# Comparing objects **URL:** \ **Category:** Ruby\ **Created:** [May 5, 2006, 4:48pm UTC](https://www.ruby-forum.com/t/comparing-objects/59699 "2006-05-05T16:48:12Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\ **Post date:** [May 5, 2006, 4:48pm UTC](https://www.ruby-forum.com/t/comparing-objects/59699/1 "2006-05-05T16:48:12Z")
Comparing objects in Ruby can be a little confusing. Let me know if I’ve got this straight. eql? - determines whether two object references refer to objects that have the same “value” - for example, my\_car.eql?(your\_car) could test whether my car and your car have the same make, model and year - need to override this method in the Car class to be meaningful, otherwise it’s the same as == equal? - determines whether two object references refer to the same object in memory (have the same object id) - for example, my\_car.equal?(your\_car) tests whether we share the same car == - sometimes same as equal?, but sometimes different - for example, mixing in the Comparable module changes it to be based on \<=\> which would be overridden I kind of wish that “eql?” was named “same\_value” and “equal?” was named “same\_object?” so it would be easier to remember. Using “==” seems somewhat dangerous unless you’re working with built-in types or types that you know mixin Comparable. Otherwise there is some uncertainty about what it does without looking at the code of the classes being compared. ---
**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\ **Post date:** [May 5, 2006, 10:45pm UTC](https://www.ruby-forum.com/t/comparing-objects/59699/2 "2006-05-05T22:45:55Z")
2006/5/5, Mark V. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > Comparing objects in Ruby can be a little confusing. Let me know if > I’ve got this straight. > > eql? > > - determines whether two object references refer to objects that > have the same “value” > - for example, my\_car.eql?(your\_car) > could test whether my car and your car have the same make, model and year > - need to override this method in the Car class to be meaningful, > otherwise it’s the same as == This is called “equivalence”. > equal? > > - determines whether two object references refer to the same object in memory > (have the same object id) > - for example, my\_car.equal?(your\_car) > tests whether we share the same car This is called “identity”. > == > > - sometimes same as equal?, but sometimes different > - for example, mixing in the Comparable module changes it to be based on > \<=\> which would be overridden This again is “equivalence”. > I kind of wish that “eql?” was named “same\_value” and “equal?” was > named “same\_object?” so it would be easier to remember. Using “==” > seems somewhat dangerous unless you’re working with built-in types or > types that you know mixin Comparable. Otherwise there is some > uncertainty about what it does without looking at the code of the > classes being compared. The most notable exception I am aware of is this: irb(main):060:0\> 2.eql? 2.0 =\> false irb(main):061:0\> 2 == 2.0 =\> true A Hash uses eql? HTH robert ---
**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\ **Post date:** [May 5, 2006, 10:52pm UTC](https://www.ruby-forum.com/t/comparing-objects/59699/3 "2006-05-05T22:52:02Z")
On May 5, 2006, at 4:44 PM, Robert K. wrote: > robert Based on this I would amend your explanation with #eql? – Equivalence with structure and type #== – Equivalence with structure #equal? – Identity ---
**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\ **Post date:** [May 5, 2006, 11:04pm UTC](https://www.ruby-forum.com/t/comparing-objects/59699/4 "2006-05-05T23:04:34Z")
2006/5/5, Logan C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > > A Hash uses eql? > > > > HTH > > > > robert > > Based on this I would amend your explanation with > > #eql? – Equivalence with structure and type > #== – Equivalence with structure > #equal? – Identity I’d leave it at “equivalence” for == and eql? because for most types they behave the same. Basically every class’s author is free what she considers “equivalence”. Equivalence is a mathematical term with clear cut meaning and all implementations that satisfy these criteria are compatible with the std lib (Hash, Enumerable methods etc.) > **[Equivalence relation](https://en.wikipedia.org/wiki/Equivalence_relation)** > > All definitions tacitly require the homogeneous relation > > R > R > > be transitive: for all > > > a > , > b > , > c > , > > a,b,c, > > if > > > > a > R > b > > > {\\displaystyle aRb} > > and > > > > b > R > c > > > {\\displaystyle bRc} > > then > > > > a > R > c > . > > > {\\displaystyle aRc.} > > > A term's definition may require additional... Kind regards robert ---
**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\ **Post date:** [May 5, 2006, 11:13pm UTC](https://www.ruby-forum.com/t/comparing-objects/59699/5 "2006-05-05T23:13:53Z")
On 5/5/06, Robert K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > > > > > #equal? – Identity > > I’d leave it at “equivalence” for == and eql? because for most types > they behave the same. Basically every class’s author is free what she > considers “equivalence”. Equivalence is a mathematical term with clear > cut meaning and all implementations that satisfy these criteria are > compatible with the std lib (Hash, Enumerable methods etc.) So why is it useful to have both == and eql? Are there cases where you want them to behave differently? ---
**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\ **Post date:** [May 6, 2006, 12:45am UTC](https://www.ruby-forum.com/t/comparing-objects/59699/6 "2006-05-06T00:45:58Z")
On May 5, 2006, at 5:11 PM, Mark V. wrote: > > > > =\> true > > > > #== – Equivalence with structure > > > > Are there cases where you want them to behave differently? > > – > R. Mark V. > Object Computing, Inc. Well the post that sparked my response and “clarification” is one such case. 2.eql? 2.0 is false, 2 == 2.0 is true I imagine the major difference is == potentially calls coerce ---
**Author:** ![markcasio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9bcb4/32.png) [@markcasio](https://www.ruby-forum.com/u/markcasio)\ **Post date:** [May 6, 2006, 12:25pm UTC](https://www.ruby-forum.com/t/comparing-objects/59699/7 "2006-05-06T12:25:01Z")
I’d like to slightly restate the differences like: > > > #eql? – Equivalence with structure and type > > > #== – Equivalence with structure > > > #equal? – Identity #eql? – Equivalence with type and value #== – Equivalence with value #equal? – Identity On 5/6/06, Logan C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > 2.eql? 2.0 is false, 2 == 2.0 is true > > I imagine the major difference is == potentially calls coerce I’d like to interpret calling coerce as a comparison based on values while ignoring types. Hence, 2.eql? 2.0 is false because their type is differnt, while 2 == 2.0 is true because their value is the same though their have differnt types. Sincerely, Minkoo S.