What is the best way to convert an array to a hash in Ruby

Question

In Ruby  given an array in one of the following forms      apple  1  banana  2    apple  1    banana  2        what is the best way to convert this into a hash in the form of      apple   gt  1  banana   gt  2

User · Accepted Answer

NOTE  For a concise and efficient solution  please see Marc-Andr   Lafortune s answer below   This answer was originally offered as an alternative to approaches using flatten  which were the most highly upvoted at the time of writing  I should have clarified that I didn t intend to present this example as a best practice or an efficient approach  Original answer follows     Warning  Solutions using flatten will not preserve Array keys or values   Building on  John Topley s popular answer  let s try   a3       apple   1     banana   2      orange   seedless    3    h3   Hash  a3 flatten    This throws an error   ArgumentError  odd number of arguments for Hash         from  irb  10 in              from  irb  10   The constructor was expecting an Array of even length  e g    k1   v1  k2   v2      What s worse is that a different Array which flattened to an even length would just silently give us a Hash with incorrect values   If you want to use Array keys or values  you can use map   h3   Hash a3 map   key  value   key  value    puts  h3    h3 inspect     This preserves the Array key   h3     orange    seedless    gt 3   apple   gt 1   banana   gt 2

User · Answer

if you have array that looks like this -  data      foo  1 2 3 4    bar  1 2    foobar  1     3 5  foo     and you want the first elements of each array to become the keys for the hash and the rest of the elements becoming value arrays  then you can do something like this -  data hash   Hash data map    key   key shift  key         gt   foo   gt  1  2  3  4    bar   gt  1  2    foobar   gt  1       3  5   foo

User · Answer

You can also simply convert a 2D array into hash using   1 9 3p362  005  gt  a    1 2   3 4       gt    1  2    3  4    1 9 3p362  006  gt  h   Hash a      gt   1  gt 2  3  gt 4

User · Answer

Summary  amp  TL DR   This answer hopes to be a comprehensive wrap-up of information from other answers   The very short version  given the data from the question plus a couple extras   flat array        apple  1    banana  2      count 4 nested array      apple  1    banana  2      count 2 of count 2 k v arrays incomplete f      apple  1    banana         count 3 - missing last value incomplete n      apple  1    banana         count 2 of either k or k v arrays     there s one option for flat array  h1    Hash  flat array                          gt   apple  gt 1  banana  gt 2     two options for nested array  h2a   nested array to h   since ruby 2 1 0      gt   apple  gt 1  banana  gt 2  h2b   Hash nested array                         gt   apple  gt 1  banana  gt 2     ok if  only  the last value is missing  h3    Hash incomplete f each slice 2  to a      gt   apple  gt 1  banana  gt nil    always ok for k without v in nested array  h4    Hash incomplete n    or  to h             gt   apple  gt 1  banana  gt nil     as one might expect  h1    h2a     gt  true h1    h2b     gt  true h1    h3      gt  false h3    h4      gt  true   Discussion and details follow     Setup  variables  In order to show the data we ll be using up front  I ll create some variables to represent various possibilities for the data  They fit into the following categories   Based on what was directly in the question  as a1 and a2    Note  I presume that apple and banana were meant to represent variables  As others have done  I ll be using strings from here on so that input and results can match      a1       apple   1     banana   2      flat input a2       apple   1     banana   2      key value paired input   Multi-value keys and or values  as a3   In some other answers  another possibility was presented  which I expand on here      keys and or values may be arrays on their own   a3        apple                     1                banana                    2                 orange   seedless        3                pear                    4  5              Unbalanced array  as a4   For good measure  I thought I d add one for a case where we might have an incomplete input   a4        apple                     1             banana                    2              orange   seedless        3             durian                          a spiky fruit pricks us  no value             Now  to work   Starting with an initially-flat array  a1   Some have suggested using  to h  which showed up in Ruby 2 1 0  and can be backported to earlier versions   For an initially-flat array  this doesn t work   a1 to h       gt  TypeError  wrong element type String at 0  expected array    Using Hash     combined with the splat operator does   Hash  a1      gt    apple   gt 1   banana   gt 2    So that s the solution for the simple case represented by a1   With an array of key value pair arrays  a2   With an array of  key value  type arrays  there are two ways to go   First  Hash     still works  as it did with  a1    Hash a2      gt    apple   gt 1   banana   gt 2    And then also  to h works now   a2 to h      gt    apple   gt 1   banana   gt 2    So  two easy answers for the simple nested array case   This remains true even with sub-arrays as keys or values  as with a3   Hash a3      gt    apple   gt 1   banana   gt 2    orange    seedless    gt 3   pear   gt  4  5    a3 to h      gt    apple   gt 1   banana   gt 2    orange    seedless    gt 3   pear   gt  4  5     But durians have spikes  anomalous structures give problems    If we ve gotten input data that s not balanced  we ll run into problems with  to h   a4 to h      gt  ArgumentError  wrong array length at 3  expected 2  was 1    But Hash     still works  just setting nil as the value for durian  and any other array element in a4 that s just a 1-value array    Hash a4      gt    apple   gt 1   banana   gt 2    orange    seedless    gt 3   durian   gt nil    Flattening - using new variables a5 and a6  A few other answers mentioned flatten  with or without a 1 argument  so let s create some new variables   a5   a4 flatten     gt    apple   1   banana   2    orange    seedless    3   durian    a6   a4 flatten 1      gt    apple   1   banana   2    orange    seedless    3   durian      I chose to use a4 as the base data because of the balance problem we had  which showed up with a4 to h   I figure calling flatten might be one approach someone might use to try to solve that  which might look like the following   flatten without arguments  a5    Hash  a5            gt    apple   gt 1   banana   gt 2   orange   gt  seedless   3  gt  durian      This is the same as calling  Hash  a4 flatten       At a na  ve glance  this appears to work     but it got us off on the wrong foot with the seedless oranges  thus also making 3 a key and durian a value   And this  as with a1  just doesn t work   a5 to h     gt  TypeError  wrong element type String at 0  expected array    So a4 flatten isn t useful to us  we d just want to use Hash a4   The flatten 1  case  a6    But what about only partially flattening   It s worth noting that calling Hash     using splat on the partially-flattened array  a6  is not the same as calling Hash a4    Hash  a6      gt  ArgumentError  odd number of arguments for Hash   Pre-flattened array  still nested  alternate way of getting a6    But what if this was how we d gotten the array in the first place    That is  comparably to a1  it was our input data - just this time some of the data can be arrays or other objects    We ve seen that Hash  a6  doesn t work  but what if we still wanted to get the behavior where the last element  important  see below  acted as a key for a nil value   In such a situation  there s still a way to do this  using Enumerable each slice to get ourselves back to key value pairs as elements in the outer array   a7   a6 each slice 2  to a     gt     apple   1     banana   2      orange    seedless    3     durian       Note that this ends up getting us a new array that isn t  identical  to a4  but does have the same values   a4 equal  a7      gt  false a4    a7          gt  true   And thus we can again use Hash       Hash a7      gt    apple   gt 1   banana   gt 2    orange    seedless    gt 3   durian   gt nil    or Hash a6 each slice 2  to a    But there s a problem   It s important to note that the each slice 2  solution only gets things back to sanity if the last key was the one missing a value   If we later added an extra key value pair   a4 plus   a4 dup   just to have a new-but-related variable name a4 plus push   lychee   4       gt     apple                  1           banana                 2            orange    seedless    3     multi-value key         durian                   missing value         lychee   4               new well-formed item  a6 plus   a4 plus flatten 1      gt    apple   1   banana   2    orange    seedless    3   durian    lychee   4   a7 plus   a6 plus each slice 2  to a     gt     apple                  1           banana                 2            orange    seedless    3     so far so good         durian                  lychee      oops  key became value         4                         and we still have a key without a value  a4 plus    a7 plus     gt  false  unlike a4    a7   And the two hashes we d get from this are different in important ways   ap Hash a4 plus    prints                          apple    gt  1                       banana    gt  2         orange    seedless      gt  3                       durian    gt  nil    correct                      lychee    gt  4      correct    ap Hash a7 plus    prints                          apple    gt  1                       banana    gt  2         orange    seedless      gt  3                       durian    gt   lychee     incorrect                            4   gt  nil         incorrect      Note  I m using awesome print s ap just to make it easier to show the structure here  there s no conceptual requirement for this    So the each slice solution to an unbalanced flat input only works if the unbalanced bit is at the very end     Take-aways    Whenever possible  set up input to these things as  key  value  pairs  a sub-array for each item in the outer array   When you can indeed do that  either  to h or Hash     will both work  If you re unable to  Hash     combined with the splat     will work  so long as inputs are balanced  With an unbalanced and flat array as input  the only way this will work at all reasonably is if the last value item is the only one that s missing      Side-note  I m posting this answer because I feel there s value to be added     some of the existing answers have incorrect information  and none  that I read  gave as complete an answer as I m endeavoring to do here   I hope that it s helpful   I nevertheless give thanks to those who came before me  several of whom provided inspiration for portions of this answer

User · Answer

If the numeric values are seq indexes  then we could have simpler ways     Here s my code submission  My Ruby is a bit rusty     input     cat   1   dog   2   wombat   3     hash   Hash new    input each with index   item  index       if  index 2    0  hash item    input index 1          hash      gt    cat   gt 1   wombat   gt 3   dog   gt 2

User · Answer

Appending to the answer but using anonymous arrays and annotating   Hash    a b c d  split      zip  1 2 3 4   flatten     Taking that answer apart  starting from the inside     a b c d  is actually a string  split on commas into an array  zip that together with the following array   1 2 3 4  is an actual array    The intermediate result is     a 1   b 2   c 3   d 4     flatten then transforms that to     a  1  b  2  c  3  d  4    and then      a  1  b  2  c  3  d  4  unrolls that into  a  1  b  2  c  3  d  4  which we can use as the arguments to the Hash   method   Hash    a b c d  split      zip  1 2 3 4   flatten     which yields     a   gt 1   b   gt 2   c   gt 3   d   gt 4

User · Answer

Simply use Hash  array variable flatten   For example   a1     apple   1   banana   2  h1   Hash  a1 flatten 1   puts  h1    h1 inspect    a2      apple   1     banana   2   h2   Hash  a2 flatten 1   puts  h2    h2 inspect     Using Array flatten 1  limits the recursion so Array keys and values work as expected

User · Answer

Update  Ruby 2 1 0 is released today  And I comes with Array to h  release notes and ruby-doc   which solves the issue of converting an Array to a Hash    Ruby docs example      foo   bar    1  2   to h        gt    foo   gt   bar  1   gt  2

User · Answer

Not sure if it s the best way  but this works   a     apple   1   banana   2  m1      for x in  a length   2  times   m1 a x 2     a x 2   1  end  b      apple   1     banana   2   m2      for x y in b   m2 x    y end

User · Answer

The best way is to use Array to h       apple 1    banana 2    to h     gt   apple  1  banana  2    Note that to h also accepts a block     apple   banana  to h    fruit   fruit   I like   fruit s            gt   apple   I like apples   banana   I like bananas     Note  to h accepts a block in Ruby 2 6 0   for early rubies you can use my backports gem and require  backports 2 6 0 enumerable to h   to h without a block was introduced in Ruby 2 1 0   Before Ruby 2 1  one could use the less legible Hash     array       apple 1    banana 2    Hash  array        gt    apple   gt  1   banana   gt  2    Finally  be wary of any solutions using flatten  this could create problems with values that are arrays themselves

User · Answer

Edit  Saw the responses posted while I was writing  Hash a flatten  seems the way to go    Must have missed that bit in the documentation when I was thinking through the response  Thought the solutions that I ve written can be used as alternatives if required    The second form is simpler   a      apple  1     banana  2   h   a inject        r  i  r i first    i last  r     a   array  h   hash  r   return-value hash  the one we accumulate in   i   item in the array  The neatest way that I can think of doing the first form is something like this   a     apple  1   banana  2  h      a each slice 2     i  h i first    i last

[ruby] What is the best way to convert an array to a hash in Ruby

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