I have the array tags, which consists of a number of Strings that I need to use to create a new Hash (content) where each value is an empty array. I currently have Hash[*tags.map {|k| [k, nil]}.flatten], but this returns:
{
"tag1" => nil,
"tag2" => nil,
"tag3" => nil
}
when I want it to be
{
"tag1" => [],
"tag2" => [],
"tag3" => []
}
sorry this is kind of a dumb question, but I've googled around and can't find the answer. Thank you!
Using flatten, map [] instead of nil like you tried, then use flatten(1). That eliminates only the first layer of array, so you get ['tag1', [], ...] to pass to Hash[].
> tags = %w[tag1 tag2 tag3]
=> ["tag1", "tag2", "tag3"]
> tags.map {|k| [k, []]}.flatten
=> ["tag1", "tag2", "tag3"]
> tags.map {|k| [k, []]}.flatten(1)
=> ["tag1", [], "tag2", [], "tag3", []]
> Hash[*tags.map {|k| [k, []]}.flatten(1)]
=> {"tag1"=>[], "tag2"=>[], "tag3"=>[]}
You can also avoid flatten altogether if you drop the splat (*) from Hash[], since ::[] also accepts a list of pairs.
> tags.map {|k| [k, []]}
=> [["tag1", []], ["tag2", []], ["tag3", []]]
> Hash[tags.map {|k| [k, []]}]
=> {"tag1"=>[], "tag2"=>[], "tag3"=>[]}
Hash[*tags.flat_map { |k| [k, []]}].Ruby's Array supports (Cartesian) product so you can take advantage of this, without need of extra logic built inside block(s):
> tags.product([[]]).to_h
=> {"tag1"=>[], "tag2"=>[], "tag3"=>[]}
Simple.
product all week :)According to the docs Hash.new accepts a block which you can use to provide a default value each time a new key, not belonging to the Hash, is accessed.
So you can use this
tags = %w(tag1 tag2 tag3)
h = Hash.new{ |hash, key| hash[key] = [] }
tags.each{ |tag| h[tag] }
# h == {"tag1"=>[], "tag2"=>[], "tag3"=>[]}
h['tag4']
# h == {"tag1"=>[], "tag2"=>[], "tag3"=>[], "tag4"=>[]}
One more way:
tags = %w|tag1 tag2 tag3|
Hash.new { |h,k| h[k] = [] }.tap { |h| h.values_at(*tags) }
#=> {"tag1"=>[], "tag2"=>[], "tag3"=>[]}