Manipulation of hash and arrays in Ruby

Question

For my first question here I would like to ask you how you'd do the following in Ruby.

I have a hash with the following aspect

variables["foo"] = [1,2,3]
variables["bar"] = [4,5,6,7]
variables[...] = ...

Update: this hash will have an arbitrary number of key => values pairs.

So it represents parameters and their possible values. I would like to "generate" now an Array containing hashes whose key=>value pairs represent each possible combination of the variables. In the case of the example above, I would have an array of 12 (=3x4) hashes like that

[ hash1, hash2, ..., hash16]

where hashi would be

 hash1["foo"] = 1
 hash1["bar"] = 4
 hash2["foo"] = 1
 hash2["bar"] = 5
 hash3["foo"] = 1
 hash3["bar"] = 6
 hash4["foo"] = 1
 hash4["bar"] = 7
 hash5["foo"] = 2
 hash5["bar"] = 4
 hash6["foo"] = 3
 hash6["bar"] = 4
 ...
 hash16["foo"] = 3
 hash16["bar"] = 7

I have a few ideas but all of them are quite complicated nested loops ...

Thanks a lot !

I would like to first have the first value of the first variable and every values of the second one, then the second value of the first variable and every values of the second one, etc. — Cedric H.
– Cedric H., Commented Aug 16, 2010 at 12:27

Jörg W Mittag · Accepted Answer · 2010-08-16 13:34:16Z

1

vars = {foo: [1, 2, 3], bar: [4, 5, 6, 7]}

(v = vars.map {|k, v| ([k] * v.length).zip(v) }).first.product(*v.drop(1)).
map {|args| args.reduce({}) {|h, (k, v)| h.tap {|h| h[k] = v }}}
# => [{:foo=>1, :bar=>4},
# =>  {:foo=>1, :bar=>5},
# =>  {:foo=>1, :bar=>6},
# =>  {:foo=>1, :bar=>7},
# =>  {:foo=>2, :bar=>4},
# =>  {:foo=>2, :bar=>5},
# =>  {:foo=>2, :bar=>6},
# =>  {:foo=>2, :bar=>7},
# =>  {:foo=>3, :bar=>4},
# =>  {:foo=>3, :bar=>5},
# =>  {:foo=>3, :bar=>6},
# =>  {:foo=>3, :bar=>7}]

This works with arbitrary many entries and arbitrary keys.

edited Aug 16, 2010 at 13:34

answered Aug 16, 2010 at 13:27

Jörg W Mittag

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Cedric H. Over a year ago

Thanks a lot ! I still have to understand it completely but it works as I wanted.

Joshua Cheek · Accepted Answer · 2010-08-16 13:06:57Z

1

Ruby syntax for an Array is brackets, and for a hash is curly braces with a key, hash rocket => value.

variables = { "foo" => [1,2,3] , "bar" => [4,5,6,7] }

results = Array.new

variables["foo"].each do |foo_variable|
  variables["bar"].each do |bar_variable|
    results << { "foo" => foo_variable , "bar" => bar_variable }
  end
end

require "pp"
pp results

answered Aug 16, 2010 at 13:06

Joshua Cheek

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Cedric H. Over a year ago

Thanks. But I forgot to say that the "variables" hash has an arbitrary number of entries.

Mladen Jablanović · Accepted Answer · 2010-08-16 14:06:38Z

1

vars.values.inject(&:product).map{|values|
  Hash[vars.keys.zip(values.flatten)]
}

answered Aug 16, 2010 at 14:06

Mladen Jablanović

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1 Comment

Jörg W Mittag Over a year ago

This is really nice. I was toying around with something similar, but then I couldn't find anything in the language specification that would guarantee that Hash#keys and Hash#values would return the items in the same order, so I backed away from it. But now I realize that I am using some 1.9 features anyway, and in 1.9, Hash es are of course guaranteed to be ordered anyhow, so your solution is much nicer, assuming that you run it on 1.9.

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