What s the difference between equal eql and

Question

I am trying to understand the difference between these four methods  I know by default that    calls the method equal  which returns true when both operands refer to exactly the same object       by default also calls    which calls equal     okay  so if all these three methods are not overridden  then I guess         and equal  do exactly the same thing   Now comes eql   What does this do  by default   Does it make a call to the operand s hash id    Why does Ruby have so many equality signs  Are they supposed to differ in semantics

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I wrote a simple test for all the above   def eq a  b    puts     a         b       a    b     puts     a         b       a     b     puts     a    eql    b       a eql  b      puts     a    equal    b       a equal  b    end  eq  all    all   eq  all   all  eq Object new  Object new  eq 3  3  eq 1  1 0

User · Answer

Ruby exposes several different methods for handling equality   a equal  b    object identity - a and b refer to the same object  a eql  b    object equivalence - a and b have the same value  a    b   object equivalence - a and b have the same value with type conversion    Continue reading by clicking the link below  it gave me a clear summarized understanding      https   www relishapp com rspec rspec-expectations v 2-0 docs matchers equality-matchers   Hope it helps others

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I love jtbandes answer  but since it is pretty long  I will add my own compact answer            eql   equal  are 4 comparators  ie  4 ways to compare 2 objects  in Ruby  As  in Ruby  all comparators  and most operators  are actually method-calls  you can change  overwrite  and define the semantics of these comparing methods yourself  However  it is important to understand  when Ruby s internal language constructs use which comparator       value comparison  Ruby uses     everywhere to compare the values of 2 objects  eg  Hash-values    a   z         a   Z          gt  false  a  1          a  1 0         gt  true        case comparison  Ruby uses      in case when constructs  The following code snippets are logically identical   case foo   when bar   p  do something  end  if bar     foo   p  do something  end   eql   Hash-key comparison  Ruby uses  eql   in combination with the method hash  to compare Hash-keys  In most classes  eql  is identical with      Knowledge about  eql  is only important  when you want to create your own special classes   class Equ   attr accessor  val   alias method   initialize   val    def hash             self val   2             end   def eql  other       self hash    other hash  end end  h    Equ new 3    gt  3   Equ new 8    gt  8   Equ new 15    gt  15      3 entries  but 2 are  eql  h size                gt  2 h Equ new 27          gt  15   Note  The commonly used Ruby-class Set also relies on Hash-key-comparison   equal   object identity comparison  Ruby uses  equal  to check if two objects are identical  This method  of class BasicObject  is not supposed to be overwritten   obj   obj2    a  obj equal  obj2           gt  true obj equal  obj dup        gt  false

User · Answer

---case equality      --- generic equality  both works similar but       even do case statements   test      test      gt  true  test       test     gt  true   here the difference  String      test       gt  true String     test      gt  false

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I m going to heavily quote the Object documentation here  because I think it has some great explanations  I encourage you to read it  and also the documentation for these methods as they re overridden in other classes  like String  Side note  if you want to try these out for yourself on different objects  use something like this  class Object   def all equals o      ops                eql    equal       Hash ops map  amp  to s  zip ops map   s  send s  o        end end   quot a quot  all equals  quot a quot      gt    quot    quot   gt true   quot     quot   gt true   quot eql  quot   gt true   quot equal  quot   gt false           generic  quot equality quot   At the Object level     returns true only if obj and other are the same object  Typically  this method is overridden in descendant classes to provide class-specific meaning   This is the most common comparison  and thus the most fundamental place where you  as the author of a class  get to decide if two objects are  quot equal quot  or not          case equality  For class Object  effectively the same as calling      but typically overridden by descendants to provide meaningful semantics in case statements   This is incredibly useful  Examples of things which have interesting     implementations   Range Regex Proc  in Ruby 1 9   So you can do things like  case some object when  a regex      The regex matches when 2  4     some object is in the range 2  4 when lambda   x  some crazy custom predicate       the lambda returned true end  See my answer here for a neat example of how case Regex can make code a lot cleaner  And of course  by providing your own     implementation  you can get custom case semantics  eql      Hash equality  The eql  method returns true if obj and other refer to the same hash key  This is used by Hash to test members for equality  For objects of class Object  eql  is synonymous with     Subclasses normally continue this tradition by aliasing eql  to their overridden    method  but there are exceptions  Numeric types  for example  perform type conversion across     but not across eql   so  1    1 0        gt  true 1 eql  1 0      gt  false   So you re free to override this for your own uses  or you can override    and use alias  eql      so the two methods behave the same way  equal      identity comparison  Unlike     the equal  method should never be overridden by subclasses  it is used to determine object identity  that is  a equal  b  iff a is the same object as b    This is effectively pointer comparison

User · Answer

I would like to expand on the     operator       is not an equality operator   Not   Let s get that point really across    You might be familiar with     as an equality operator in Javascript and PHP  but this just not an equality operator in Ruby and has fundamentally different semantics   So what does     do       is the pattern matching operator        matches regular expressions     checks range membership     checks being instance of a class       calls lambda expressions     sometimes checks equality  but mostly it does not   So how does this madness make sense    Enumerable grep uses     internally case when statements use     internally Fun fact  rescue uses     internally   That is why you can use regular expressions and classes and ranges and even lambda expressions in a case when statement   Some examples  case value when  regexp      value matches this regexp when 4  10     value is in range when MyClass     value is an instance of class when - gt  value              lambda expression returns true when a  b  c  d     value matches one of a through d with       when  array     value matches an element in array with       when x     values is equal to x unless x is one of the above end   All these example work with pattern     value too  as well as with grep method   arr     the    quick    brown    fox   1  1  2  3  5  8  13  arr grep   qx                                                                                                                                    gt    quick    fox   arr grep 4  10      gt   5  8  arr grep String      gt    the    quick    brown    fox   arr grep 1      gt   1  1

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Equality operators     and     The    operator  also known as equality or double equal  will return true if both objects are equal and false if they are not    koan      koan    Output    gt  true   The    operator  also known as inequality  is the opposite of     It will return true if both objects are not equal and false if they are equal    koan      discursive thought    Output    gt  true   Note that two arrays with the same elements in a different order are not equal  uppercase and lowercase versions of the same letter are not equal and so on    When comparing numbers of different types  e g   integer and float   if their numeric value is the same     will return true   2    2 0   Output    gt  true   equal   Unlike the    operator which tests if both operands are equal  the equal method checks if the two operands refer to the same object  This is the strictest form of equality in Ruby   Example      a    zen      b    zen   a object id    Output    gt  20139460 b object id    Output    gt  19972120  a equal  b    Output    gt  false   In the example above  we have two strings with the same value  However  they are two distinct objects  with different object IDs  Hence  the equal  method will return false   Let s try again  only this time b will be a reference to a  Notice that the object ID is the same for both variables  as they point to the same object   a    zen  b   a  a object id    Output    gt  18637360 b object id    Output    gt  18637360  a equal  b    Output    gt  true   eql   In the Hash class  the eql  method it is used to test keys for equality  Some background is required to explain this  In the general context of computing  a hash function takes a string  or a file  of any size and generates a string or integer of fixed size called hashcode  commonly referred to as only hash  Some commonly used hashcode types are MD5  SHA-1  and CRC  They are used in encryption algorithms  database indexing  file integrity checking  etc  Some programming languages  such as Ruby  provide a collection type called hash table  Hash tables are dictionary-like collections which store data in pairs  consisting of unique keys and their corresponding values  Under the hood  those keys are stored as hashcodes  Hash tables are commonly referred to as just hashes  Notice how the word hashcan refer to a hashcode or to a hash table  In the context of Ruby programming  the word hash almost always refers to the dictionary-like collection   Ruby provides a built-in method called hash for generating hashcodes  In the example below  it takes a string and returns a hashcode  Notice how strings with the same value always have the same hashcode  even though they are distinct objects  with different object IDs     meditation  hash    Output    gt  1396080688894079547  meditation  hash    Output    gt  1396080688894079547  meditation  hash    Output    gt  1396080688894079547   The hash method is implemented in the Kernel module  included in the Object class  which is the default root of all Ruby objects  Some classes such as Symbol and Integer use the default implementation  others like String and Hash provide their own implementations    Symbol instance method  hash  owner    Output    gt  Kernel Integer instance method  hash  owner   Output    gt  Kernel  String instance method  hash  owner    Output    gt  String Hash instance method  hash  owner    Output    gt  Hash   In Ruby  when we store something in a hash  collection   the object provided as a key  e g   string or symbol  is converted into and stored as a hashcode  Later  when retrieving an element from the hash  collection   we provide an object as a key  which is converted into a hashcode and compared to the existing keys  If there is a match  the value of the corresponding item is returned  The comparison is made using the eql  method under the hood     zen  eql   zen       Output    gt  true   is the same as  zen  hash     zen  hash   Output    gt  true   In most cases  the eql  method behaves similarly to the    method  However  there are a few exceptions  For instance  eql  does not perform implicit type conversion when comparing an integer to a float   2    2 0      Output    gt  true 2 eql  2 0      Output    gt  false 2 hash    2 0 hash    Output    gt  false   Case equality operator          Many of Ruby s built-in classes  such as String  Range  and Regexp  provide their own implementations of the     operator  also known as case-equality  triple equals or threequals  Because it s implemented differently in each class  it will behave differently depending on the type of object it was called on  Generally  it returns true if the object on the right  belongs to  or  is a member of  the object on the left  For instance  it can be used to test if an object is an instance of a class  or one of its subclasses    String      zen     Output    gt  true Range      1  2      Output    gt  true Array      1 2 3      Output    gt  true Integer     2     Output    gt  true   The same result can be achieved with other methods which are probably best suited for the job  It s usually better to write code that is easy to read by being as explicit as possible  without sacrificing efficiency and conciseness    2 is a  Integer     Output    gt  true 2 kind of  Integer    Output    gt  true 2 instance of  Integer   Output    gt  false   Notice the last example returned false because integers such as 2 are instances of the Fixnum class  which is a subclass of the Integer class  The      is a  and instance of  methods return true if the object is an instance of the given class or any subclasses  The instance of method is stricter and only returns true if the object is an instance of that exact class  not a subclass    The is a  and kind of  methods are implemented in the Kernel module  which is mixed in by the Object class  Both are aliases to the same method  Let s verify   Kernel instance method  kind of      Kernel instance method  is a     Output     true  Range Implementation of      When the     operator is called on a range object  it returns true if the value on the right falls within the range on the left    1  4      3    Output    gt  true  1  4      2 345   Output    gt  true  1  4      6    Output    gt  false    a    d        c    Output    gt  true   a    d        e    Output    gt  false   Remember that the     operator invokes the     method of the left-hand object  So  1  4      3 is equivalent to  1  4      3  In other words  the class of the left-hand operand will define which implementation of the     method will be called  so the operand positions are not interchangeable    Regexp Implementation of      Returns true if the string on the right matches the regular expression on the left       zen       practice zazen today     Output     true       is the same as      practice zazen today     zen   Implicit usage of the     operator on case when statements  This operator is also used under the hood on case when statements  That is its most common use   minutes   15  case minutes   when 10  20     puts  match    else     puts  no match  end    Output  match   In the example above  if Ruby had implicitly used the double equal operator       the range 10  20 would not be considered equal to an integer such as 15  They match because the triple equal operator       is implicitly used in all case when statements  The code in the example above is equivalent to   if  10  20      minutes   puts  match  else   puts  no match  end   Pattern matching operators     and        The     equal-tilde  and     bang-tilde  operators are used to match strings and symbols against regex patterns    The implementation of the    method in the String and Symbol classes expects a regular expression  an instance of the Regexp class  as an argument     practice zazen      zen      Output    gt  11  practice zazen      discursive thought    Output    gt  nil   zazen     zen       Output    gt  2  zazen     discursive thought     Output    gt  nil   The implementation in the Regexp class expects a string or a symbol as an argument     zen      practice zazen     Output    gt  11  zen      discursive thought    Output    gt  nil   In all implementations  when the string or symbol matches the Regexp pattern  it returns an integer which is the position  index  of the match  If there is no match  it returns nil  Remember that  in Ruby  any integer value is  truthy  and nil is  falsy   so the    operator can be used in if statements and ternary operators   puts  yes  if  zazen      zen    Output    gt  yes  zazen      zen   yes   no    Output    gt  yes   Pattern-matching operators are also useful for writing shorter if statements  Example   if meditation type     zazen     meditation type     shikantaza     meditation type     kinhin    true end Can be rewritten as  if meditation type       zazen shikantaza kinhin      true end   The    operator is the opposite of     it returns true when there is no match and false if there is a match   More info is available at this blog post

[ruby] What's the difference between equal?, eql?, ===, and ==?

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