# What's String.hash **URL:** \ **Category:** Ruby\ **Created:** [January 6, 2010, 3:57am UTC](https://www.ruby-forum.com/t/whats-string-hash/181135 "2010-01-06T03:57:04Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\ **Post date:** [January 6, 2010, 3:57am UTC](https://www.ruby-forum.com/t/whats-string-hash/181135/1 "2010-01-06T03:57:04Z")
irb(main):038:0\* x=“hello” =\> “hello” irb(main):039:0\> x.object\_id =\> 69352346 irb(main):040:0\> x.hash =\> 267522041 irb(main):041:0\> y=“hello”.intern =\> :hello irb(main):042:0\> y.object\_id =\> 246184 irb(main):043:0\> y.hash =\> 246184 What’s the meanings of String.hash method? does it have any relation to object\_id? Thanks. ---
**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\ **Post date:** [January 6, 2010, 4:16am UTC](https://www.ruby-forum.com/t/whats-string-hash/181135/2 "2010-01-06T04:16:58Z")
On Tue, Jan 5, 2010 at 9:56 PM, Ruby N. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > irb(main):043:0\> y.hash > =\> 246184 > > What’s the meanings of String.hash method? > does it have any relation to object\_id? Thanks. Actually all Objects have both a hash and an eql? method. They’re there to support the implementation of the Hash class, and there’s a strict definition of the relationship between those two methods. Hash uses the eql? method to determine if two keys are considered the same. The Object class makes eql? the same as ==, but some Objects override this, for example, 1 == 1.0 returns true, but 1.eql?(1.0) does not. For some objects eql? might be somewhat expensive, for example comparing two large arrays or strings which have many initial elements equal requires scanning until either the end of one of them or until an unequal element is found. So the hash method is used to avoid the Hash object needing to compare everything. It must return a fixnum. The definition of Object#hash states that any overriding implementation must ensure that self.eql?(another) implies self.hash == other.hash Comparing two fixnums for equality is very fast, so when a Hash object wants to determine if it contains a particular object as a key, it compares the hash of that object to the hash of each of its keys. If the two hash values aren’t == then the key and the object can’t be eql? so the key can be rejected as a match. The potentially expensive comparison using eql? is only done when the hash of the object is == to the hash of a key in the hash. There’s a bit more to it than that, the fixnum hash values can also be used to compute where to look for a key in the hash table, but the basic idea is that the hash value is a quick test to rule out two objects being eql? HTH – Rick DeNatale Blog: [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/) Twitter: [http://twitter.com/RickDeNatale](http://twitter.com/RickDeNatale) WWR: [http://www.workingwithrails.com/person/9021-rick-denatale](http://www.workingwithrails.com/person/9021-rick-denatale) LinkedIn: [Rick DeNatale - Developer - IBM | LinkedIn](http://www.linkedin.com/in/rickdenatale) ---
**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\ **Post date:** [January 6, 2010, 4:48pm UTC](https://www.ruby-forum.com/t/whats-string-hash/181135/3 "2010-01-06T16:48:58Z")
A Hash puts objects into “hash chains”, to make it quick to find a particular key amongst a very large collection. Object#hash gives a number, which after a little more processing points directly to the right hash chain. You then only have to search along that one chain to find the object. So it’s not really that eql? is expensive, but rather than you want to minimise the number of eql?'s that are done, by only looking along one chain. > for example > comparing two large arrays or strings which have many initial elements > equal requires scanning until either the end of one of them or until > an unequal element is found. But calculating a hash value for an array or string also involves iterating through the whole object, so that argument doesn’t really hold. For example: to ensure that the following two arrays have the same hash, Array#hash is having to calculate the hash of each element, and combine those hashes together to make the final value. > > a = [1,2,“foo”] > > =\> [1, 2, “foo”] > > > b = [1,2,“foo”] > > =\> [1, 2, “foo”] > > > a.hash > > =\> 876516209 > > > b.hash > > =\> 876516209 If I mutate one of the objects contained, the hash changes (which shows that it’s being recalculated across the whole array) > > b[2] \<\< “x” > > =\> “foox” > > > b.hash > > =\> -1093306170 ---
**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\ **Post date:** [January 7, 2010, 12:05am UTC](https://www.ruby-forum.com/t/whats-string-hash/181135/4 "2010-01-07T00:05:11Z")
On Wed, Jan 6, 2010 at 4:07 PM, Ryan D. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > 502 % ri Object.hash > ------------------------------------------------------------ Object#hash > obj.hash =\> fixnum > > ``` > From Ruby 1.8 > > ``` > > * * * > > ``` > Generates a +Fixnum+ hash value for this object. This function must > > ``` I’m new to Ruby and have been wondering what +…+ is. Does it mark emphasis in some markup language? ---
**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\ **Post date:** [January 7, 2010, 12:42am UTC](https://www.ruby-forum.com/t/whats-string-hash/181135/5 "2010-01-07T00:42:04Z")
On Jan 6, 2010, at 6:04 PM, Eric C. wrote: > > must > > I’m new to Ruby and have been wondering what +…+ is. Does it mark > emphasis in some markup language? yes, and if you’re using fastri (it’s a gem), you’ll get actual highlighting rather than seeing the + markup (which is from the RDoc). -Rob Rob B. [http://agileconsultingllc.com](http://agileconsultingllc.com) removed\_email\_address@domain.invalid ---
**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\ **Post date:** [January 6, 2010, 11:07pm UTC](https://www.ruby-forum.com/t/whats-string-hash/181135/6 "2010-01-06T23:07:49Z")
On Jan 5, 2010, at 18:56 , Ruby N. wrote: > irb(main):043:0\> y.hash > =\> 246184 > > What’s the meanings of String.hash method? > does it have any relation to object\_id? Thanks. You should be using ri to answer these questions: 501 % ri String.hash ------------------------------------------------------------ String#hash str.hash =\> fixnum ``` From Ruby 1.8 ``` * * * ``` Return a hash based on the string's length and content. ``` 502 % ri Object.hash ------------------------------------------------------------ Object#hash obj.hash =\> fixnum ``` From Ruby 1.8 ``` * * * ``` Generates a +Fixnum+ hash value for this object. This function must have the property that +a.eql?(b)+ implies +a.hash == b.hash+. The hash value is used by class +Hash+. Any hash value that exceeds the capacity of a +Fixnum+ will be truncated before being used. ``` 503 % ri Object.object\_id ------------------------------------------------------- Object#object\_id obj. **id** =\> fixnum obj.object\_id =\> fixnum ``` From Ruby 1.8 ```