Inverting a hash with array values

Question

For ease of authoring I'm writing my hash like this:

h = {
    :key1: [:val1, :val2, :val3],
    :key2: [:val4, :val5, :val6]
}

But everywhere I use it I need to look up the key associated with a value. Currently I'm doing the following to transform it:

h = Hash[*{
    :key1: [:val1, :val2, :val3],
    :key2: [:val4, :val5, :val6]
}.map {|key, vals| vals.map {|val| [val, key]}}.flatten]

Which gives me what I want:

{ :val1 => :key1, :val2 => key1, :val3 => key1, :val4 => key2, :val5 => :key2, :val6 => :key2 }

But is there a simpler way to achieve the same goal?

Chris Heald · Accepted Answer · 2013-07-08 10:30:38Z

10

Array#product is pretty badass for this. :)

h = {
    key1: [:val1, :val2, :val3],
    key2: [:val4, :val5, :val6]
}

p Hash[h.flat_map {|k,v| v.product [k]}]
# {:val1=>:key1, :val2=>:key1, :val3=>:key1, :val4=>:key2, :val5=>:key2, :val6=>:key2}

edited Jul 8, 2013 at 10:30

answered Jul 8, 2013 at 10:22

Chris Heald

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1 Comment

Bastien D Over a year ago

I think it is a readable and efficient solution compared to other

Arup Rakshit · Accepted Answer · 2013-07-08 10:16:41Z

3

h = {
    :key1 => [:val1, :val2, :val3],
    :key2 => [:val4, :val5, :val6]
}

p Hash[h.flat_map{|k,v| v.zip [k]*v.size }]
# >> {:val1=>:key1, :val2=>:key1, :val3=>:key1, :val4=>:key2, :val5=>:key2, :val6=>:key2}
p Hash[h.flat_map{|k,v| v.zip [k].cycle }]
# >> {:val1=>:key1, :val2=>:key1, :val3=>:key1, :val4=>:key2, :val5=>:key2, :val6=>:key2}

edited Jul 8, 2013 at 10:16

answered Jul 8, 2013 at 10:10

Arup Rakshit

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3 Comments

tessi Over a year ago

I think the code is hard to read (thus, hard to maintain for production systems). But its cool and short :) (+1)

michael_wu Over a year ago

Very cool. v.zip([k]*v.size) could be simplified to v.zip([k].cycle).

Arup Rakshit Over a year ago

@undur_gongor Nice usecase I have seen today for #cycle. Thank you very much.

johnsyweb · Accepted Answer · 2013-07-08 10:09:42Z

2

I was trying to do just this yesterday. This was my solution:

h = {
key1: [:val1, :val2, :val3],    
key2: [:val4, :val5, :val6],    
}  
=> {:key1=>[:val1, :val2, :val3], :key2=>[:val4, :val5, :val6]}

hp = {}
=> {}

h.each { |k, v| v.each{ |e| hp[e] = k } }
=> {:key1=>[:val1, :val2, :val3], :key2=>[:val4, :val5, :val6]}

hp
=> {:val1=>:key1,
:val2=>:key1,
:val3=>:key1,
:val4=>:key2,
:val5=>:key2,
:val6=>:key2}

answered Jul 8, 2013 at 10:09

johnsyweb

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