How can I combine hashes in Perl

Question

What is the best way to combine both hashes into  hash1   I always know that  hash2 and  hash1 always have unique keys   I would also prefer a single line of code if possible      hash1  1      red    hash1  2      blue    hash2  3      green    hash2  4      yellow

User · Accepted Answer

Quick Answer  TL DR          hash1     hash1   hash2          or else           hash1 keys  hash2    values  hash2          or with references           hash ref1       hash ref1    hash ref2       Overview   Context  Perl 5 x Problem  The user wishes to merge two hashes1 into a single variable    Solution   use the syntax above for simple variables use Hash  Merge for complex nested variables   Pitfalls   What do to when both hashes contain one or more duplicate keys    see e g   Perl - Merge hash containing duplicate keys   see e g  Perl hashes  how to deal with duplicate keys and get possible pair   Should a key-value pair with an empty value ever overwrite a key-value pair with a non-empty value    What constitutes an empty vs non-empty value in the first place   eg  undef  zero  empty string  false  falsy         See also   PM post on merging hashes PM Categorical Q amp A hash union Perl Cookbook 5 10  Merging Hashes websearch   perlfaq  merge two hashes  websearch   perl merge hash https   metacpan org pod Hash  Merge     Footnotes  1    aka associative-array  aka dictionary

User · Answer

Check out perlfaq4  How do I merge two hashes  There is a lot of good information already in the Perl documentation and you can have it right away rather than waiting for someone else to answer it        Before you decide to merge two hashes  you have to decide what to do if both hashes contain keys that are the same and if you want to leave the original hashes as they were   If you want to preserve the original hashes  copy one hash   hash1  to a new hash   new hash   then add the keys from the other hash   hash2 to the new hash  Checking that the key already exists in  new hash gives you a chance to decide what to do with the duplicates   my  new hash    hash1    make a copy  leave  hash1 alone  foreach my  key2   keys  hash2             if  exists  new hash  key2                      warn  Key   key2  is in both hashes              handle the duplicate  perhaps only warning                      next                else                    new hash  key2     hash2  key2                     If you don t want to create a new hash  you can still use this looping technique  just change the  new hash to  hash1   foreach my  key2   keys  hash2             if  exists  hash1  key2                      warn  Key   key2  is in both hashes              handle the duplicate  perhaps only warning                      next                else                    hash1  key2     hash2  key2                     If you don t care that one hash overwrites keys and values from the other  you could just use a hash slice to add one hash to another  In this case  values from  hash2 replace values from  hash1 when they have keys in common    hash1  keys  hash2     values  hash2

User · Answer

Check out perlfaq4  How do I merge two hashes  There is a lot of good information already in the Perl documentation and you can have it right away rather than waiting for someone else to answer it        Before you decide to merge two hashes  you have to decide what to do if both hashes contain keys that are the same and if you want to leave the original hashes as they were   If you want to preserve the original hashes  copy one hash   hash1  to a new hash   new hash   then add the keys from the other hash   hash2 to the new hash  Checking that the key already exists in  new hash gives you a chance to decide what to do with the duplicates   my  new hash    hash1    make a copy  leave  hash1 alone  foreach my  key2   keys  hash2             if  exists  new hash  key2                      warn  Key   key2  is in both hashes              handle the duplicate  perhaps only warning                      next                else                    new hash  key2     hash2  key2                     If you don t want to create a new hash  you can still use this looping technique  just change the  new hash to  hash1   foreach my  key2   keys  hash2             if  exists  hash1  key2                      warn  Key   key2  is in both hashes              handle the duplicate  perhaps only warning                      next                else                    hash1  key2     hash2  key2                     If you don t care that one hash overwrites keys and values from the other  you could just use a hash slice to add one hash to another  In this case  values from  hash2 replace values from  hash1 when they have keys in common    hash1  keys  hash2     values  hash2

User · Answer

For hash references  You should use curly braces like the following     hash ref1      hash ref1    hash ref2     and not the suggested answer above using parenthesis     hash ref1     hash ref1   hash ref2

User · Answer

This is an old question  but comes out high in my Google search for  perl merge hashes  - and yet it does not mention the very helpful CPAN module Hash  Merge

User · Answer

For hash references  You should use curly braces like the following     hash ref1      hash ref1    hash ref2     and not the suggested answer above using parenthesis     hash ref1     hash ref1   hash ref2

User · Answer

This is an old question  but comes out high in my Google search for  perl merge hashes  - and yet it does not mention the very helpful CPAN module Hash  Merge

User · Answer

Check out perlfaq4  How do I merge two hashes  There is a lot of good information already in the Perl documentation and you can have it right away rather than waiting for someone else to answer it        Before you decide to merge two hashes  you have to decide what to do if both hashes contain keys that are the same and if you want to leave the original hashes as they were   If you want to preserve the original hashes  copy one hash   hash1  to a new hash   new hash   then add the keys from the other hash   hash2 to the new hash  Checking that the key already exists in  new hash gives you a chance to decide what to do with the duplicates   my  new hash    hash1    make a copy  leave  hash1 alone  foreach my  key2   keys  hash2             if  exists  new hash  key2                      warn  Key   key2  is in both hashes              handle the duplicate  perhaps only warning                      next                else                    new hash  key2     hash2  key2                     If you don t want to create a new hash  you can still use this looping technique  just change the  new hash to  hash1   foreach my  key2   keys  hash2             if  exists  hash1  key2                      warn  Key   key2  is in both hashes              handle the duplicate  perhaps only warning                      next                else                    hash1  key2     hash2  key2                     If you don t care that one hash overwrites keys and values from the other  you could just use a hash slice to add one hash to another  In this case  values from  hash2 replace values from  hash1 when they have keys in common    hash1  keys  hash2     values  hash2

User · Answer

Check out perlfaq4  How do I merge two hashes  There is a lot of good information already in the Perl documentation and you can have it right away rather than waiting for someone else to answer it        Before you decide to merge two hashes  you have to decide what to do if both hashes contain keys that are the same and if you want to leave the original hashes as they were   If you want to preserve the original hashes  copy one hash   hash1  to a new hash   new hash   then add the keys from the other hash   hash2 to the new hash  Checking that the key already exists in  new hash gives you a chance to decide what to do with the duplicates   my  new hash    hash1    make a copy  leave  hash1 alone  foreach my  key2   keys  hash2             if  exists  new hash  key2                      warn  Key   key2  is in both hashes              handle the duplicate  perhaps only warning                      next                else                    new hash  key2     hash2  key2                     If you don t want to create a new hash  you can still use this looping technique  just change the  new hash to  hash1   foreach my  key2   keys  hash2             if  exists  hash1  key2                      warn  Key   key2  is in both hashes              handle the duplicate  perhaps only warning                      next                else                    hash1  key2     hash2  key2                     If you don t care that one hash overwrites keys and values from the other  you could just use a hash slice to add one hash to another  In this case  values from  hash2 replace values from  hash1 when they have keys in common    hash1  keys  hash2     values  hash2

[perl] How can I combine hashes in Perl?

Quick Answer (TL;DR)

Overview

Solution

Pitfalls

See also

Footnotes

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