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.

You can always extend (open at runtime) the built in Hash to add this custom method if you really need it. — dbryson
Ruby 3 will have this. ruby-lang.org/en/news/2020/09/25/ruby-3-0-0-preview1-released — B Seven

Peter Brown · Accepted Answer · 2012-06-19T17:03:50

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

You don’t have to use the full Rails stack. You can include include ActiveSupport in any Ruby application. — Fryie
To add to Fryie’s answer, you don’t even need to load all of ActiveSupport; you can just include them then require "active_support/core_ext/hash/except" — GMA
too late to edit: I meant “include the gem” not “include them” — GMA
@GMA: when your five-minutes-of-editing are up, you can always copy, delete, modify, and repost a comment. — iconoclast

Fabio · Accepted Answer · 2012-01-27T00:35:21

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}

+1 It’s worth mentioning that this method is destructive on h. Hash#except will not modify the original hash. — Mulan
Use h.dup.tap { |hs| hs.delete(:a) } to avoid modifying the original hash. — Jimbali

Srini · Accepted Answer · 2011-06-03T13:23:16

Why not just use:

hash.delete(key)

hash is now the “remaining hash” you’re looking for.

Srini

dbryson

8

@dbryson: I agree that sometimes it doesn’t worth it. I just wonder why there are merge, merge!, delete, but no detele!…
–
if you realy need it as a one liner do: foo(hash.delete(key) || hash)
– Bert Goethals
It would be more consistent with Ruby conventions if delete did not modify its parameter and if delete! existed and did modify its parameter.
– David J.
This doesn’t return the remaining hash as mentioned in the question, it will return the value associated with the deleted key.
– MhdSyrwan
delete returns the key but it does also alter the hash. As to why there is no delete!, my guess is that it semantically doesn’t make sense to call delete on something and not actually delete it. calling hash.delete() as opposed to hash.delete!() would be a no-op.
– eggmatters

techdreams · Accepted Answer · 2016-08-30T15:05:36

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

.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}

.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}

.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}

.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}

.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.

slice and except are added by using ActiveSupport::CoreExtensions::Hash. They are not part of Ruby core. They can be used by require 'active_support/core_ext/hash' — Madis Nõmme
Since Ruby 2.5 Hash#slice is in the standard library. ruby-doc.org/core-2.5.0/Hash.html#method-i-slice Yay! — Madis Nõmme

Yura Taras · Accepted Answer · 2012-12-19T13:55:03

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}

Max Williams · Accepted Answer · 2011-06-03T14:42:11

#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)
=> {}

rewritten · Accepted Answer · 2011-10-13T00:25:32

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.

Just require 'facets/hash/except' and their are no “issues” (not sure what issues they would be anyway other than not 100% AS API). If you are doing a Rails project using AS makes sense, if not Facets has a much smaller footprint. — trans
@trans ActiveSupport nowadays has a quite small footprint too, and you can require only parts of it. Just like facets, but with many more eyes on it (so I suppose it gets better reviews). — rewritten

Mohamad · Accepted Answer · 2014-12-12T19:19:50

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

gamov · Accepted Answer · 2012-09-07T04:25:13

in pure Ruby:

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

Marian13 · Accepted Answer · 2020-10-04T12:29:17

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.

Community · Accepted Answer · 2014-04-07T04:18:19

See Ruby on Rails: Delete multiple hash keys

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

CommunityBot

Nakilon

2

keys_to_delete.each {|k| hash.delete(k)} is much faster for large datasets. correct me if wrong.
– Vignesh Jayavel
@VigneshJayavel, you are right but OP wanted the hash to be returned. each would return the array.
– Nakilon

frenesim · Accepted Answer · 2018-09-02T09:48:38

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}

Sachin Singh · Accepted Answer · 2021-04-04T17:18:06

Try the except! method.

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

frediy · Accepted Answer · 2014-01-12T15:33:02

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)

frediy

2

Hash#except and Hash#except! have been mentioned enough already. The Proc.new version is not very readable as you mention and also more complicated than use_remaining_hash_for_something(begin hash.delete(:key); hash end). Maybe just delete this answer.
– Michael Kohl
Shortened my answer and removed what had already been said. Keeping my answer along with your comment because they answer the question and make good recommendations for use.
– frediy

Ketan Mangukiya · Accepted Answer · 2019-05-14T07:27:52

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

fdghdfg · Accepted Answer · 2014-02-02T19:29:10

This would also work: hash[hey] = nil

fdghdfg

2

h = {:a => 1, :b => 2, :c => 3}; h[:a]=nil; h.each{|k,v| puts k} Is not the same as: h = {:a => 1, :b => 2, :c => 3}; h.delete(:a); h.each{|k,v| puts k}
– obaqueiro
To remove a key from a hash isn’t the same as removing the value of a key from a hash. As this might lead people to confuse, it’d be better to remove this answer.
– Sebastian Palma

16 Answers
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Hash#except (Ruby 3.0+)

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

16 Answers 16

Hash#except (Ruby 3.0+)

16 Answers
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