Iterate nested hash to get an array of strings

Question

I have a nested hash like this:

s = Struct.new :topic_version

s_blank = s.new
s_bar = s.new "bar"
s_foo = s.new "foo"

h = {
  :aa=>{:services=>[nil, s_blank, s_bar]},
  :ab=>{:services=>[s_blank, nil, s_bar, s_bar]},
  :ac=>{:services=>[nil, s_foo, s_blank]}
}

And I'd like to get an array of string values eg. ["bar", baz"]

My attempt is:

@topic_version_collection = []
h.each do |key, value| 
  value[:services].each do |v|
    @topic_version_collection << v.topic_version
  end
end

And it works if there aren't any nil values in the array.

But how can I make it work with nils?

Where does "baz" come from? Can you add an if !v.nil? check? — ggorlen
– ggorlen, Commented Aug 27, 2019 at 16:29
Note it is more idiomatic to capitalize classes: S = Struct.new :topic_version — steenslag
– steenslag, Commented Aug 27, 2019 at 19:15

tadman · Accepted Answer · 2019-08-27 16:30:56Z

The lazy way here is to use the conditional navigation operator, or in other words:

v&.topic_version

Where that returns nil if v is nil.

That's just scratching the surface, though. When you see the pattern:

x = [ ]
y.each do |z|
  x << z.a
end

What you really want is:

x = y.map(&:a)

Where map helps transform values in one array to values in another using a 1:1 mapping.

To build up to a solution here first strip out the values from the Hash, then pull the :services key in that nested Hash, then call topic_version on all non-nil entries. Since those can return nil, strip out nil with compact at the end.

In other words:

h.values.map do |v|
  v[:services]
end.flat_map do |v|
  v.compact.map(&:topic_version)
end.compact

Now if you want them unique, add .uniq at the end.

The flat_map here is a way of combining multiple result arrays into a singular final result.

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