max_depth = states_by_depth.keys.max
min_depth.upto(max_depth) do |depth|
  next unless states = states_by_depth[depth]
  puts "Depth: #{depth}"
  states.each{ |state| puts "..#{state}" }
end
#=> Depth: 0
#=> ..S0
#=> Depth: 1
#=> ..S1
#=> Depth: 2
#=> ..S3
#=> ..S4
#=> ..S2
#=> Depth: 3
#=> ..S5

Upvotes: 1

J-_-L
J-_-L

Reputation: 9177

require 'set'

depths_with_states = []

max_level = states_with_depths.max_by{|e|e[1]}[1]

states_with_depths.map{ |state, level|
  (level..max_level).each{ |i|
    depths_with_states[i] ||= Set.new  # ...or just array
    depths_with_states[i] << state
  }
}

depths_with_states.each{ |states|
  # do whatever you want to do ;)
}

Upvotes: 1

Michael Kohl
Michael Kohl

Reputation: 66837

You can just use group_by with the value:

>> states_with_depths.group_by { |k,v| v }
 #=> {0=>[["S0", 0]], 1=>[["S1", 1]], 2=>[["S2", 2], ["S3", 2], ["S4", 2]], 3=>[["S5", 3]]}

This can also be shortened to:

states_with_depths.group_by(&:last)

To use this you can do something like:

states_with_depths.group_by(&:last).each do |depth, arrs| 
  puts "Values with depth #{depth}: #{arrs.map(&:first)}"                                                                                                                        
end

Which outputs:

Values with depth 0: ["S0"]
Values with depth 1: ["S1"]
Values with depth 2: ["S2", "S3", "S4"]
Values with depth 3: ["S5"]

Upvotes: 4

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