return keys from array having same values in ruby

Question

How to find out elements of array having same value_entries. As code is in ruby, looking better approach.

Input

"block_device": {
  "sda": {
    "size": "83886080",
    "removable": "0",
    "model": "VBOX HARDDISK",
    "rev": "1.0",
    "state": "running",
    "timeout": "30",
    "vendor": "ATA",
    "rotational": "1"
  },
  "sdb": {
    "size": "16384",
    "removable": "0",
    "model": "VBOX HARDDISK",
    "rev": "1.0",
    "state": "running",
    "timeout": "30",
    "vendor": "ATA",
    "rotational": "1"
  },
  "sdc": {
    "size": "16384",
    "removable": "0",
    "model": "VBOX HARDDISK",
    "rev": "1.0",
    "state": "running",
    "timeout": "30",
    "vendor": "ATA",
    "rotational": "1"
  }
}

Sample Code Block:

devicesForRaid = []
deviceHolder = []
node['block_device'].each do |deviceName,deviceProperty|
   deviceHolder.push(deviceName,deviceProperty['size'])       #['sda'=>'83886080','sdb'=>'16384','sdc'=>'16384']
end

deviceHolder.each do | deviceName,deviceSize|

    # how to get deviceName who all having same size

    if(deviceSize_match_found){
        devicesForRaid.push(deviceName)
    }
end

Expected Output:

devicesForRaid = ['sdb','sdc']

Trial way:

using stack, push 1st element onto stack, and comparing with rest of array element.

if match found, push that element onto stack.

Sample code block completion or better code highly appreciated.

Cary Swoveland · Accepted Answer · 2015-10-19 06:05:28Z

2

You can do this:

input_hash[:block_device].each_with_object({}) { |(k,g),h|
  h.update(g[:size]=>[k]) { |_,o,n| o+n } }
  #=> {"83886080"=>[:sda], "16384"=>[:sdb, :sdc]}

This uses the form of Hash#update (aka merge!) that employs the block:

{ |_,o,n| o+n }

to determine the values of keys that are present in both hashes being merged.

answered Oct 19, 2015 at 6:05

Cary Swoveland

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7 Comments

John La Rooy Over a year ago

Isn't o+n going to create a new array each time? Can you use o<<n.first instead?

Cary Swoveland Over a year ago

@John, yes, though each will be marked for garbage collection as soon as it's replaced by the next one.

Drenmi Over a year ago

If we're talking array allocation, each_with_object already allocates one for each enumerated key, value pair. To be super optimized, it could use each_pair and a pre-instantiated object instead.

John La Rooy Over a year ago

@Drenmi, That's still just extra O(n) complexity. The quadratic is potentially more serious - especially if it is widely used.

John La Rooy Over a year ago

@CarySwoveland, It's definitely quadratic in 2.2.1 according to a quick test I just did comparing { |_,o,n| o+n } to { |_,o,n| o<<n.first}. So something to keep in mind when using this pattern with mutables

|

John La Rooy · Accepted Answer · 2015-10-19 05:53:16Z

1

res = Hash.new { |h, k| h[k] = [] }
node['block_device'].each{|k, v| res[v[:size]]<<k}

gives:

=> {"83886080"=>[:sda], "16384"=>[:sdb, :sdc]}

I guess you want to look through res for values with length of 2 or more

res.to_a.select{|k, v| v.size > 1}

answered Oct 19, 2015 at 5:53

John La Rooy

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1 Comment

K M Rakibul Islam Over a year ago

I think, {"83886080"=>[:sda], "16384"=>[:sdb, :sdc]}.to_a.select{|k, v| v.size > 1} will give the output: [["16384", [:sdb, :sdc]]] which is slightly off from the expected output. You may want to do something like this: {"83886080"=>[:sda], "16384"=>[:sdb, :sdc]}.select{|k, v| v.size > 1}.values.flatten.

K M Rakibul Islam · Accepted Answer · 2015-10-19 06:24:55Z

0

You can do it this way (assuming block_device is a key in your input data hash):

hash = input_data[:block_device]
new_hash = Hash.new{ |h, k| h[k] = [] }
hash.each do |k, v|
  new_hash[v[:size]] << k
end
p new_hash
# => {"83886080"=>[:sda], "16384"=>[:sdb, :sdc]}

From this new_hash, you can extract your required data easily.

e.g. if you want to extract the elements that has a size more than 1, you can do this:

p new_hash.select { |k,v| v.length > 1 }.values.flatten
# => [:sdb, :sdc]

edited Oct 19, 2015 at 6:24

answered Oct 19, 2015 at 5:55

K M Rakibul Islam

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Comments

itmammoth · Accepted Answer · 2015-10-20 03:58:55Z

0

How about using group_by?

node[:block_device]
  .group_by {|device, attributes| attributes[:size] }
  .map {|size, devices| devices.size > 1 ? devices.map(&:first) : nil }
  .compact
  .flatten
=> [:sdb, :sdc]

I think this way is easy to understand what you are doing.

answered Oct 20, 2015 at 3:58

itmammoth

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