How to check if a value exists in an array in Ruby

Question

I have a value  Dog  and an array   Cat    Dog    Bird      How do I check if it exists in the array without looping through it  Is there a simple way of checking if the value exists  nothing more

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If you want to check by a block  you could try any  or all       w ant bear cat  any    word  word length  gt   3       gt  true    w ant bear cat  any    word  word length  gt   4       gt  true     nil  true  99   any                                gt  true     See Enumerable for more information   My inspiration came from  evaluate if array has any items in ruby

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For what it s worth  The Ruby docs are an amazing resource for these kinds of questions   I would also take note of the length of the array you re searching through  The include  method will run a linear search with O n  complexity which can get pretty ugly depending on the size of the array    If you re working with a large  sorted  array  I would consider writing a binary search algorithm which shouldn t be too difficult and has a worst case of O log n     Or if you re using Ruby 2 0  you can take advantage of bsearch

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it has many ways to find a element in any array but the simplest way is  in    method    example  arr    1 2 3 4  number   1 puts  yes   number  is present in arr  if number in  arr

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You re looking for include     gt  gt    Cat    Dog    Bird   include   Dog    gt  true

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if you don t want to use include  you can first wrap the element in an array and then check whether the wrapped element is equal to the intersection of the array and the wrapped element   This will return a boolean value based on equality   def in array  array  item      item    item  unless item is a  Array      item    array  amp  item end

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Several answers suggest Array include   but there is one important caveat  Looking at the source  even Array include  does perform looping   rb ary includes VALUE ary  VALUE item        long i       for  i 0  i lt RARRAY LEN ary   i              if  rb equal RARRAY AREF ary  i   item                 return Qtrue                      return Qfalse      The way to test the word presence without looping is by constructing a trie for your array  There are many trie implementations out there  google  ruby trie    I will use rambling-trie in this example    a    w cat dog bird   require  rambling-trie    if necessary  gem install rambling-trie trie   Rambling  Trie create    trie  a each do  e  trie  lt  lt  e end     And now we are ready to test the presence of various words in your array without looping over it  in O log n  time  with same syntactic simplicity as Array include   using sublinear Trie include    trie include   bird     gt  true trie include   duck     gt  false

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If you re trying to do this in a MiniTest unit test  you can use assert includes  Example   pets     Cat    Dog    Bird   assert includes pets   Dog          - gt  passes assert includes pets   Zebra        - gt  fails

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If you need to check multiples times for any key  convert arr to hash  and now check in O 1    arr     Cat    Dog    Bird   hash   arr map   x   x true   to h    gt    Cat   gt true   Dog   gt true   Bird   gt true  hash  Dog      gt  true hash  Insect      gt  false   Performance of Hash has key  versus Array include    Parameter              Hash has key                  Array include   Time Complexity         O 1  operation                O n  operation   Access Type             Accesses Hash key  if it      Iterates through each element                         returns any value then        of the array till it                         true is returned to the       finds the value in Array                         Hash has key  call                         call       For single time check using include  is fine

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There are multiple ways to accomplish this  A few of them are as follows   a    1 2 3 4 5   2 in  a     gt  true  8 in  a    gt  false  a member  1    gt  true  a member  8    gt  false

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Try    Cat    Dog    Bird   include   Dog

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If you don t want to loop  there s no way to do it with Arrays  You should use a Set instead   require  set  s   Set new 100 times  i  s  lt  lt   foo  i    s include   foo99      gt  true  1 2 3 4 5 6 7 8  to set include  4       gt  true   Sets work internally like Hashes  so Ruby doesn t need to loop through the collection to find items  since as the name implies  it generates hashes of the keys and creates a memory map so that each hash points to a certain point in memory  The previous example done with a Hash   fake array      100 times  i  fake array  foo  i      1  fake array has key   foo99       gt  true   The downside is that Sets and Hash keys can only include unique items and if you add a lot of items  Ruby will have to rehash the whole thing after certain number of items to build a new map that suits a larger keyspace  For more about this  I recommend you watch  MountainWest RubyConf 2014 - Big O in a Homemade Hash by Nathan Long    Here s a benchmark   require  benchmark  require  set   array      set     Set new  10 000 times do  i    array  lt  lt   foo  i     set    lt  lt   foo  i   end  Benchmark bm do  x    x report  array     10 000 times   array include   foo9999         x report  set       10 000 times   set include   foo9999         end   And the results         user     system      total        real array  7 020000   0 000000   7 020000    7 031525  set    0 010000   0 000000   0 010000    0 004816

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Use Enumerable include   a    w Cat Dog Bird   a include   Dog    Or  if a number of tests are done 1 you can get rid of the loop  that even include  has  and go from O n  to O 1  with   h   Hash  a  a  transpose  h  Dog      1  I hope this is obvious but to head off objections  yes  for just a few lookups  the Hash   and transpose ops dominate the profile and are each O n  themselves

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There s the other way around this   Suppose the array is    edit   update   create   show    well perhaps the entire seven deadly restful sins   And further toy with the idea of pulling a valid action from some string    my brother would like me to update his profile    Then      edit   update   create   show   select  v  v if  my brother would like me to update his profile  downcase               v to s

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Ruby has eleven methods to find elements in an array   The preferred one is include  or  for repeated access  creat a Set and then call include  or member    Here are all of them   array include  element    preferred method array member  element  array to set include  element  array to set member  element  array index element   gt  0 array find index element   gt  0 array index    each  each    element    gt  0 array find index    each  each    element    gt  0 array any     each  each    element   array find    each  each    element      nil array detect    each  each    element      nil   They all return a trueish value if the element is present    include  is the preferred method  It uses a C-language for loop internally that breaks when an element matches the internal rb equal opt rb equal functions  It cannot get much more efficient unless you create a Set for repeated membership checks   VALUE rb ary includes VALUE ary  VALUE item      long i    VALUE e     for  i 0  i lt RARRAY LEN ary   i          e   RARRAY AREF ary  i       switch  rb equal opt e  item           case Qundef          if  rb equal e  item   return Qtrue          break        case Qtrue          return Qtrue              return Qfalse      member  is not redefined in the Array class and uses an unoptimized implementation from the Enumerable module that literally enumerates through all elements   static VALUE member i RB BLOCK CALL FUNC ARGLIST iter  args       struct MEMO  memo   MEMO CAST args      if  rb equal rb enum values pack argc  argv   memo- gt v1         MEMO V2 SET memo  Qtrue       rb iter break          return Qnil     static VALUE enum member VALUE obj  VALUE val      struct MEMO  memo   MEMO NEW val  Qfalse  0      rb block call obj  id each  0  0  member i   VALUE memo     return memo- gt v2      Translated to Ruby code this does about the following   def member  value    memo    value  false  0    each with object memo  do  each  memo      if each    memo 0        memo 1    true        break     end   memo 1  end   Both include  and member  have O n  time complexity since the both search the array for the first occurrence of the expected value    We can use a Set to get O 1  access time at the cost of having to create a Hash representation of the array first  If you repeatedly check membership on the same array this initial investment can pay off quickly  Set is not implemented in C but as plain Ruby class  still the O 1  access time of the underlying  hash makes this worthwhile   Here is the implementation of the Set class   module Enumerable   def to set klass   Set   args   amp block      klass new self   args   amp block    end end  class Set   def initialize enum   nil   amp block     yields  o      hash     Hash new     enum nil  and return     if block       do with enum enum     o  add block o         else       merge enum      end   end    def merge enum      if enum instance of  self class         hash update enum instance variable get   hash       else       do with enum enum     o  add o        end     self   end    def add o       hash o    true     self   end    def include  o       hash include  o    end   alias member  include         end   As you can see the Set class just creates an internal  hash instance  maps all objects to true and then checks membership using Hash include  which is implemented with O 1  access time in the Hash class   I won t discuss the other seven methods as they are all less efficient   There are actually even more methods with O n  complexity beyond the 11 listed above  but I decided to not list them since they scan the entire array rather than breaking at the first match    Don t use these     bad examples array grep element  any   array select    each  each    element   size  gt  0

User · Answer

Here is one more way to do this   arr     Cat    Dog    Bird   e    Dog   present   arr size     arr -  e   size

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This is another way to do this  use the Array index method   It returns the index of the first occurrence of the element in the array   For example   a     cat   dog   horse   if a index  dog       puts  dog exists in the array  end   index   can also take a block   For example   a     cat   dog   horse   puts a index   x  x match  o     This returns the index of the first word in the array that contains the letter  o

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Fun fact    You can use   to check array membership in a case expressions    case element when  array        else       end   Notice the little   in the when clause  this checks for membership in the array    All the usual magic behavior of the splat operator applies  so for example if array is not actually an array but a single element it will match that element

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There is an in  method in ActiveSupport  part of Rails  since v3 1  as pointed out by  campaterson  So within Rails  or if you require  active support   you can write    Unicorn  in    Cat    Dog    Bird        gt  false   OTOH  there is no in operator or  in  method in Ruby itself  even though it has been proposed before  in particular by Yusuke Endoh a top notch member of ruby-core   As pointed out by others  the reverse method include  exists  for all Enumerables including Array  Hash  Set  Range     Cat    Dog    Bird   include   Unicorn       gt  false     Note that if you have many values in your array  they will all be checked one after the other  i e  O n    while that lookup for a hash will be constant time  i e O 1    So if you array is constant  for example  it is a good idea to use a Set instead  E g   require  set  ALLOWED METHODS   Set  to s   to i   upcase   downcase                          etc                         def foo what    raise  Not allowed  unless ALLOWED METHODS include  what to sym    bar send what  end   A quick test reveals that calling include  on a 10 element Set is about 3 5x faster  than calling it on the equivalent Array  if the element is not found    A final closing note  be wary when using include  on a Range  there are subtleties  so refer to the doc and compare with cover

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This will tell you not only that it exists but also how many times it appears    a     Cat    Dog    Bird    a count  Dog       gt  1

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If you want to return the value not just true or false  use  array find  x  x     Dog     This will return  Dog  if it exists in the list  otherwise nil

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If we want to not use include  this also works     cat   dog   horse   select   x  x     dog    any

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array      Cat    Dog    Bird    array include   Dog

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You can try   Example  if Cat and Dog exist in the array      Cat   Dog   Bird    amp    Cat   Dog     size    2    or replace 2 with   Cat   Dog  size   Instead of     Cat   Dog   Bird   member   Cat   and   Cat   Dog   Bird   include   Dog     Note  member  and include  are the same   This can do the work in one line

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How about this way     Cat    Dog    Bird   index  Dog

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I always find it interesting to run some benchmarks to see the relative speed of the various ways of doing something   Finding an array element at the start  middle or end will affect any linear searches but barely affect a search against a Set   Converting an Array to a Set is going to cause a hit in processing time  so create the Set from an Array once  or start with a Set from the very beginning   Here s the benchmark code     frozen string literal  true  require  fruity  require  set   ARRAY    1  20 000  to a SET   ARRAY to set  DIVIDER    -    20  def array include  elem    ARRAY include  elem  end  def array member  elem    ARRAY member  elem  end  def array index elem    ARRAY index elem   gt   0 end  def array find index elem    ARRAY find index elem   gt   0 end  def array index each elem    ARRAY index    each  each    elem    gt   0 end  def array find index each elem    ARRAY find index    each  each    elem    gt   0 end  def array any each elem    ARRAY any     each  each    elem   end  def array find each elem    ARRAY find    each  each    elem      nil end  def array detect each elem    ARRAY detect    each  each    elem      nil end  def set include  elem    SET include  elem  end  def set member  elem    SET member  elem  end  puts format  Ruby v  s   RUBY VERSION        First    gt  ARRAY first     Middle    gt   ARRAY size   2  to i     Last    gt  ARRAY last   each do  k  element    puts DIVIDER  k  DIVIDER    compare do      array include           array include  element                array member            array member  element                 array index             array index element                   array find index        array find index element              array index each        array index each element              array find index each   array find index each element         array any each          array any each element                array find each         array find each element               array detect each       array detect each element          end end  puts     DIVIDER   Sets vs  Array include    DIVIDER      First    gt  ARRAY first     Middle    gt   ARRAY size   2  to i     Last    gt  ARRAY last   each do  k  element    puts DIVIDER  k  DIVIDER    compare do      array include    array include  element         set include      set include  element           set member       set member  element         end end    Which  when run on my Mac OS laptop  results in   Ruby v 2 7 0 -------------------- First -------------------- Running each test 65536 times  Test will take about 5 seconds   array include  is similar to  array index  array index is similar to  array find index  array find index is faster than  array any each by 2x    1 0  array any each is similar to  array index each  array index each is similar to  array find index each  array find index each is faster than  array member  by 4x    1 0  array member  is faster than  array detect each by 2x    1 0  array detect each is similar to  array find each -------------------- Middle -------------------- Running each test 32 times  Test will take about 2 seconds   array include  is similar to  array find index  array find index is similar to  array index  array index is faster than  array member  by 2x    0 1  array member  is faster than  array index each by 2x    0 1  array index each is similar to  array find index each  array find index each is similar to  array any each  array any each is faster than  array detect each by 30 000000000000004     10 0   array detect each is similar to  array find each -------------------- Last -------------------- Running each test 16 times  Test will take about 2 seconds   array include  is faster than  array find index by 10 000000000000009     10 0   array find index is similar to  array index  array index is faster than  array member  by 3x    0 1  array member  is faster than  array find index each by 2x    0 1  array find index each is similar to  array index each  array index each is similar to  array any each  array any each is faster than  array detect each by 30 000000000000004     10 0   array detect each is similar to  array find each  -------------------- Sets vs  Array include  -------------------- -------------------- First -------------------- Running each test 65536 times  Test will take about 1 second   array include  is similar to  set include   set include  is similar to  set member  -------------------- Middle -------------------- Running each test 65536 times  Test will take about 2 minutes   set member  is similar to  set include   set include  is faster than  array include  by 1400x    1000 0 -------------------- Last -------------------- Running each test 65536 times  Test will take about 4 minutes   set member  is similar to  set include   set include  is faster than  array include  by 3000x    1000 0   Basically the results tell me to use a Set for everything if I m going to search for inclusion unless I can guarantee that the first element is the one I want  which isn t very likely  There s some overhead when inserting elements into a hash  but the search times are so much faster I don t think that should ever be a consideration  Again  if you need to search it  don t use an Array  use a Set   Or a Hash    The smaller the Array  the faster the Array methods will run  but they re still not going to keep up  though in small arrays the difference might be tiny    First    Middle  and  Last  reflect the use of first  size   2 and last for ARRAY for the element being searched for  That element will be used when searching the ARRAY and SET variables   Minor changes were made for the methods that were comparing to  gt  0 because the test should be  gt   0 for index type tests   More information about Fruity and its methodology is available in its README

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Cat    Dog    Bird   detect    x  x     Dog     gt   Dog     Cat    Dog    Bird   detect    x  x     Dog   nil    gt  true

[ruby] How to check if a value exists in an array in Ruby

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