merging ruby hash with array of values into another hash with array of values

Question

I can't seem to find anywhere that talks about doing this.

Say I have a hash {"23"=>[0,3]} and I want to merge in this hash {"23"=>[2,3]} to result with this hash {"23"=>[0,2,3]}

Or how about {"23"=>[3]} merged with {"23"=>0} to get {"23"=>[0,3]}

Thanks!

Well, what have you tried? Anyway, looked at inject? There might be a more clever zip-by-key approach, though... — user166390
– user166390, Commented Jun 23, 2012 at 18:15
I've looked at both inject and reduce but I honestly don't understand how those function work and what they are capable of doing. They have always been a mystery to me. :\ — bfcoder
– bfcoder, Commented Jun 23, 2012 at 18:31
What do you mean you looked at both, they are aliased methods. I guess @SergioTulentsev was trying to make a joke. If you want to understand how they work, you should read up on folds: en.wikipedia.org/wiki/Fold_(higher-order_function) — Michael Kohl
– Michael Kohl, Commented Jun 23, 2012 at 19:00

megas · Accepted Answer · 2018-11-05 15:53:15Z

38

{ "23" => [0,3] }.merge({ "23" => [2,3] }) do |key, oldval, newval| 
  oldval | newval
end
#=> {"23"=>[0, 3, 2]}

More generic way to handle non-array values:

{ "23" => [0,3] }.merge({ "23" => [2,3] }) do |key, oldval, newval|
  (newval.is_a?(Array) ? (oldval + newval) : (oldval << newval)).uniq
end

Updated with a Marc-André Lafortune's hint .

answered Jun 23, 2012 at 18:24

megas

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5 Comments

Didn't know merge could take a block. Sweet!

That is awesome. Works perfectly! +1 for megas! Thanks!

Better, shorter, faster to use oldval | newval then (oldval + newval).uniq

Awesome, thanks Marc-André Lafortune Faster and shorter is always better. :]

How to merge everything? I mean if there is 23 => [], 24 => [], I want to merge all to one single array []