How do I copy a hash in Ruby

Question

I ll admit that I m a bit of a ruby newbie  writing rake scripts  now   In most languages  copy constructors are easy to find  Half an hour of searching didn t find it in ruby  I want to create a copy of the hash so that I can modify it without affecting the original instance    Some expected methods that don t work as intended   h0       John   gt  Adams   Thomas   gt  Jefferson   Johny   gt  Appleseed   h1 Hash new h0  h2 h1 to hash   In the meantime  I ve resorted to this inelegant workaround  def copyhash inputhash    h   Hash new   inputhash each do  pair      h store pair 0   pair 1     end   return h end

User · Answer

As others have pointed out  clone will do it   Be aware that clone of a hash makes a shallow copy   That is to say   h1     a   gt   foo    h2   h1 clone h1  a   lt  lt   bar  p h2                    gt    a  gt  foobar     What s happening is that the hash s references are being copied  but not the objects that the references refer to   If you want a deep copy then   def deep copy o    Marshal load Marshal dump o   end  h1     a   gt   foo   h2   deep copy h1  h1  a   lt  lt   bar  p h2                    gt    a  gt  foo     deep copy works for any object that can be marshalled   Most built-in data types  Array  Hash  String   amp c   can be marshalled   Marshalling is Ruby s name for serialization   With marshalling  the object--with the objects it refers to--is converted to a series of bytes  those bytes are then used to create another object like the original

User · Answer

Use Object clone   h1   h0 clone    Confusingly  the documentation for clone says that initialize copy is the way to override this  but the link for that method in Hash directs you to replace instead

User · Answer

Since Ruby has a million ways to do it  here s another way using Enumerable   h0       John   gt  Adams   Thomas   gt  Jefferson   Johny   gt  Appleseed   h1   h0 inject     do  new   name  value        new name    value      new  end

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If you re using Rails you can do   h1   h0 deep dup   http   apidock com rails Hash deep dup

User · Answer

you can use below to deep copy Hash objects   deeply copied hash   Marshal load Marshal dump original hash

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Since standard cloning method preserves the frozen state  it is not suitable for creating new immutable objects basing on the original object  if you would like the new objects be slightly different than the original  if you like stateless programming

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I am also a newbie to Ruby and I faced similar issues in duplicating a hash  Use the following  I ve got no idea about the speed of this method   copy of original hash   Hash new merge original hash

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Clone is slow  For performance should probably start with blank hash and merge  Doesn t cover case of nested hashes     require  benchmark   def bench  Benchmark bm do  b          test     a    gt  1   b    gt  2   c    gt  3  4   gt   d       b report  clone  do       1 000 000 times do  i          h   test clone         h  new     5       end     end     b report  merge  do       1 000 000 times do  i          h              h  new     5         h merge  test       end     end     b report  inject  do       1 000 000 times do  i          h   test inject     do  n   k  v             n k    v            n         end         h  new     5       end     end   end end       bench  user      system      total          real    clone  1 960000   0 080000    2 040000       2 029604    merge  1 690000   0 080000    1 770000       1 767828    inject 3 120000   0 030000    3 150000       3 152627

User · Answer

As mentioned in Security Considerations section of Marshal documentation       If you need to deserialize untrusted data  use JSON or another   serialization format that is only able to load simple     primitive      types such as String  Array  Hash  etc    Here is an example on how to do cloning using JSON in Ruby   require  json   original     John   gt  Adams   Thomas   gt  Jefferson   Johny   gt  Appleseed   cloned   JSON parse JSON generate original      Modify original hash original  John    lt  lt    Sandler  p original     gt    John   gt  Adams Sandler    Thomas   gt  Jefferson    Johny   gt  Appleseed      cloned remains intact as it was deep copied p cloned      gt    John   gt  Adams    Thomas   gt  Jefferson    Johny   gt  Appleseed

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This is a special case  but if you re starting with a predefined hash that you want to grab and make a copy of  you can create a method that returns a hash   def johns          John   gt  Adams   Thomas   gt  Jefferson   Johny   gt  Appleseed   end  h1   johns   The particular scenario that I had was I had a collection of JSON-schema hashes where some hashes built off others   I was initially defining them as class variables and ran into this copy issue

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Hash can create a new hash from an existing hash   irb main  009 0 gt  h1    1   gt  2    gt   1  gt 2  irb main  010 0 gt  h2   Hash h1    gt   1  gt 2  irb main  011 0 gt  h1 object id   gt  2150233660 irb main  012 0 gt  h2 object id   gt  2150205060

User · Answer

Alternative way to Deep Copy that worked for me   h1     a   gt   foo    h2   Hash h1 to a    This produced a deep copy since h2 is formed using an array representation of h1 rather than h1 s references

User · Answer

The clone method is Ruby s standard  built-in way to do a shallow-copy   irb main  003 0 gt  h0     John    gt   Adams    Thomas    gt   Jefferson     gt    John   gt  Adams    Thomas   gt  Jefferson   irb main  004 0 gt  h1   h0 clone   gt    John   gt  Adams    Thomas   gt  Jefferson   irb main  005 0 gt  h1  John      Smith    gt   Smith  irb main  006 0 gt  h1   gt    John   gt  Smith    Thomas   gt  Jefferson   irb main  007 0 gt  h0   gt    John   gt  Adams    Thomas   gt  Jefferson     Note that the behavior may be overridden      This method  may have class-specific behavior  If so  that behavior will be documented under the  initialize copy method of the class

[ruby] How do I copy a hash in Ruby?

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