How to remove a key from Hash and get the remaining hash in Ruby/Rails?

Question

To add a new pair to Hash I do:

{:a => 1, :b => 2}.merge!({:c => 3})   #=> {:a => 1, :b => 2, :c => 3}

Is there a similar way to delete a key from Hash ?

This works:

{:a => 1, :b => 2}.reject! { |k| k == :a }   #=> {:b => 2}

but I would expect to have something like:

{:a => 1, :b => 2}.delete!(:a)   #=> {:b => 2}

It is important that the returning value will be the remaining hash, so I could do things like:

foo(my_hash.reject! { |k| k == my_key })

in one line.

Answer 1

For those of you who just came here to know how to delete a key/value pair from a hash, you can use:
hash.delete(key)

For the rest of you who came here to read a wall of text about something entirely different, you can read the rest of this answer:

Rails has an except/except! method that returns the hash with those keys removed. If you're already using Rails, there's no sense in creating your own version of this.

class Hash
  # Returns a hash that includes everything but the given keys.
  #   hash = { a: true, b: false, c: nil}
  #   hash.except(:c) # => { a: true, b: false}
  #   hash # => { a: true, b: false, c: nil}
  #
  # This is useful for limiting a set of parameters to everything but a few known toggles:
  #   @person.update(params[:person].except(:admin))
  def except(*keys)
    dup.except!(*keys)
  end

  # Replaces the hash without the given keys.
  #   hash = { a: true, b: false, c: nil}
  #   hash.except!(:c) # => { a: true, b: false}
  #   hash # => { a: true, b: false }
  def except!(*keys)
    keys.each { |key| delete(key) }
    self
  end
end

Answer 2

Why not just use:

hash.delete(key)

hash is now the "remaining hash" you're looking for.

Answer 3

Oneliner plain ruby, it works only with ruby > 1.9.x:

1.9.3p0 :002 > h = {:a => 1, :b => 2}
 => {:a=>1, :b=>2} 
1.9.3p0 :003 > h.tap { |hs| hs.delete(:a) }
 => {:b=>2} 

Tap method always return the object on which is invoked...

Otherwise if you have required active_support/core_ext/hash (which is automatically required in every Rails application) you can use one of the following methods depending on your needs:

➜  ~  irb
1.9.3p125 :001 > require 'active_support/core_ext/hash' => true 
1.9.3p125 :002 > h = {:a => 1, :b => 2, :c => 3}
 => {:a=>1, :b=>2, :c=>3} 
1.9.3p125 :003 > h.except(:a)
 => {:b=>2, :c=>3} 
1.9.3p125 :004 > h.slice(:a)
 => {:a=>1} 

except uses a blacklist approach, so it removes all the keys listed as args, while slice uses a whitelist approach, so it removes all keys that aren't listed as arguments. There also exist the bang version of those method (except! and slice!) which modify the given hash but their return value is different both of them return an hash. It represents the removed keys for slice! and the keys that are kept for the except!:

1.9.3p125 :011 > {:a => 1, :b => 2, :c => 3}.except!(:a)
 => {:b=>2, :c=>3} 
1.9.3p125 :012 > {:a => 1, :b => 2, :c => 3}.slice!(:a)
 => {:b=>2, :c=>3} 

Answer 4

There are many ways to remove a key from a hash and get the remaining hash in Ruby.

1. .slice => It will return selected keys and not delete them from the original hash. Use slice! if you want to remove the keys permanently else use simple slice.

   2.2.2 :074 > hash = {"one"=>1, "two"=>2, "three"=>3}
    => {"one"=>1, "two"=>2, "three"=>3} 
   2.2.2 :075 > hash.slice("one","two")
    => {"one"=>1, "two"=>2} 
   2.2.2 :076 > hash
    => {"one"=>1, "two"=>2, "three"=>3} 
   

2. .delete => It will delete the selected keys from the original hash(it can accept only one key and not more than one).

   2.2.2 :094 > hash = {"one"=>1, "two"=>2, "three"=>3}
    => {"one"=>1, "two"=>2, "three"=>3} 
   2.2.2 :095 > hash.delete("one")
    => 1 
   2.2.2 :096 > hash
    => {"two"=>2, "three"=>3} 
   

3. .except => It will return the remaining keys but not delete anything from the original hash. Use except! if you want to remove the keys permanently else use simple except.

   2.2.2 :097 > hash = {"one"=>1, "two"=>2, "three"=>3}
    => {"one"=>1, "two"=>2, "three"=>3} 
   2.2.2 :098 > hash.except("one","two")
    => {"three"=>3} 
   2.2.2 :099 > hash
    => {"one"=>1, "two"=>2, "three"=>3}         
   

4. .delete_if => In case you need to remove a key based on a value. It will obviously remove the matching keys from the original hash.

   2.2.2 :115 > hash = {"one"=>1, "two"=>2, "three"=>3, "one_again"=>1}
    => {"one"=>1, "two"=>2, "three"=>3, "one_again"=>1} 
   2.2.2 :116 > value = 1
    => 1 
   2.2.2 :117 > hash.delete_if { |k,v| v == value }
    => {"two"=>2, "three"=>3} 
   2.2.2 :118 > hash
    => {"two"=>2, "three"=>3} 
   

5. .compact => It is used to remove all nil values from the hash. Use compact! if you want to remove the nil values permanently else use simple compact.

   2.2.2 :119 > hash = {"one"=>1, "two"=>2, "three"=>3, "nothing"=>nil, "no_value"=>nil}
    => {"one"=>1, "two"=>2, "three"=>3, "nothing"=>nil, "no_value"=>nil} 
   2.2.2 :120 > hash.compact
    => {"one"=>1, "two"=>2, "three"=>3}
   

Results based on Ruby 2.2.2.

Answer 5

If you want to use pure Ruby (no Rails), don't want to create extension methods (maybe you need this only in one or two places and don't want to pollute namespace with tons of methods) and don't want to edit hash in place (i.e., you're fan of functional programming like me), you can 'select':

>> x = {:a => 1, :b => 2, :c => 3}
=> {:a=>1, :b=>2, :c=>3}
>> x.select{|x| x != :a}
=> {:b=>2, :c=>3}
>> x.select{|x| ![:a, :b].include?(x)}
=> {:c=>3}
>> x
=> {:a=>1, :b=>2, :c=>3}

Answer 6

Hash#except (Ruby 3.0+)

Starting from Ruby 3.0, Hash#except is a build-in method.

As a result, there is no more need to depend on ActiveSupport or write monkey-patches in order to use it.

h = { a: 1, b: 2, c: 3 }
p h.except(:a) #=> {:b=>2, :c=>3}

Sources:

• Hash#except from official Ruby docs.
• Link to the PR.
• Ruby 3.0 adds Hash#except and ENV.except.

Answer 7

#in lib/core_extensions.rb
class Hash
  #pass single or array of keys, which will be removed, returning the remaining hash
  def remove!(*keys)
    keys.each{|key| self.delete(key) }
    self
  end

  #non-destructive version
  def remove(*keys)
    self.dup.remove!(*keys)
  end
end

#in config/initializers/app_environment.rb (or anywhere in config/initializers)
require 'core_extensions'

I've set this up so that .remove returns a copy of the hash with the keys removed, while remove! modifies the hash itself. This is in keeping with ruby conventions. eg, from the console

>> hash = {:a => 1, :b => 2}
=> {:b=>2, :a=>1}
>> hash.remove(:a)
=> {:b=>2}
>> hash
=> {:b=>2, :a=>1}
>> hash.remove!(:a)
=> {:b=>2}
>> hash
=> {:b=>2}
>> hash.remove!(:a, :b)
=> {}

Answer 8

You can use except! from the facets gem:

>> require 'facets' # or require 'facets/hash/except'
=> true
>> {:a => 1, :b => 2}.except(:a)
=> {:b=>2}

The original hash does not change.

EDIT: as Russel says, facets has some hidden issues and is not completely API-compatible with ActiveSupport. On the other side ActiveSupport is not as complete as facets. In the end, I'd use AS and let the edge cases in your code.

Answer 9

Instead of monkey patching or needlessly including large libraries, you can use refinements if you are using Ruby 2:

module HashExtensions
  refine Hash do
    def except!(*candidates)
      candidates.each { |candidate| delete(candidate) }
      self
    end

    def except(*candidates)
      dup.remove!(candidates)
    end
  end
end

You can use this feature without affecting other parts of your program, or having to include large external libraries.

class FabulousCode
  using HashExtensions

  def incredible_stuff
    delightful_hash.except(:not_fabulous_key)
  end
end

Answer 10

in pure Ruby:

{:a => 1, :b => 2}.tap{|x| x.delete(:a)}   # => {:b=>2}

Answer 11

See Ruby on Rails: Delete multiple hash keys

hash.delete_if{ |k,| keys_to_delete.include? k }

Answer 12

It's was great if delete return the delete pair of the hash. I'm doing this:

hash = {a: 1, b: 2, c: 3}
{b: hash.delete(:b)} # => {:b=>2}
hash  # => {:a=>1, :c=>3} 

Answer 13

Try the except! method.

{:a => 1, :b => 2}.except!(:a)   #=> {:b => 2}

Answer 14

This is a one line way to do it, but it's not very readable. Recommend using two lines instead.

use_remaining_hash_for_something(Proc.new { hash.delete(:key); hash }.call)

Answer 15

Multiple ways to delete Key in Hash. you can use any Method from below

hash = {a: 1, b: 2, c: 3}
hash.except!(:a) # Will remove *a* and return HASH
hash # Output :- {b: 2, c: 3}

hash = {a: 1, b: 2, c: 3}
hash.delete(:a) # will remove *a* and return 1 if *a* not present than return nil

So many ways is there, you can look on Ruby doc of Hash here.

Thank you

Answer 16

Use delete, except, or except!

sample_hash = {hey: 'hey', hello: 'hello'}

Delete:

sample_hash.delete(:hey)
=> 'hey'

sample_hash
=> {hello: 'hello'}

Returns value of the key and deletes the key in the original object, returns nil if no such key

Except:

sample_hash.except(:hey)
=> {hello: 'hello'}

sample_hash
=> {hey: 'hey', hello: 'hello'}

Returns the entire hash without the specified keys, but does not update the original hash

Except!: except! is the same as except but it permanently changes the state of the original hash like all bang operated methods do

sample_hash.except!(:hey)
=> {hello: 'hello'}

sample_hash
=> {hello: 'hello'}

Answer 17

I want to delete a list of keys, and get back the deleted "slice" of the hash:

Rails:

hash = {a: 1, b: 2, c: 3}

def delete_slice!(hash, *keys)
  hash.slice(*keys).tap { hash.except!(*keys) }
end

delete_slice!(hash, :a, :b)
# => {a: 1, b: 2}
hash
# => {c: 3}

Pure Ruby:

hash = {a: 1, b: 2, c: 3}

def delete_slice!(hash, *keys)
  hash.slice(*keys).tap { keys.each{ hash.delete _1 } }
end

delete_slice!(hash, :a, :b)
# => {a: 1, b: 2}
hash
# => {c: 3}

Answer 18

This would also work: hash[hey] = nil