Removing Elements from a Hash

Problem

Certain elements of your hash have got to go!

Solution

Most of the time you want to remove a specific element of a hash. To do that, pass the key into Hash#delete.

	h = {}
	h[1] = 10
	h # => {1=>10}
	h.delete(1)
	h # => {}

 

Discussion

Don't try to delete an element from a hash by mapping it to nil. It's true that, by default, you get nil when you look up a key that's not in the hash, but there's a difference between a key that's missing from the hash and a key that's present but mapped to nil. Hash#has_key? will see a key mapped to nil, as will Hash#each and all other methods except for a simple fetch:

	h = {}
	h[5] # => nil
	h[5] = 10
	h[5] # => 10
	h[5] = nil
	h[5] # => nil
	h.keys # => [5]
	h.delete(5)
	h.keys # => []

Hash#delete works well when you need to remove elements on an ad hoc basis, but sometimes you need to go through the whole hash looking for things to remove. Use the Hash#delete_if iterator to delete key-value pairs for which a certain code block returns true (Hash#reject works the same way, but it works on a copy of the Hash). The following code deletes all key-value pairs with a certain value:

	class Hash
	 def delete_value(value)
	 delete_if { |k,v| v == value }
	 end
	end

	h = {'apple' => 'green', 'potato' => 'red', 'sun' => 'yellow',
	 'katydid' => 'green' }
	h.delete_value('green')
	h # => {"sun"=>"yellow", "potato"=>"red"}

This code implements the opposite of Hash#merge; it extracts one hash from another:

	class Hash
	 def remove_hash(other_hash)
	 delete_if { |k,v| other_hash[k] == v }
	 end
	end

	squares = { 1 => 1, 2 => 4, 3 => 9 }
	doubles = { 1 => 2, 2 => 4, 3 => 6 }
	squares.remove_hash(doubles)
	squares # => {1=>1, 3=>9}

Finally, to wipe out the entire contents of a Hash, use Hash#clear:

	h = {}
	1.upto(1000) { |x| h[x] = x }
	h.keys.size # => 1000
	h.clear
	h # => {}

 

See Also

  • Recipe 5.3, "Adding Elements to a Hash"
  • Recipe 5.7, "Iterating Over a Hash"


Strings

Numbers

Date and Time

Arrays

Hashes

Files and Directories

Code Blocks and Iteration

Objects and Classes8

Modules and Namespaces

Reflection and Metaprogramming

XML and HTML

Graphics and Other File Formats

Databases and Persistence

Internet Services

Web Development Ruby on Rails

Web Services and Distributed Programming

Testing, Debugging, Optimizing, and Documenting

Packaging and Distributing Software

Automating Tasks with Rake

Multitasking and Multithreading

User Interface

Extending Ruby with Other Languages

System Administration



Ruby Cookbook
Ruby Cookbook (Cookbooks (OReilly))
ISBN: 0596523696
EAN: 2147483647
Year: N/A
Pages: 399

Similar book on Amazon

Flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net