How to assign hash[‘a’][‘b’]= ‘c’ if hash[‘a’] doesn’t exist?

by

The easiest way is to construct your Hash with a block argument:

hash = Hash.new { |h, k| h[k] = { } }
hash['a']['b'] = 1
hash['a']['c'] = 1
hash['b']['c'] = 1
puts hash.inspect
# "{"a"=>{"b"=>1, "c"=>1}, "b"=>{"c"=>1}}"

This form for new creates a new empty Hash as the default value. You don’t want this:

hash = Hash.new({ })

as that will use the exact same hash for all default entries.

Also, as Phrogz notes, you can make the auto-vivified hashes auto-vivify using default_proc:

hash = Hash.new { |h, k| h[k] = Hash.new(&h.default_proc) }

UPDATE: I think I should clarify my warning against Hash.new({ }). When you say this:

h = Hash.new({ })

That’s pretty much like saying this:

h = Hash.new
h.default = { }

And then, when you access h to assign something as h[:k][:m] = y, it behaves as though you did this:

if(h.has_key?(:k))
    h[:k][:m] = y
else
    h.default[:m] = y
end

And then, if you h[:k2][:n] = z, you’ll end up assigning h.default[:n] = z. Note that h still says that h.has_key?(:k) is false.

However, when you say this:

h = Hash.new(0)

Everything will work out okay because you will never modified h[k] in place here, you’ll only read a value from h (which will use the default if necessary) or assign a new value to h.

More Related Contents:

  1. Accessing elements of nested hashes in ruby [duplicate]
  2. How to remove empty string in hash
  3. Strange, unexpected behavior (disappearing/changing values) when using Hash default value, e.g. Hash.new([])
  4. How to avoid NoMethodError for missing elements in nested hashes, without repeated nil checks?
  5. Is there any difference between the `:key => “value”` and `key: “value”` hash notations?
  6. Converting a nested hash into a flat hash
  7. Is Hash Rocket deprecated?
  8. Ruby Style: How to check whether a nested hash element exists
  9. Why is `a = a` `nil` in Ruby?
  10. Group hashes by keys and sum the values
  11. How do I search within an array of hashes by hash values in ruby?
  12. Ruby Hash with duplicate keys?
  13. Performance of Arrays and Hashes in Ruby
  14. Comparing ruby hashes [duplicate]
  15. Find key/value pairs deep inside a hash containing an arbitrary number of nested hashes and arrays
  16. Double Pipe Symbols in Ruby Variable Assignment? [duplicate]
  17. Ruby method Array#
  18. How to get a specific output iterating a hash in Ruby?
  19. How do I create a SHA1 hash in ruby?
  20. Convert array of 2-element arrays into a hash, where duplicate keys append additional values
  21. Is order of a Ruby hash literal guaranteed?
  22. How do methods use hash arguments in Ruby?
  23. ruby using the “&:methodname” shortcut from array.map(&:methodname) for hash key strings rather than methodname
  24. How do I compare two hashes?
  25. EventSource / Server-Sent Events through Nginx
  26. Ruby objects and JSON serialization (without Rails)
  27. class
  28. How can I generate zip file without saving to the disk with Ruby?
  29. What’s the precedence of ruby’s method call
  30. In Ruby, why does nil[1]=1 evaluate to nil?

Leave a Comment